the production of aluminium grain refining master …
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THE PRODUCTION OF ALUMINIUM GRAIN REFINING MASTER ALLOYS
by
Mark Simon Lee
A thesis submitted for the degree o f Doctor o f Philosophy of London University andthe Diploma o f Imperial College
Department o f Materials,Imperial College o f Science, Technology and M edicine,
London SW 7 2BP
Novem ber 1989
A B ST R A C T
The grain refinem ent o f aluminium and its alloys during solidification is h ighly desirable; since it improves the properties o f the metal and allow s the use o f efficient direct chill casting techniques. Grain refinem ent is achieved industrially by the addition o f titanium and boron to the m elt in the form o f an A l-T i-B master alloy. Such alloys are produced com m ercially by the reduction o f the com plex inorganic salts potassium fluoroborate, K BF4, and potassium fluorotitanate, K2T iF 6, by aluminium in the temperature range 700-800°C.
The present investigation is concerned with a number o f aspects o f the production process for A l-T i-B m aster a lloys. Firstly, an apparatus for m easuring the decom position pressures o f com plex alkali metal fluorides has been established and used to investigate the volatilities o f KBF4, K2T iF6 and their sodium equivalents. Secondly , studies o f the decom position pressures o f KF-A1F3 fluxes, w hich are produced during the manufacture o f A l-T i-B alloys, have been made as part o f an investigation o f the thermodynamics o f the KF-A1F3 system . Thirdly, a m odified sessile drop technique has been used to investigate interfacial phenom ena in the follow ing systems: Al-Ti/K F-AlF3-TiF4, A1-B/KF-A1F3-BF3 and Al-Ti-B/KF-A1F3- TiF4-BF3. Such studies have made it possible to propose solutions to the industrial problems o f m etal-flux entrapment and boride agglomeration. Fourthly, the effect o f the process history o f the alloy on its microstructure has been studied, since the grain refining ability o f an A l-T i-B alloy is known to depend on its microstructure. B y relating the process history o f an alloy to microstructural change it has been possib le to specify the process parameters required to produce efficient and reliable grain refiners.
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C O N TE N T S
Page
1. THE GRAIN REFINEMENT OF ALUMINIUM 1
2 . THE MEASUREMENT OF DECOMPOSITION PRESSURES OF COMPLEX ALKALI METAL FLUORIDES
2.1 Introduction 6
2 .2 Thermodynamic Treatment o f Vapour Pressure Data 6
2 .3 Techniques 82.3.1 The Boiling Point Method 9
2 .3 .2 The Transportation Method 9
2 .4 Experimental Method 112 .4 .1 Apparatus 112 .4 .2 Calibration 17
2 .5 Conclusions 21
3 . SALTS
3.1 Introduction 22
3 .2 Production3 .2 .1 Alkali Metal Fluoroborates 2 23 .2 .2 Alkali Metal Fluorotitanates 23
3 .3 Phase Equilibria 24
3 .3 .1 Alkali Metal Fluoroborates 2 63 .3 .2 Alkali Metal Fluorotitanates 30
Page
3 .4 Decom position Pressures 343 .4 .1 Alkali Metal Fluoroborates 343 .4 .2 Alkali Metal Fluorotitanates 59
3 .5 Reactions with Aluminium 423 .5 .1 Chemistry 4 23 .5 .2 Thermodynamics 433 .5 .3 Kinetics 45
3 .6 D iscussion and Conclusions 47
4 . THE KF-AIF3 SYSTEM
4 .1 Introduction 50
4 .2 Phase Equilibria 50
4 .3 M elt Activities and Structure 524 .3 .1 Activities in Molten Salts 524 .3 .2 Techniques for the Determination o f Activities in Molten Salts 524 .3 .3 Determination o f MF-A1F3 Melt Activities from Calorimetric Data 534 .3 .4 Structural Constituents o f MF-A1F3 M elts 61
4 .4 Decom position Pressures o f KF-A1F3 M elts4 .4 .1 Vapour species over KF-AIF3 M elts 624 .4 .2 Previous Work on Decom position Pressures o f KF-AIF3 M elts 6 54 .4 .3 Experimental Programme and Results 654 .4 .4 D iscussion 65
4 .5 Equilibrium Reactions in the A1/KF-A1F3 System 65
4 .6 Conclusions and Industrial Implications ^
Page
5 . INTERFACIAL PHENOMENA
5 .1 Introduction5 .1 .1 Interfacial Phenomena and Metallurgical Processes 715 .1 .2 Interfacial Phenomena and the Production o f Al-Ti-B A lloys 71
5 .2 Interfacial Energy5 .2 .1 Theory 725 .2 .2 Interfacial Energy in Ternary Systems 76
5 .2 .2 .1 Principles 765 .2 .2 .2 Ternary Interfacial Energy Diagrams 78
5 .2 .3 Influence o f M ass Transfer on Interfacial Phenomena 805 .2 .4 Techniques o f Measurement 815 .2 .5 Experimental Method 82
5 .3 Metal-Flux Systems5 .3 .1 Introduction 835 .3 .2 The Al/NaF-AlF3 System 845 .3 .3 Experimental Programme and Results 86
5 .3 .3 .1 AI/KF-AIF3 865 .3 .3 .2 Al/NaF-AlF3 86
5 .3 .3 .3 Al-Ti/KF-A1F3 875 .3 .3 .4 Al-Ti/NaF-AlF3 915 .3 .3 .5 A1-B/KF-A1F3 915 .3 .3 .6 A l-B/NaF-AlF3 995 .3 .3 .7 Al-Ti-B/KF-A1F3 955 .3 .3 .8 Al-Ti-B/NaF-AlF3 985 .3 .3 .9 Al-Zr-B/KF-A1F3 101
5 .3 .4 D iscussion5 .3 .4 .1 A1/KF-A1F3 1015 .3 .4 .2 Al/NaF-AlF3 1015 .3 .4 .3 Al-Ti/KF-A1F3 1015 .3 .4 .4 Al-Ti/NaF-AlF3 1° 45 .3 .4 .5 A1-B/KF-A1F3 1° 45 .3 .4 .6 Al-B/NaF-AlF3 1°75 .3 .4 .7 Al-Ti-B/KF-A1F3 1075 .3 .4 .8 Al-Ti-B/NaF-AlF3 1095 .3 .4 .9 Al-Zr-B/KF-A1F3 1° 9
Page
5 .4 Conclusions and Industrial Implications 109
6 . ALLOY STRUCTURE
6 .1 Introduction 111
6 .2 Al-Ti Master A lloys 111
6 .3 Al-Zr Master A lloys 116
6 .4 Al-Ti-B Master A lloys 116
6 .5 The Effects o f Process Parameters on A lloy Microstructure6 .5 .1 Theory 1186 .5 .2 A lloy Structure - Experimental Work 1206 .5 .3 Results and D iscussion 12 0
6 .6 A l-B Master Alloys6 .6 .1 Introduction 1246 .6 .2 Phase Equilibria 124
6.7 Conclusions and Industrial Implications 125
7 . CONCLUSIONS AND INDUSTRIAL IMPLICATIONS 129
8 . REFERENCES 132
9 . APPENDIX 136
1 0 . A C K N O W L E D G E M E N T S 138
LIST OF FIGURES
1.1 The grain refining characteristics o f a typical Al-Ti-B master alloy.
2 .1 Typical vapour pressure vs. flow rate curve for a transportation apparatus.
2 .2 Transportation technique apparatus.
2 .3 Graphite boat.
2 .4 Reaction zone in the transportation technique apparatus.
2 .5 The apparent decom position pressure o f a 0.45 A1F3 - 0.55 KF flux at 690°C measured at various argon flow rates.
3 .1 Phase diagram for the system KF-KBF4.
3 .2 Phase diagram for the system s KBF4-KBF3OH and KF-KBF3OH.
3 .3 Phase diagram for the system NaF-NaBF4.
3 .4 Phase diagrams for the systems:
(a) KF-TiF4(b) NaF-TiF4
3 .5 Plot o f lnp vs. (1/T) for KBF4.
3 .6 Plot o f lnp vs. (1/T) for NaBF4.
3 .7 Plot o f lnp vs. (1/T) for K2TiF6.
3 .8 Plot o f lnp vs. (1/T) for Na2TiF6.
3 .9 Free energy diagram for the formation o f M eF2.
4 .2 AH for the reaction M F ^ + A1F3(-S) at 1298K.
4 .3 (a) Partial enthalpies and free energies for the systems LiF-AlF3 andN aF-A lF3.
(b) Partial entropies for the system s LiF-AlF3 and NaF-AlF3.
4 .4 (a) L iF ^ iso-activity lines on the LiF-AlF3 phase diagram.
(b) A1F3(s) iso-activity lines on the LiF-AlF3 phase diagram.
4 .5 (a) N aF ^ iso-activity lines on the NaF-AlF3 phase diagram.
(b) A1F3(s) iso-activity lines on the NaF-AlF3 phase diagram.
4 .6 (a) K F ^ iso-activity lines on the KF-A1F3 phase diagram.
(b) A1F3(s) iso-activity lines on the KF-A1F3 phase diagram.
4 .7 KA1F4 partial pressure isobars on the KF-A1F3 phase diagram.
4 .8 Activities in the KF-A1F3 system at 600°C.
4 .9 Plot o f lnp vs. (1/T) for a 0.45 A1F3 - 0 .55 KF melt.
4 .1 0 Plot o f lnp vs. (1/T) for a 0 .50 A1F3 - 0 .5 0 KF melt.
5 .1 The Al-Ti-B/KF-A1F3 interface.
5 .2 Definition o f contact angle.
5 .3 Possible shapes o f a liquid drop.
5 .4 Diagram o f a drop o f phase 3 at phase 1/2 interface.
5 .6 Plot o f a vs. t for a typical reaction involving mass transfer at the slag-metal interface.
5 .7 X-Radiography technique to determine interfacial tension.
5 .8 The influence o f different additives on the interfacial tension at the electrolyte/Al boundary at 1273K. Electrolyte consists o f 88 wt% o f NaF-AlF3 mixture and 12 wt% AI2O3.
5 .9 The reaction o f A1 with 7.5 wt% K2TiF6 in (KF-A1F3)E for various times at 690°C.
5 .1 0 The reaction o f A1 with various flux compositions for 15 minutes at 720°C.
5 .11 The reaction o f A1 with 7.5 wt% K2TiF6 in (KF-A1F3)E for various tim es at 740°C.
5 .1 2 The influence o f CaF2 addition levels on the reaction o f A1 with 7.5 wt% K2TiF6 in (KF-A1F3)e for 15 minutes at 790°C .
5 .1 3 The reaction o f A1 with 15 wt% KBF4 in (KF-A1F3)e for various times at 740°C.
5 .1 4 Distribution o f boron between metal and flux with varying concentration o f KBF4 in initial flux.
5 .1 5 The reaction o f A1 with 15 wt% K2TiF6 and 15 wt% KBF4 in (KF-A1F3)E for various times at 740°C system.
5 .1 6 The effect o f order o f addition o f K 2TiF6 and KBF4 to A1 at 740°C .
5 .1 7 The influence o f K2TiF6 and KBF4 level in the flux on the interfacial tension and agglomeration o f borides.
5 .1 8 (a) Ternary interfacial energy diagram for the system A l-T iA l3-(KF-A1F3).
(b) Interfacial phenomena represented on the KF-TiF4-A lF3 ternary phase diagram.
5 .1 9 (a) Ternary interfacial energy diagram for the system A1-A1B 12-(K F-A1F3).
(b) Interfacial phenomena represented on the KF-BF3-A1F3 ternary phase diagram.
5 .2 0 Ternary interfacial energy diagram for the system Al-TiB2-(KF-AlF3).
6 .1 Aluminium rich end o f the Al-Ti phase diagram.
6 .2 Typical aluminide (TiAl3) morphologies in an A1 - 5 wt% - 1 wt% B alloy.
6 .3 The effect o f ageing at 700°C on aluminide morphology in an A1 - 5 wt% Ti -0 .2 wt% B alloy.
6 .4 Aluminium rich end o f the Al-B phase diagram.
6 .5 Free energy - com position relationship in the Al-B system.
LIST OF TABLES
2 .1 The apparent decomposition pressure o f a 0.45 A1F3 - 0.55 KF flux at 692°C measured at various argon flow rates.
2 .2 The vapour pressure o f lead measured at 9 10°C.
3.1 Properties o f KBF4 and K 2TiF6.
3 .2 The m elting point o f KBF4.
3 .3 The melting point o f NaBF4.
3 .4 The m elting point o f K^TiFg.
3 .5 The decom position pressure o f KBF4.
3 .6 The decom position pressure o f NaBF4.
3 .7 The decom position pressure o f K2T iF6.
3 .8 The decom position pressure o f N a2TiF6.
3 .9 Species present in the salt phase during the salt-metal reaction at 750°C .
3 .1 0 Titanium and boron transfer efficiencies for laboratory scale melts.
4 .1 Partial enthalpy and entropy data for LiF-AlF3, NaF-AlF3 and KF-A1F3 liquid- solid mixtures.
4 .2 The decom position pressure o f a 0 .45 A1F3 - 0.55 KF melt.
4 .3 The decom position pressure o f a 0 .5 0 A1F3 - 0 .50 KF melt.
6 .1 The variation in aluminide morphology with process conditions.
1. THE GRAIN REFINEMENT OF ALUMINIUM
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1 . THE GRAIN REFINEMENT OF ALUMINIUM
The prom otion o f a fine, uniform grain structure during the so lid ifica tion o f aluminium and its alloys is highly desirable. The benefits include:
(a) Improved mechanical properties(b) Uniform surface finish.(c) Reduction in internal stresses.(d) D irect Chill (D.C.) Casting
Grain refinem ent permits a substantial increase in casting speed by the direct chill continuous m ethod. It also leads to a reduction in segregation cracking and the number o f cold shuts produced by the D.C. technique. Grain refinem ent o f a metal or alloy during solid ification m ay be achieved in a number o f w ays e .g . m elt inoculation, vibration or stirring. In the case o f aluminium, m elt inoculation with titanium and boron is the method predominantly used. This m ethod o f alum inium grain refinem ent offers substantial benefits in both continuous casting by direct chill (D .C .) and in cast to shape products. Perhaps the m ost important benefit is that grain refinem ent can permit a radical increase in casting speed. In the aluminium industry this has led to the bulk o f grain refining agents being used in the production o f ingots, extrusion billets and sheet for fabrication, using either semi-continuous or continuous m ethods o f casting. Here, inadequate rates o f nucleation w ould g iv e coarse structures, which in extreme cases result in ingot cracking or, in other alloys, surface defects which are detrimental in sheet and other products requiring a superior finish.
The inoculants titanium and boron are usually introduced in the form o f an aluminium master alloy. A typical master alloy com position is A1 - 5 wt% T i - 1 wt% B and the levels o f titanium and boron in the final grain refined alloy are o f the order o f 0 .01 wt% T i and 0 .0 0 2 wt% B. In other w ords, the m aster a lloy is d iluted by approxim ately 500 times. In D . C. casting the grain refiner is introduced into the launder ju st before the m ould and so a fast grain refining action is required. H ow ever, if the grain refining addition is made to a holding furnace and held for several hours, then it is necessary for the grain refining performance to be retained. Such time dependent grain refining characteristics o f a master alloy are best illustrated on a grain refining curve - see Figure 1.1. A typical grain refining curve has two distinct sections:-
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(a) After addition o f the grain refiner, the grain size initially decreases with time (the line AO on Figure 1.1) reaching a m inim um - known as the 'ultimate grain size' - at 0. The tim e to reach this point is referred to as the ’contact time' o f the grain refiner.
(b) Further hold ing g iv es on ly increasing grain size , the upward line OB indicating the phenomenon known as 'fade'.
A s outlined above, the optim um contact time o f a grain refiner depends on the application for w hich it w as intended. In D. C. casting a short contact tim e is obviously desirable, but when the master alloy addition is made to a holding furnace then a long contact time and good fade characteristics are required.
A great deal o f work [1-15] has been done over the years in the field o f aluminium grain refinement. A com prehensive review o f this work has been given by G uzowski et al. [15]. This work has addressed tw o main questions. Firstly, why is it that an A l-T i-B alloy is so m uch more effective as a grain refiner than an A l-T i alloy? Secondly, w hy is it that two master alloys o f exactly the same chem ical com position can behave quite differently as grain refiners? In attempting to answ er these questions much work has been done in trying to elucidate the m echanism o f grain refinem ent. D espite the enorm ous amount o f attention devoted to this area, no conclusive answers have yet emerged. A lum inium grain refinem ent rem ains, to a large extent, as an art rather than a science.
The production o f the grain refining master alloys them selves is also very m uch an art. A l-T i-B alloys are produced com m ercially by the addition o f the com plex inorganic salts potassium fluorotitanate, K^TiFg, and potassium fluoroborate, K BF4, to m olten aluminium. The temperature o f the process varies from 700 to 800°C . Alum inium reduces the fluoride salts to yield an A l-T i-B alloy under a KF-A1F3 flux layer. This production route has both econom ic and technical advantages over the introduction o f titanium and boron in elem ental form . H ow ever, there are also problems associated with this production route. Firstly, the salts K2TiF6 and KBF4 are volatile and so the transfer o f Ti and B to the m elt is not com plete. Secondly, problem s occur with the separation o f the alloy from the KF-AIF3 flux. This can result in entrapment o f flux in metal, w hich is unacceptable from the master alloy users point o f view . Thirdly, agglom eration o f T iB 2 particles can occur. N ot only does this affect the ability o f the alloty to grain refine, but also causes problem s i f such boride agglomerates enter the finished product. Despite these problems, and the
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Log (Holding Time)
Figure 1. 1 Master alloy grain refining characteristics.
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im portance o f the process, very little has appeared in the open literature that is concerned with master alloy production. Recognising that a lack o f understanding o f the production process o f A l-T i-B alloys from fluoride salts exists, Laporte Fluorides Ltd., as producers o f K^TiFg and KBF4, are com m itted to pursuing research in this area. A s part o f that commitment, Laporte Fluorides have initiated and funded this project, which is concerned with the optimisation o f the production o f A l-T i-B alloys from K 2T iF6 and KBF4.
In order to attempt to optim ise the production o f aluminium grain refining master alloys it is necessary first to have an understanding o f the chemistry and m etallurgy underlying the process. The project has, therefore, been con cerned w ith investigations in the follow ing areas:-
(a) Salts
The fluoride salts K2T iF6 and KBF4 have been investigated. O f particular interest here is the thermal stability o f the salts, the chem istry o f their reactions w ith aluminium and the kinetics o f uptake o f Ti and B by aluminium.
(b) KF-AIF3 Flux
The physico-chem ical properties o f KF-A1F3, such as density, v iscosity and surface tension, are o f great importance in the process and have received m uch attention in this project.
(c) The interface between the alloy and flux is also a particularly important area and has received much attention in this study.
(d) A lloy Structure
The grain refining performance o f a master alloy is thought to be related to the alloy microstructure. The microstructure in turn is related to the process history o f the alloy. B y studying the affect o f process parameters on alloy structure it m ay then be possible to produce reliable grain refiners.
This thesis consists o f four main sections which are concerned with the properties o f the fluoride salts, the KF-A1F3 flux, the alloy-flux interface and the alloy structure. The technique o f vapour pressure measurement has been used to investigate both the
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salts and the KF-A1F3 flux. The technique has, therefore, been described in a separate chapter which precedes those chapters concerned with the fluoride salts and the KF-AIF3 flux.
2. THE MEASUREMENT OF DECOMPOSITION PRESSURES OF COMPLEX ALKALI
METAL FLUORIDES
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2 . THE MEASUREMENT OF DECOMPOSITION PRESSURES OF COMPLEX ALKALI METAL FLUORIDES
2 .1 Introduction
In the area o f h igh temperature therm ochem ical research, vapour pressure m easurements have been determined extensively. The reason for this is that the vapour pressures g ive, directly, standard G ibbs free energy changes for chem ical reactions involving the production o f gaseous species in equilibrium with a system o f condensed phases. The temperature dependence o f the vapour pressure can be used to determ ine changes in the standard enthalpy and standard entropy for these reactions. Vapour pressure measurement constitutes, therefore, a powerful tool in the area o f high temperature thermochemistry.
T his chapter is concerned w ith the analysis o f vapour pressure data to g iv e therm odynam ic in form ation and w ith the tech n iq u es o f vapour pressure determ ination. A n apparatus has been d evelop ed in this laboratory for the measurement o f decom position pressures o f com plex alkali metal fluorides and w ill be described.
The apparatus has been used to determine the decom position pressures o f the fluoride salts K2T iF 6, K BF4, N a2T iF 6 and N aB F4 and o f KF-A1F3 flu xes o f varying KF:A1F3 ratio. The results o f these investigations on fluoride salts and KF-A1F3 fluxes w ill be given in Chapters 3 and 4 respectively.
2 .2 The Thermodynamic Treatment of Vapour Pressure Data
Evaporation is the process o f escape o f atoms or m olecules from the surface o f a liquid; sublimination the escape o f species from the surface o f a solid. In a c losed system the space above a condensed phase becom es saturated with vapour and a dynamic equilibrium is established. The vapour m olecules exert a saturation vapour pressure w hose value depends only on the temperature; it is virtually independent o f external pressure. If the volum e o f the space is reduced, som e o f the vapour condenses, but there is no change o f vapour pressure.
For vapour pressures le ss than one atmosphere, vapour species may be considered
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ideal and the ideal gas law s apply. The equilibrium constant, K, m ay then be calculated from the partial pressure, P. For the simple reaction
M (s or 1) -> M (g)
K = P M (g) an<i the standard free energy change at the absolute temperature o f reaction, T, is:
A G °t = - RTlnK (2 .1)
Enthalpies and entropies o f reaction can be obtained from the variation o f equilibrium constants with temperature by two different theoretical methods. Firstly, there is the 'second law' approach in which:
A G °t = AHt ° - TASt ° (2 .2)
Where AH°T = change in standard enthalpy at T A S°t = change in standard entropy at T AG°t = change in standard free energy at T
Com bining (2.1) and (2.2)
InK = -AH °t + AS°t (2 .3)“R T “ ~R
The function InK is plotted versus 1/T and the enthalpy and entropy o f reaction are obtained from the slope and intercept o f the straight line through the experimental points.
S econdly , there is the 'third law' m ethod in w hich A H 0 is calculated at som e reference temperature, generally 298K, from each experimental point by computing the change in free energy function (f.e.f.).
Af.e.f. = -A G °t - H ° 298 (2 .4)
AH°298 = -RTlnp + TA(f.e.f.) (2 .5)
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A com m only used test for system atic errors in temperature m easurem ent or in measured lnp values is to exam ine A H °298 as calculated from Ink values at various tem peratures. If a temperature dependent trend in A H ° 298 is apparent, som e system atic error is present in the experiments or in the free energy functions used. If the available f .e .f.’s are calculated from actual spectroscopic data for atom s and m olecu les and from measured Cp values for condensed phases, then the third law method is the more accurate method for obtaining enthalpies.
H owever, if the f.e.f.'s are not available or have only been estimated, then the second law approach must be used. Significant errors can be introduced by using such an approach. This may be appreciated by differentiating (2.3):-
dp = AH dT (2.6)*p" R T2
For AH = 1 0 0 K cal/m ole T = 1000K dT = 5K
the fractional error in p is 25% . Typical enthalpies o f vaporization o f m etals and com pounds o f interest in m etals research m ay range from 2 0 K cal/m ole to 150 Kcal/m ole. Clearly, the accurate control o f temperature is an important consideration w hen measuring vapour pressure.
2 . 3 T e c h n iq u e s
A variety o f experim ental m ethods have been d evelop ed for vapour pressure measurements at high temperature as reviewed by Kubaschewski, Evans and A lcock [16] and by Margrave [17]. Vapour pressures may be measured either directly or indirectly. D irect vapour pressure m easurem ent techniques include the sim ple m anom eter, boiling point observations and the d ew point m ethod. Indirect techniques include the transportation technique, the Langmuir m ethod and the Knudsen method. The preferred technique for any system depends on a number o f factors, including the magnitude o f the pressure being measured and the nature o f the system being investigated. For measurements o f the vapour pressures o f sodium cryolite (NaF - A1F3) melts, two methods have com m only been used [18], nam ely the boiling point method and the transportation method.
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2 .3 .1 T h e B o i l i n g P o i n t M e t h o d
The boiling point method has been applied in various forms, these differing mainly in the means o f detecting the point o f balance between the inert gas pressure and the vapour pressure. One method successfully applied is the m odified thermogravimetric procedure. Here, the substance to be investigated is contained in a crucible with a sm all opening in the lid. The crucible is suspended from a balance in the isothermal zone o f the furnace, and an inert gas atmosphere, w hich is in itially at a higher pressure than the equilibrium vapour pressure o f the system , surrounds the crucible. W hen the inert gas pressure is low ered (or the vapour pressure increased by increasing the temperature) the rate o f vapour transport through the open ing increases. The rate o f increase becom es particularly marked w hen the inert gas pressure becom es sm aller than the vapour pressure. From the data for the rate o f m ass loss from the sample as a function o f the inert gas pressure, the equilibrium vapour pressure m ay be determined. The boiling point m ethod has been used extensively in the measurement o f vapour pressures over NaF-AlF3 melts [18].
2 .3 .2 T h e T r a n s p o r t a t i o n M e t h o d
In essence, the transportation m ethod for the m easurem ent o f vapour pressures con sists o f passing an inert carrier gas over the sam ple o f interest at constant temperature and total pressure. The vapour pressure o f the sam ple m ay then be calculated from the amount o f material lost from the sample or the amount o f material condensed, after a known volum e o f carrier gas has been passed. The transportation technique has the follow ing advantages for vapour pressure measurements:
(a) It provides a m eans for measuring individual vapour pressures o f several components o f a mixture at the same time - providing that suitable methods o f analysis are available.
(b) The apparatus has structural simplicity.
The use o f the transportation m ethod for the measurem ent o f vapour pressures is based on the follow ing assumptions:
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(a) Ideal Gas Behaviour
If the vapour and carrier gas behave ideally then Dalton's Law o f Partial Pressures w ill hold.
PT = £ P i (2 .7 )i
nq1 = £ nj (2 .8 )i
where
PT = Total pressure (usually one atmosphere)Pj = Partial pressure o f component i nT = Total number o f m oles o f gas
= Number o f m oles o f component i
Thus, for flow o f an inert gas A , and one vapour species V , w e have:
P y = Py = nv (2 .9 )PT Pv + PA ny + nA
(b) A knowledge o f the molecular species
The calculation o f n y in ( 2 .9 ) depends on a know ledge o f the m ass o f the vapour transported, M v , and the molecular weight o f the vapour, Mr(V).
nv = M v (2 .10 )Mr(V)
The vapour species may be identified either by a technique such as mass spectrometry or by perform ing vapour pressure m easurem ents by the techniques o f both transportation and boiling point. The boiling point method gives a direct value for the vapour pressure and m akes no assum ptions concerning the vapour species. The
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transportation technique in v o lv es an assum ption o f the vapour sp ecies. B y com paring results from both m ethods it is then p ossib le to determ ine the vapour species present.
(c) That the carrier gas is saturated with vapour
B y using the transportation m ethod many in vestigation have calculated apparent vapour pressures and displayed their results as a function o f flow rate as show n in Figure 2.1. At low total flow rate the fraction nv/(n v + nA) in equation 2 .9 becom es large because m olecules leave the reaction zone by m echanism s other than bulk transport e .g. by diffusion dow n a thermal gradient. A t high flow rates, ny/(n v + nA) becom es small because kinetic limitations prevent saturation o f the carrier gas. It is necessary to establish a plateau region on such a curve. Vapour pressures measured in this region may then be taken as the saturated vapour pressures.
The flo w rate range o f the plateau region w ill vary according to the experim ental technique and the system studied. The effects o f sam ple geometry, carrier gas flow rate and absolute magnitude o f vapour pressure have been studied by A lcock and H ooper [19]. Previous workers using the transportation m ethod em ployed large sam ples and reaction tubes in w hich there w as a com paratively large dead volum e above the sample. This space between the sample and the reaction tube made it difficult to obtain saturation o f the carrier gas. To overcom e this problem A lcock and H ooper em ployed an alumina plug device. The plugs were seen to affect both the low and high flow rate parts o f the curve. It w as suggested that at high flow rates the alumina plug stirs up the approaching carrier gas. The turbulence produced allow s higher flow rates to be used before under-saturation begins. At low flow rates it was suggested that the alum ina plug device at the mouth o f the reaction tube w as to prevent a barrier to m olecu les o f the volatile species leaving the reaction zone by diffusion.
2 . 4 E x p er im en ta l M eth o d
2 .4 .1 A p p a r a t u s
An apparatus was established in the laboratory for the m easurem ent o f vapour pressures by the transportation technique. Figure 2 .2 is a schematic diagram o f the overall technique. The technique involves passing a known volum e o f argon carrier gas over the m olten fluoride sam ple, which is contained in a graphite boat. The
ApparentVapourPressure
hoI
Flow Rate of Carrier Gas
Figure 2.1 Typical vapour pressure versus flow rate curve for a transportation apparatus.
A Kanthal Resistance Furnace E Gas Meter
B Cooling Coils Hi Capillary Flow MetersC Titanium Furnace Ti Controlling ThermocoupleD Drying Tubes T2 Measuring Thermocouple
FIGURE 2.2 Transportation technique apparatus
-1 4 -
cylindrical boat - which is illustrated in Figure 2.3 - also serves as a plug.
The volum e o f high purity argon carrier gas w hich w as p assed during each experim ent was measured by means o f an 'Alexander Wright' com m ercial gas m eter and regulated by a capillary flow meter. In the gas meter the stream b ecom es saturated with water vapour and so it is necessary to then dry the gas by passing it over silica gel and then anhydrone. The gas meter is guaranteed to V2% and can be read to ± 10 m l. H ow ever, since the gas in the gas meter becom es saturated with water vapour a correction factor has to be applied when calculating the volum e o f dry gas passed over the sam ple. In addition to the carrier gas stream, a stream o f deoxygenated argon is passed around the outside o f the reaction tube to carry the volatile species away. An enlarged diagram o f the reaction zone is given in Figure2 .4 .
It is important that the graphite boat does not oxidise during the course o f a run, since this w ill lead to a w eight change and thus it w ill not be possib le to ascertain the w eight loss o f the sample. A preliminary test, in which no fluoride w as present, gave no w eight loss within the lim its o f experimental error i.e. the graphite boat was not oxidised in a typical experiment.
The temperature o f the furnace is controlled by a Euro-therm temperature controller with a chrom el-alum el thermocouple in a m ullite tube (T x). The temperature inside the furnace tube is m easured by another chrom el-alum el therm ocouple (T2). The temperature profile o f the furnace was determined by placing a thermocouple inside the inner reaction tube. A constant temperature hot zone, 5 cm in length, w as located in the furnace. The passage o f 500 ml min"* argon (at N.T.P.) through the tube did not alter the position o f the hot zone.
The experimental procedure for each run was as follows:-
(i) The graphite boat is weighed empty and then with the sample.(ii) The furnace is brought to temperature and flushed with argon for at least
one hour.(iii) The reaction tube, containing the graphite boat, is inserted into the
furnace and sits in the hot zone.(iv) A measured quantity o f argon gas is passed over the graphite boat over a
two hour period.(v) At the end o f the run the reaction tube is withdrawn from the furnace
A D
E
E
A Kanthal Windings E Mullite Reaction TubeB Alumina Winding Tube F Graphite BoatC Alumina Furnace Tube
Ti Controlling ThermocoupleD Mullite Thermocouple Tube
T2 Measuring Thermocouple
FIGURE 2.4 Reaction Zone in the transportation technique apparatus
-17-
and cooled in an atmosphere o f argon.(vi) The graphite boat is removed and weighed.
2 . 4 . 2 C a l i b r a t i o n
In order to establish the flow range for which the decom position pressure m easured m ay be taken as a saturation decom position pressure the apparatus w as calibrated using a KF-A1F3 flux as sample. The com position o f the sample w as 45 m ole A1F3 i.e. o f eutectic com position. A s discussed in Chapter 4 the vapour above a KF-A1F3 m elt is the m olecule KA1F4. Thus, the weight loss o f the sample may be converted into the number o f m oles o f vapour carried away by the argon. The apparent decom position pressure o f the sample, at 692°C , w as measured over a variety o f argon flow rates ranging from 30 m l min"^ to 250 m l min"^ m easured at normal temperature and pressure (N.T.P.). The results obtained are given in Table 2.1 and illustrated in Figure 2.5 . It was seen that a plateau in the decom position pressure vs. flow rate curve occured betw een flow rates o f argon o f 60 and 120 m l m in' ^. Decom position pressures measured in this flow region m ay then be taken as saturated decom position pressures.
In order to check the validity o f the decom position pressure values obtained in these experiments it was necessary to use the apparatus to measure the vapour pressure o f a compound or elem ent which has a w ell defined vapour pressure. Lead at 910°C was chosen. Lead is a suitable candidate material since it does not react with graphite and at 910°C its vapour pressure is in the same range as that determined for the KF-A1F3 slag at 690°C. The results obtained are given in Table 2.2.
From the data o f K im and Cosgarea [20] the vapour pressure o f lead at 910°C is 0.41 m m Hg. The vapour pressures calculated in Table 3.2 are based on the assum ption that the m olecular w eight o f the vapour is 207 .2 i.e. that lead vapour is m onatom ic. The work o f K im and Cosgarea show s that lead vapour is 100% m onatom ic and hence the assumption is valid. The values for the vapour pressure o f lead at 910°C by the transportation m ethod are low er than the actual vapour pressure. From (2.6) w e have that:-
= AH dT R T 2
dPP
THE APPARENT DECOMPOSITION PRESSURES OF A KF-AIF FLUX (45 mole% A1F 3 - 55 mole% KF) AT 690°C MEASURED AT VARIOUS ARGON FLOW RATES
R U N Am/g n(f/moles VAl/d n i3 VAr (ml m irr^ n^/m ole I^/mmHg
1 0.0228 1.600 x 10 “4 3.85 32 0 .1 6 0 .7 62 0.0265 1.870 x 10 6.28 52 0 .26 0 .5 43 0.0225 1.580 x 1 0 ' 4 7 .32 60 0.305 0 .3 94 0.0261 1.838 x 1 0 ' 4
-48.83 73 0.367 0.38
5 0.0289 2 .0 3 0 x 10 10.58 88 0.441 0 .356 0.0339 2.387 x 10 ~4 10.7 89 0.446 0 .4 07 0.0323 2.275 x 10 “4 10.92 91 0.455 0 .3 88 0.0357 2.500 x 10 - 4 11.9 99 0.496 0 .389 0.0415 2.920 x 10 “4 13.8 106 0.575 0 .3 910 0.0422 2.970 x 10 14.4 120 0 .6 0 .3811 0.045 3.170 x 10 ‘ 4 17.2 143 0.717 0 .3 412 0.0408 2.875 x 10 ' 4 22.8 190 0.95 0 .2313 0.0487 3.43 x 10 ‘ 4 31.44 260 1.31 0 .1 9
T A B L E 2.1
Vapour Pressure (mm Hg)
Figure 2.5 The apparent vapour pressures of a KF - AIF3 flux measured at 690°C and at varying argon flow rates.
19-
APPARENT VAPOUR PRESSURE OF LEAD AT 910°C
RUN AnVg n pAnoles VAr/dm 3 V ^/m l min n/\^moles Pp/mm Hg
1 0 .0602 2.90 x 10"4 13.58 113 0 .5 66 0 .3 92 0.0562 2.71 x 10‘4 13.58 113 0 .5 66 0 .3 73 0.0667 3.22 x 10'4 13.68 76 0.57 0 .4 34 0.0621 3.01 x 10'4 14.5 81 0 .6 04 0 .3 85 0 .042 2.03 x 10"4 9 .64 80 0 .4 02 0 .38
TABLE 2.2
-2 1 -
For lead at 910°C w e have
T = 1183K, AH = 4 6 .6 K cal m ole '1, dT = 5K.
H ence, the fractional error in P is 8%. The vapour pressure o f lead measured by the transportation method is, therefore, within the limits o f experimental error.
2 .5 C onclusions
For the transportation method apparatus a region exists, between flow rates o f 60 and 120 m l min"l argon, in w hich the apparent vapour pressure is independent o f flow rate. Thus, it may be concluded that the apparatus constructed satisfies the conditions necessary for the measurement o f saturated vapour pressures, o f magnitudes in the order o f 1/2 m m Hg.
The apparatus has been used to measure decom position pressures, o f this order, for the fluoride salts K 2TiF6, KBF4, Na2TiF6 and N A BF4 and flux com positions in the system KF-A1F3. The results obtained are given in the follow ing Chapters.
3. SALTS
-2 2 -
3 . SALTS
3 .1 Introduction
The predominant industrial production route for the manufacture o f A l-T i-B alloys is the reaction o f the inorganic salts potassium fluorotitanate, K^TiFg, and potassium fluoroborate, K BF4, with aluminium. This production route has econom ic and technical advantages over the introduction o f titanium and boron in elem ental form. H ow ever, the reduction o f fluoride salts by alum inium has a number o f its ow n particular process problems. Firstly, the salts are volatile. Hence, com plete transfer o f titanium and boron from the salt to the metal is not seen in the current production process, where the salts are added to the surface o f the molten aluminium. Secondly, problem s are encountered with the separation o f the a lloy and flux. Thirdly, unwanted agglomeration o f T iB 2 particles is seen. In order to try to overcom e these problem s, as part o f a process optim isation, it is necessary first to have an understanding o f the properties o f the fluoride salts and o f their reactions w ith aluminium. This section o f the thesis is, therefore, devoted to the salts.
The potassium salts w ill receive the m ost attention in this chapter, since these constitute the bulk o f salts used com m ercially. H owever, consideration w ill also be given to other alkali metal salts and in particular the sodium salts. This w ill a llow a consideration o f the possibility o f substituting sodium salts for potassium salts and may also shed light on the behaviour o f alkali metal-fluoroborates and -fluorotitanates in general.
3.2 Production
3.2.1 A lk a li M e ta l F lu o ro b o ra te s
The process route for the production o f potassium fluoroborate is outlined below . The production route for other alkali metal fluoroborates is similar [21].
The production o f potassium fluoroborate involes the solution o f pentahydrate borax in hydrofluoric acid to form tetrafluoroboric acid, H BF4, and boric acid, H3B 0 3. Tetrafluoroboric acid is a strong acid, dissociating to form the hydroxonium ion, H 30 + , and the fluoroborate ion, BF4\ The solution is then reacted w ith sodium hydroxide to form sodium fluoroborate, and potassium fluoroborate is then
-2 3 -
precipitated out by addition o f potassium chloride.
The basic reactions are as follow s.
2N a2B 40 7.5H 20 ^ + 3 2 H F ^ —>
4N aF(aq) + 7 H B F 4(aq) + H3B 0 3 (aq) + 21H 20 (1)
?H BF4(aq) + 7NaO H (aq) -» 7NaBF4(aq) + 7H 20 (1)
7N aB F4(aq) + 7K Cl(aq) 7KBF4(s) + 7N aC l(aq)
Overall:
2N a2B 40 7.5H 20 (s) + 32H F(aq) + 7NaO H (aq) + 7K C l(aq) -»
7K BF4(s) + 4N aF(aq) + H j B O ^ , + 7NaCl(aq) + 28H 20 (1)
3 . 2 . 2 A lk a li M e ta l F lu o ro tita n a te s
The process route for the production o f potassium fluorotitanate is outlined below . The production route for other alkali metal fluorotitanates is similar.
The synthesis o f K 2TiF6 in vo lves the solution o f T i, T i02 or TiF4 in hydrofluoric acid . The so lu tion produced conta ins various fluoro co m p lex io n s , but predominantly the TiF62~ ion. There is no sim ple hydrated Ti4+ ion, because o f the high charge to radius ratio. This ion may be crystallised out by the addition o f potassium hydroxide to g ive potassium hexafluorotitanate. Industrially, titaniferrous raw materials, ilm enite ores or a slag concentrate m ay be used as starting materials [22]. However, this g ives rise to a contamination o f the solution which is difficult to correct. Hence, pure titanium dioxide is preferred as the starting material.
Basic reactions:-
™ 2<w + 6HF(aq) - » H2TiF6(aq) + 2H20(1)
-2 4 -
H2TiF6(aq) + 2K OH (aq) -» K2TiF6(s) -i- 2H20 a)
Overall:-
2K O H (aq )+ Ti02(s) + 6HF(aq) - » K J . F ^ + 4H 20 (1)
Side Reaction:-
H 2 ^ i F 6 ( a q ) + 2 K O H ( a q ) K 2 ^ i F 6 ( s ) + 2 H 2 ^ ( 1 )
The typical properties o f K BF4 and K2TiF6, produced by Laporte F luorides, are given in Table 3.1.
3.3 Phase Equilibria
3.3.1 I n tr o d u c t io n
Many investigations o f the equilibrium phase diagrams o f metal fluoride system s took place during the 1950's, 60's and 70's in the U .S .S .R . and the U .S .A . T h ese investigations were conducted in order to define the phase relationships in numerous metal fluoride system s as a m eans, primarily, o f evaluating their potential as nuclear fission enriching solutions, fuel solvents, coolants and reprocessing m edia. The results o f these investigations are, o f course, o f great interest to other workers in the field o f molten salts.
A number o f techniques are available for use in the determination o f phase diagrams for metal fluorides. They are based essentially on the four follow ing techniques:
(a) Thermal A nalysis - this in vo lves the exam ination o f the co o lin g curves obtained by thermal analysis with relatively large samples.
(b) Thermal Gradient Quenching - this involves the examination o f sm all samples quenched after equilibration at a known temperature. Phases present in each sample are identified by examination in the polarizing m icroscope and/or by use o f X ray analysis.
-25
PROPERTIES OF POTASSIUM FLUOROTITANATE, K2TiF6
Physical Form Crystalline PowderColour WhiteChemical Analysis: K 2T iF6 97.5 wt%K 2S iF 6 1 wt%Iron as Fe 75 ppmH eavy M etals as Pb 50 ppmM g, Ca 400 ppmParticle size 80% > 75 |imBulk Density: Solid - 2.5 g/cnr3
Solubility in WaterL oose - 1.41 g/cm 3 approx. 1.25 g per 100 cm 3 at 20°C
PROPERTIES OF POTASSIUM FLUOROBORATE, KBF4
Physical Form Crystalline PowderColour WhiteChemical Analysis:k b f 4 98-99 wt%Free boric acid as H3B 0 3 0.1 wt%Fluorosilicate as K2SiF6 0.2 wt%Sodium as Na 0.1 wt%Chloride as Cl 0 .2 wt%Particle size 50% > 75}imBulk Density: Solid - 2.85 g/cm 3
L oose - 1.36 g/cm 3 approx.Solubility in Water 0 .4 g per 100 cm 3 at 20°C
T A B L E 3.1
-2 6 -
(c) Visual Observation - this involves the visual inspection o f samples w hile they are held at known temperatures e.g. by hot stage microscopy.
(d) Differential Thermal Analysis (D.T.A.) - this involves the detection o f thermal changes in a sample, relative to a standard, as the temperature is varied.
The determ ination o f phase diagram s for alkali m etal-fluoroborates and - fluorotitanates by these, or any other techniques is com plicated by a number o f factors. Firstly, these com pounds are volatile. The com position o f the sam ple w ill, therefore, vary with tim e i f the vapour species is different in com position to the melt i.e . vaporization is incongruent. This can have a profound effect on the data obtained. This is w ell illustrated in sections 3.3.1.1 and 3.3 .1 .2 on the system s KF- K BF4 and N aF-N aBF4. Here, loss o f the volatile com ponent B F3 m eans that the sample becom es progressively enriched in the alkali fluoride. The problem can be overcom e by using D .T .A . em ploying a sample contained in a sealed crucible.
Secondly, the presence o f moisture, and hence hydroxy fluorides, can drastically lower the m elting point o f the sample. These two factors - volatility and hydrolysis - have led to major d ifferences being observed by different workers for the same system . If any phase diagram determination is to be accepted then any D .T .A . investigations must be carried out with the sample in a sealed capsule and great care must be taken to avoid introducing moisture into the sample.
3 . 3 . 2 A l k a l i M e t a l F l u o r o b o r a t e s
The phase diagrams for the M F-M BF4 system s (M = Li, Na, K, Rb, Cs) are all seen to be sim ple eu tectics. N o interm ediate phases have been observed . The determination o f phase relations in alkali metal fluoroborate system s is particularly sensitive to the problem s outlined above in 3.3.1 i.e. volatalisation and hydrolysis. This is reflected in the w idely varying melting points reported for NaBF4 and KBF4 - see Table 3 .2 - and the w id ely ranging eutectic temperatures and com positions reported.
3 .3 .2 .1 K F - K B F 4
The melting point o f KBF4 is reported in the literature as ranging from 514°C [26] to 580°C [24]. H ow ever, m any o f these determinations were carried out in open
- 2 7 -
system s and hence must be discounted. Tw o sets o f investigators have used D.T.A . with a sealed capsule to study K BF4. Barton et al. [23] report a m elting point o f 570°C and Pistorius [24] gives a value o f 580°C.
Barton et al. [23] have conducted what is probably the best and m ost thorough investigation o f the KF-KBF4 system , by means o f D .T .A . and gradient quenching techniques. Their value for the eutectic com position 74.5 ± 1 m ole % K BF4, agrees with that reported by Pawlenkco [28], 75 m ole % KBF4. However, Barton's
SALT MPT/°C METHOD OF DETERMINATION REFERENCE
570 D.T.A. - Sealed Cm cible 23580 D.T.A. - Sealed C m cible 24537 D.T.A. - Open Cm cible 25514 D.T.A. - Open Cm cible 26532 D.T.A. - Open Cm cible 27567 D.T.A. - Open Cm cible 28552 Thermal Analysis 25531 Thermal Analysis 29
TABLE 3.2
eutectic m elting point, 460 ± 2°C , is som ew hat higher than that reported by Pawlenkco, 441°C. Barton et al. were w ell aware o f the problems o f hydrolysis and volatalisation and took care to avoid them. Additionally, the agreement between theirD .T .A . and quenching data for the eutectic temperature was good. Their diagram w ill, therefore, be taken as the best available. It is given in Figure 3.1.
In addition to his investigation o f the KBF4-KF system , Pawlenkco also considered the KBF4-KBF3OH and the K F-K BF3OH systems. H is diagrams for these system s are given in Figure 3.2. Although his absolute values m ay be in error, comparison o f his eutectic temperatures for K F-K BF4 (441°C ) and K F-K BF3OH (310°C ) is nevertheless a good illustration o f the effect o f hydrolysis.
-2 8 -
Phase diagram for the system KF-KBF^ [23]
FIGURE 3.1
-2 9 -
MOL% B
Phase diagrams for the systems KBF^-KBF^OH and KF-KBF^OH [28]
FIGURE 3.2
-3 0 -
3 .3 .2 .2 N a F - N a B F 4
The m elting point o f NaBF4 is reported in the literature as ranging from 368°C [26] to 408°C [23]. The literature values are given in Table 3.3.
SALT MPT/°C METHOD OF DETERMINATION REFERENCE
N aB F4 393 D.T.A. - Open Crucible 25368 D.T.A. - Open Cm cible 26408 D.T.A. - Sealed C m cible 23399 D.T.A. - Sealed C m cible 30
TABLE 3.3
H ow ever, as w ith KBF4, som e o f these determinations were carried out in open system s and thus the m elting points obtained can be expected to be low er than the actual value. U sing D .T .A . with sealed crucibles Pistorius et al. [30] reported a m elting point o f 399°C and Barton et al. [23], 408°C. O nce again, Barton's diagram is probably the best and is preferred. It is given in Figure 3.3.
3 . 3 . 3 A l k a l i M e t a l F l u o r o t i t a n a t e s
The com plex form ing ability o f the fluorides o f the elem ents silicon , titanium, zirconium and hafnium in ionic m elts increases from silicon to titanium and then to hafnium. The fluorosilicates are the least thermally stable. In v iew o f the fact that the specific ionic charge decreases in the sequence: S i —> Ti —» Zr, H f, it m ay be concluded that the decisive factor governing the com plex forming ability o f MF4 with respect to MF are the geom etrical parameters o f the metal ion M4+, which is capable o f coordinating a specific number o f fluoride ions.
On passing from lithium to caesium fluorides, the thermal stability o f the com plexes increases. This increase in stability is confirm ed not only by the corresponding enthalpies o f fusion, but also by the shapes o f the minim a on the D .T .A . trace. The radii o f curvature o f these peaks decrease from Rb to Cs [31] which demonstrates an increase in the thermal stability o f these salts. The stability o f M 3TiF7 also increases on going from Li through to Rb. The heptafluorotitanates are not formed in the LiF-
-3 1 -
NoBF4 (mol* X)
Phase diagram for the system NaF-NaBF, 4 [23]
FIGURE 3.3
-3 2 -
Li2TiF6 and NaF-Na2TiF6 system s. On the other hand, K3TiF7 exists in KF-K2TiF6 m elts and has a fairly high melting point (775°C). On passing from the potassium to caseiu m salts the increase in thermal stability is confirm ed not on ly by the corresponding heats o f fusion , but also by the shapes o f the fusion peaks on the D .T .A . trace.
3 .3 .3 .1 K F - K 2T iF 6
The m elting point o f K2TiF6 is reported in the literature as ranging from 780°C to 899°C. The various literature values are given in Table 3.4.
The KF-K^TiFg system w as first reported by Ono et al. [34], w ho investigated the system using thermal analysis. They described a sim ple eutectic system with eutectic point 7 10°C, 65 m ole % K2TiF6. Antipin et al. [35] reported the existence o f the phase K 3TiF7, with a m elting point o f 890°C. Their K F-K2TiF6 diagram
SALT MPTA>C METHOD OF DETERMINATION REFERENCE
K 2T iF6 780 D.T.A. - Open Crucible 32820 D.T.A. - Open Crucible 33840 Thermal Analysis 34899 D.T.A. - Sealed Crucible 31
TABLE 3.4
w as then divided into tw o sim ple eutectic system s, nam ely KF-K3TiF7 and K3TiF7- K2TiF6. Chernov and Erm olenko [31] reported the existence o f the phases K3TiF7 and KTiF5. In v iew o f the precautions and techniques used in their work, it would seem that the diagram o f Chernov and Ermolenko is the m ost reliable. Their diagram, drawn as a KF-TiF4 phase diagram, is given in Figure 3.4(a).
3.3.3.2 N a F - N a 2 T iF 6
The on ly phase diagram published for this system is that due to Chernov and Ermolenko. It is given in Figure 3.4(b). The system is a sim ple eutectic one, with no intermediate compounds.
-33-
Phase diagrams for the systems:
(a) KF-TiF4(b) NaF-TiF4 [31]
FIGURE 3.4
-3 4 -
3 .4 Decomposition Pressures
3.4.1 A lk a l i M e ta l F lu o ro b o ra te s
(a) K BF4
D e B oer and van Liem pt [29] studied the thermal dissociation o f the alkali metal fluoroborates N aB F 4, K BF4, R bBF4 and C sB F4. They m easured equilibrium dissociation pressures over a range o f temperatures by a direct manometric technique. From their measurements they calculated the enthalpy o f reaction:
K B F 4(s,l) —» KF(s.1) + BF.3(g)
to be 28.9 Kcal/mole. They obtained a linear relationship between lnp and 1/T over a temperature range from 510°C to 930°C. This temperature range, however, includes both the melting point o f KBF4, which they report to be 530°C, and the m elting point o f the KF reaction product, 858°C. The enthalpies o f fusion w ill alter the enthalpy o f reaction and so the slope o f the lnp vs. 1/T line should change at the m elting points o f K BF4 and KF.
U sing the decom position pressure data o f D e Boer and van Liempt, the enthalpy o f formation o f K BF4 at 25°C has been calculated as -424 K cal/m ole [36] and -433 K cal/m ole [37]. Independent o f these vapour pressure data B ills and Cotton [38] have determined the enthalpy o f formation o f KBF4 at 298K, A H f° (KBF4^ ) , to be -451.6 K cal/m ole and Gross [39] has determined the value to be -450.5 K cal/m ole. The values obtained by B ills and Cotton [38] and by Gross [39] are consistent within the lim its o f experimental uncertainty. Janaf [40] adopts an average value i.e. A H °f (KBF4(s)) = -451.0 K cal/m ole.
The decom position pressure o f KBF4 was measured over the temperature range 490- 570°C . The results are given in Table 3.5. A plot o f lnp vs. 1/T is given in Figure3.5. Measurements o f the slope and intercept o f the line o f best fit yield the follow ing enthalpy and entropy for the reaction:
(KBF4) (KF) + BF3(g)
DECOMPOSITION PRESSURE OF K BF4
(ASSUM ING BF^ TO BE ONLY VAPOUR SPECIES)
R U N Am/g ry /m oles VAr/dm V4r/ml min i^j/m oles i ° C P F/mm Hg (1/T) K ln(P/atm)
1 0.0097 1.43 x 10 10.44 87 0.435 490 0.25 13.11 x 10 -8 .032 0.0105 1 .5 4 x 1 0 , 10.42 87 0.434 493 0.27 13.05 x 1 0 ? -7 .943 0.0108 1.60 x 10 ' 4 10.40 87 0.433 494 0.28 13.04 x 10 4 -7 .894 0.0149 2.19 x 10 12.50 89 0.521 499 0 .32 12.95 x 10"4 -7 .765 0.0153 2 .2 5 x 1 0 7 12.10 88 0.504 500 0 .34 12.94 x 10"4 -7 .706 0.0154 2 .2 6 x 1 0 4 10.32 86 0.43 508 0 .4 12.81 x 10~4 -7 .547 0.0169 2.49 x 10 ~4 10.08 84 0.42 508 0.45 12.80 x 10“4 -7 .438 0.0194 2.86 x 10 10.23 81 0.426 514 0.51 12.70 x 10"4
12.70 x 10“4-7.31
9 0.0211 3.10 x 10 ~4 10.66 86 0.444 514 0.53 -7.2710 0.0245 3.50 x 10 "4 10.42 88 0.434 520 0.63 12.61 x 10"4 -7 .1011 0.0251 3.69 x 10 ’ 4 10.54 89 0.439 523 0 .64 12.57 x 10^7 -7 .0912 0.0315 4.63 x 10 11.40 82 0.475 527 0 .74 12.50 x 10 4 -6.9313 0.312 4.59 x 10 "4 10.08 84 0.42 534 0.83 12.39 x 10“4
12.39 x 10 4-6 .82
14 0.0361 5.31 x 10 10.42 87 0.434 534 0.93 -6.7115 0.044 6.47 x 10 "4 12.29 91 0.512 535 0.96 12.38 x 10 -6.6716 0.0452 6.65 x 10 "4 11.66 90 0.486 536 1.04 12.37 x 10 ?
12.30 x 10 4-6 .59
17 0.0518 7.62 x 10 10.38 87 0.432 540 1.34 -6 .3418 0 .5 6 8.24 x 10 "4 10.38 84 0.432 545 1.45 12.32 x 10~4 -6 .2619 0.0731 10.75 x 10 "4
1 0 .0 5 x 1 0 411.40 88 0.475 546 1.59 12.22 x 10"4
12.15 x 10"4-6.17
20 0.0683 10.66 89 0.444 550 1.72 -6 .0921 0 .076 11.16 x 10 "4 10.85 90 0.452 555 1.88 12.08 x 10 "4 -6 .0022 0.1091 16.04 x 10 "4 11.04 88 0.46 571 2.65 11.85 x 1 0 ‘ 4 -5 .66
T A B L E 3.5
-35-
-3 6 -
ln(P/atm)
ln(P/atm)
Figure 3.6 Plot of InP versus 1/T for NaBF4
-3 7 -
where the parenthesis indicate that the component is part o f the flux phase.
AH = 35.9 Kcal/m ole AS = 31.0 cal/m oleK
H ence, the free energy change, AG , for the above reaction at 500°C is 11.9 K cal/m ole. The Janaf values are as follow s.
AH = 38.0 K cal/m ole AS = 35.8 cal/m oleK AG5oo°C = 10.3 K cal/m ole
These values assume that both KBF4 and KF are solids. The larger measured value for AG can be explained by the fact that above the eutectic temperature KBF4 and KF are not at unit activity.
(b) NaBF4
The decomposition pressure o f NaBF4 s has been measured between 329 and 376°C - below the eutectic m elting point. The results are given in Table 3 .6 . A plot o f lnp vs. 1/T is given in Figure 3.6. Measurement o f the slope and intercept o f the line o f best fit yield the enthalpy and entropy values for the follow ing reaction.
NaBF4(s) -* NaF(s) + BF3(g)
AH = 27.6 K cal/m ole AS = 30.5 cal/m oleK A G 350°C = 8-6 K cal/m ole
The enthalpy o f formation o f NaBF4 has been determined to be -440 .6 K cal/m ole [39]. U sing the approach o f Latimer [41] the entropy o f formation o f NaBF4 may be estim ated to be -75 cal/m oleK . T hese values g ive the enthalpy, entropy and free energy change at 350°C for the above reaction to be as follow s.
AH = 30.9 Kcal/m ole AS = 35.4 cal/m oleK A G 350°C = 8*9 K cal/m ole
DECOMPOSITION PRESSURE OF N aB F4
(ASSUM ING BF3 THE ONLY VAPOUR SPECIES)
R U N Am/g np/moles VAl/dm3 \^ r/m lm in -1 n/\j/moles t/°C F^/mmHg (1/T) K InP
1 0.0089 1.32 x 10 ~4 10.44 87 0 .435 329 0.23 1.66 x l O ' 3 - 8 .1 0
2 0 .0 2 1 2 3.13 x 10 ~4 1 1 .2 0 93 0 .466 337 0.51 1.64 x l O ' 3 -7 .303 0.0207 3.04 x 10 ^ 10.26 86 0.428 348 0.54 1.61 x 1 0 ‘ 3 -7 .254 0.0457 6.72 x 10 ~4 10.74 90 0.448 356 1.14 1.59 x 1 0 ' 3 -6 .505 0.0449 6.60 x 10 ‘ 4 10 .0 2 84 0.418 372 1 .20 1.55 x 10 "3 -6 .456 0.0782 1.15 x 1 0 ' 3 10 .88 91 0.453 376 1.87 1.54 x 1 0 ' 3 -6 .01
T A B L E 3.6
-39-
The values for A G 3 5 QOQ, and hence the decom position pressure, are within the limits o f experimental uncertainty.
3 . 4 . 2 A l k a l i M e t a l F l u o r o t i t a n a t e s
(a) K 2TiF6
There is only one value for the enthalpy o f formation o f K2T iF6 reported in the literature [26], A H f (K 2T iF 6) = -696 .2 K cal/m ole. Jenkins and Pratt [43] have calculated theoretically the lattice energy o f K2TiF6 to be 386.8 K cal/m ole. U sing these values and a Bom -H aber cycle, the enthalpy o f formation o f the gaseous ion TiF62‘(g) may be estimated as -552.6 Kcal/m ole. U sing the approach o f Latimer [24] the entropy o f formation o f K2TiF6 may be estimated as -127 cal/moleK.
The decomposition pressure o f K2TiF6 has been measured over the temperature range 826-917°C i.e. above the eutectic temperature. The results are given in Table 3.7. A plot o f lnp vs. 1/T is given in Figure 3.7. The slope and intercept o f the line o f best fit give the enthalpy and entropy change for the follow ing reaction:
(K^TiFg) ^ 2(KF) + TiF4(g)
where the parenthesis indicate that the component is part o f the flux phase.
AH = 31.5 Kcal/m ole AS = 14.1 cal/m oleK
A t 850°C , this g ives the free energy change for the above reaction to be 15.7 K cal/m ole. The free energy change for this reaction according to the literature value is -1.5 Kcal/mole. This is equivalent to a decom positon pressure at this temperature o f tw o atmospheres. The vapour pressure o f K2TiF6 at 850°C is significantly lower than 2 atms and clearly the data o f Karapet'yants and Karapet'yants [42] are in error. The enthalpy measured for the decom position o f K2TiF6 is not a standard enthalpy change, because K2TiF6 and KF are not in their standard states i.e. pure solid K2TiF6 and KF. H owever, the value obtained w ill be taken as a rough estim ate o f the standard enthalpy o f reaction. The enthalpy o f formation o f K2TiF6 m ay then be calculated to be -713 K cal/m ole as opposed to -696 K cal/m ole. The enthalpy o f
DECOMPOSITION PRESSURE OF K2TiF6(ASSUM ING TiF4 TO BE THE ONLY VAPOUR SPECIES)
R U N Am/g qf /m oles VAr/dm \^ r/m lm in iftj/m oles \P C PF/mmHg (1/T) K ln(P/atm)
1 0.0386 3.11 x 10~4 10.00 83 0 .417 830 0.56 9.066 x 10~4 -7.212 0.0365 2.94 x 10‘ 4 11.34 81 0 .4 74 826 0.47 9.099 x 10"4 -7 .393 0.0463 3.73 x 1 0 '4 12.74 85 0.531 826 0.53 9.099 x 1 0 '4 -7 .274 0.0393 3.17 x 10"4 10.60 88 0 .4 42 827 0.55 9.398 x 1 0 '4 -7 .235 0.1097 8.85 x 10"4 10.90 91 0 .4 54 903 1.48 8.503 x 1 0 '4 -6 .636 0.0977 7.88 x 1 0 '4 11.00 92 0.458 910 1.30 8.453 x 10"4 -6 .377 0.1138 9.18 x 1 0 '4 10.68 89 0.445 917 1.57 8.403 x 10"4 -6.818 0.0627 5.06 x 1 0 '4 10.92 91 0.455 871 0.84 8.741 x 1 0 ' 4 -6.819 0.0704 5.68 x 1 0 '4 10.66 89 0 .4 44 873 0.97 8.726 x 1 0 '4 -6 .6 610 0.0969 7.81 x 10‘ 4 10.50 88 0.438 900 1.36 8.525 x 1 0 ' 4 -6 .3311 0.0448 3.61 x 10 ’ 4 10.44 87 0.435 855 0.63 8.865 x 1 0 ' 4 -7 .10
T A B L E 3.7
-017
-
-41 -
ln(P/atm)
In(P/atm)
I
- 4 2 -
formation o f TiF62~(g) m ay then be recalculated as -569.6 K cal/m ole.
(b) N a2TiF6
Jenkins and Pratt [43] have calculated theoretically the lattice energy o f N a^ iF g to be402 .4 K cal/m ole. U sin g this value and the enthalpy o f formation o f TiF62_(g) as -596 .6 K cal/m ole the enthalpy o f formation o f Na2TiF6^ m ay be calculated to be -682.3 K cal/m ole. U sing the approach o f Latimer [41] the entropy o f formation o f N a^ iF g may be estimated as -123 cal/moleK.
The decom position pressure o f N a^ iF g has been measured between 644 and 693°C . The results are given in Table 3.8. A plot o f lnp vs. 1/T is given in Figure 3.8 . The slope and intercept o f the line o f best fit g ive the follow ing enthalpies and entropies for the reaction:
N a2TiF6(s) 2N aF(s) + TiF4(g)
AH = 63.3 K cal/m ole AS = 53.2 cal/m oleK
T hese values g ive the free energy charge for the reaction at 650°C to be 14.2 K cal/m ole. The estim ated thermodynamic data for N a2TiF6 g ive the enthalpy and entropy charge for reaction to be AH = 54 K cal/m ole and AS = 43.7 cal/m oleK . The free energy charge at 650°C may then be calculated as 13.7 K cal/m ole. Although the estimated and measured enthalpies and entropies differ in magnitude, the free energy changes, and hence the decom position pressure, are in reasonably good accord.
3 . 5 R ea ctio n s w ith A lu m in iu m
3 . 5 . 1 C h e m i s t r y
Alum inium reduces K^TiFg and KBF4 to form an A l-T i-B alloy and a KF-A1F3 flux. B y measuring the transfer efficiencies o f T i and B and by determining the chem ical com position o f the final flu x , Squires [25] w as able to put forward chem ical equations to represent the reactions taking place.
(a) A l-K 2TiF6
DECOMPOSITION PRESSURE OF N^Tif?
(ASSUM ING TiF4TO BE THE ONLY VAPOUR SPECIES)
R U N Am/g iip/moles 'VAr/m lm in i^j/m oles t!°C If- /m m Hg (l/T ) x id* K ln(P/atm)
1 0.0089 1.32 x 10 10.96 91 0.457 644 0 .2 2 10.91 -8 .152 0.0133 1.96 x 10 10.48 87 0.437 650 0 .3 4 10.83 -7 .703 0.0189 2.79 x 10 10.36 86 0 .4 32 655 0.49 10.78 -7 .354 0.0191 2.81 x 10 10.06 84 0 .419 669 0.51 10.62 -7 .305 0.0353 5 .1 9 x 1 0 10.08 84 0 .4 2 674 0 .94 10.56 -6 .706 0.0329 4.85 x 10 10.42 87 0 .4 34 683 0.85 10.46 -6 .807 0.0488 7.18 x 10 10.40 87 0.433 693 1.26 10.32 -6 .40
T A B L E 3.8
-43-
-4 4 -
95% transfer o f Ti from salt to metal. KA1F4 and K3A1F6 present in a 3:1 molar ratio
18(K2TiF6) + 24A1(1) -> 6(K 3A1F6) + 18(KA1F4) + 18 J i
(K 2TiF6) - » 2(KF) + TiF4(g)
where the underline represents an elem ent in the m etallic phase and the brackets represent a constituent dissolved in the slag.
(b) A1-KBF4
65% transfer o f B from salt to metal. KA1F4 flux formed.
2(KBF4) + 2A1(1) -> 2(KA1F4) + 2 B
(KBF4) - * (KF) + BF3(g)
(c) A l-K 2TiF6-K BF4
90% B transfer efficiency, 80% Ti transfer efficiency. KA1F4 and K3A1F6 formed in a 10:1 molar ratio.
9(K BF4) + 9A1(1) 9(KA1F4) + 9 B
(KBF4) -> (KF) + BF3(g)
9(K 2TiF6) + 12A1(1) -> 9(KA1F4) +■ 3(K 3A1F6) + 9 J i
2(K 2TiF6) 4(K F) + 2TiF4(g)
-45-
3 . 5 . 2 T h e r m o d y n a m i c s
Figure 3.9 represents the free energy charge when one m ole o f gaseous fluorine at 1 atm. pressure com bines with a pure elem ent to form a fluoride [44]. The diagram illustrates the following points.
(a) The lines for the formation o f TiF4 and BF3 lie above that for A1F3. In other words, aluminium has a greater affinity for fluorine than does either titanium or boron. Hence, A1 w ill reduce TiF4 and BF3.
(b) KF is more stable than A1F3. Hence, KF w ill not be reduced by aluminium and so it remains in the flux.
(c) M gF2 is more stable than A1F3. Hence, M g w ill reduce A1F3. This explains w hy a KF-A1F3 flux may be used to 'de-mag' aluminium scrap.
3 . 5 . 3 K i n e t i c s
The alm ost com plete chem ical reduction o f KBF4 and K^TiFg by alum inium is known to be achieved quickly, relative that is to other metallurgical processes e.g. steelm aking reactions. A previous study by Kiusalass [45] involved the preparation o f an A l-5 wt% Ti - 1 wt% B alloy by the addition o f K2TiF6 and KBF4 to m olten aluminium at 750°C. The melt was agitated and then the salt layer w as sampled after 20s and then subsequently each 15s up to 2 minutes. Further sam ples were taken after 5, 30, 90 and 180 minutes. It was observed that K^TiFg and K BF4 were found on ly in the initial 20s sample and there only in very small amounts. In order to investigate the kinetics o f reduction further and in order to investigate the effects o f single salt addition as com pared to sim ultaneous salt addition, it w as decided to perform experiments similar to those o f Kiusalass.
400 g o f com m ercial purity aluminium was m elted in a Salamander crucible in a Leybold-H eraeus vacuum induction furnace. The furnace can be operated under vacuum or with a gaseous atmosphere in the chamber. In view o f the fact that the fluoride salts are volatile the experiments were conducted under an argon atmosphere and not under vacuum. The follow ing salt additions were made when the temperature o f the aluminium was 750°C .
Free energy diagram for the formation of MeF2 [44]
(Change in free energy AG° when one mole of fluoride
(F2) gas at 1 atm pressure combines with a pure element
to form fluoride)
FIGURE 3.9
-4 7 -
(a) A l-K 2TiF6
106g K^TiFg were added to 400g o f aluminium at 750°C in order to prepare an A l-5 wt% Ti alloy.
(b) A1-KBF4
49 g KBF4 were added to 400g o f aluminium at 750°C in order to prepare an Al-1 wt% B alloy.
(c) A l-K B F4-K 2TiF6
106g K2TiF6 and 49 g KBF4 were added to 400g o f aluminium at 750°C in order to prepare an A l-5 wt% T i- 1 wt% B alloy.
The salt layer in each case w as sampled after 30s, 45s, 60s, 90s and 120s by pipetting approximately 1 cm 3 o f molten salt into a graphite tube. Each sample w as then exam ined by X ray analysis in order to determ ine the phases present. The results o f the X ray analysis are summarised in Table 3.9.
The reduction o f KBF4 and K2TiF6 by aluminium is seen to be rapid:- K BF4 within 45s and K^TiFg within 90s. For simultaneous salt addition the time for com plete reduction is increased slightly:- KBF4 within 60s and K 2TiF6 within 90s. These increased tim es for reaction could be due to two reasons. Firstly, the ratio o f salt to aluminium volum e is higher for the case o f simultaneous salt addition. Secondly, the activities o f KBF4 and K 2TiF6 are lowered.
3 . 6 D isc u ss io n a n d C o n c lu s io n s
Sodium hexafluorotitanate (N a2T iF6) and sodium tetrafluoroborate (NaBF4) have been assessed as alternatives to the currently used potassium equivalents for the manufacture o f A l-T i-B master alloys. The sodium salts offer possib le advantages over the potassium salts because the difference in the atomic w eights o f sodium and potassium means that proportionately less sodium salt is needed to introduce the same w eight o f boron or titanium to the alloy. H ow ever, the vapour pressures o f the sodium salts have been seen to be significantly higher than those o f the potassium salts. This may adversely affect the transfer efficiency o f Ti and B i.e. the w eight o f T i/B found in the m etal over the w eight contained in the salt, expressed as a
SPECIES PRESENT IN THE SALT PHASE DURING THE SALT-METAL REACTION AT 750°C
PHASESPRESENT
30s 45s 60s 90s 1 2 0 s
K 2TiF6 X X
K 2TiF6 + Al k a if 4 X X X X X
X X X X X
k b f 4 X
KBF4 + A1 k a i f 4 X X X X X
K ^ 1F6
KBF4 X X
KBF4 + K2TiF6 + A1 K2TiF6 X X X
KAIF4 X X X X X
K3A1F6 X X X X X
K 3AH6
T A B L E 3.9
-48-
-4 9 -
percentage. The work o f Squire [25] has indeed show n this to be the case. In a series o f laboratory scale (200g A l) m elts the transfer efficiency o f T i and B w as determined. Squire's results are summarised in Table 3.10.
SYSTEM Ti TRANSFER EFFICIENCY (%)
B TRANSFER EFFICIENCY (%)
NaBF4-A l - 35KBF4-A1 - 65Na2TiF6-A l 85 -
K2TiF6-A l 95 -
N a2T iF6-NaBF4-A l 95 65K 2TiF6-K BF4-A l 100 100
T ITA N IU M A N D B O R O N TRANSFER EFFICIENCIES FOR LA BO R A TO R Y SCALE MELTS [25]
TABLE 3.10
The transfer effic ien cy o f Ti and B from the sodium salts is considerably less than from the equivalent potassium salts. Thus, the potassium salts are superior to the sodium salts for master alloy production. Squire’s results are o f interest in that they show that the Ti and B transfer efficiencies, are greatly im proved by em ploying dual as opposed to single salt additions. This can be understood in terms o f low er activities o f BF3 and TiF4 in a molten solution and hence the reduced volatility o f these components.
4. THE KF-AIF3 SYSTEM
-5 0 -
4 . THE KF-AIF3 SYSTEM
4 .1 Introduction
The reduction o f K2TiF6 and KBF4 by aluminium yields an Al-Ti-B alloy under a KF- AIF3 flux layer. The chemistry o f the alloy-flux interaction and the physico-chem ical properties o f the flux w ill have a great influence on the process. An understanding o f the properties o f KF-A1F3 m elts is therefore o f great im portance to an overall understanding o f master alloy manufacture.
Comparatively little work has been done on the KF-A1F3 system, although much work has been done on the N aF-A lF3 system because o f its obvious importance in the production o f aluminium by the Hall-Heroult process. This work and the work in general on the M F-A IF3 system (M = Li, K, N a) w ill be review ed. Further investigations have been carried out on the KF-A1F3 system and w ill be described.
N .B . The term 'flux' w ill be used to describe a KF-AIF3 m elt that is in equilibrium with the metal, as opposed to the term 'salt' for a K2T iF 6, KBF4, KF-A1F3 mixture which has not yet achieved chemical equilibrium with the metal.
4.2 Phase Equilibria
Phillips et al. [46] and Jensen [47] have investigated phase equilibria in the KF-A1F3 system. Jensen's phase diagram is given in Figure 4.1. Tw o com pounds are found in th is sy stem : p o ta ss iu m te tra flu o ro a lu m in a te , K A1F4 , and p o ta ss iu m h exafluoroalum inate, K 3A1F6. KA1F4 m elts at 580°C and K3A1F6 at 1000°C . K 3A1F6 m elts congruently, but it is not entirely clear as to whether KA1F4 m elts congruently. T w o eutectic points exist at 7 .0 m ole % A1F3, 820°C , and at 45 .0 m ole % A1F3, 560°C.
Reaction o f K2TiF6 with aluminium gives a flux with KA1F4 and K3A1F3 in a molar ratio o f 3:1. Reaction o f KBF4 with aluminium gives a KA1F4 flux. Reaction o f K B F4 and K2T iF 6 w ith aluminium so as to yield an A l-5 T i-IB alloy g ives a 10:1 KA1F4:K3A1F6 flux. This final flux is o f eutectic com position, 45 m ole % A1F3, Mpt. 560°C .
1200
1000
800
iL
600
400
200
-51-
^Alf^ =1 a t m
l P - A IR «■ sm e lte
- k f f y -k 3a i f 6
o vKAlF^-P-AIFj
‘Y-K3AIFg+KAIf
K F .p - K 3A lF 6
s l
KF*CUK3A IF 6
KAIF^ ♦ CL-A1F3
P - K 3A1F6.KA IF^
a - K 3AIF6*KA IF^
KF20 40
Mol 7. A!F3
60
3hase diagram for the system KF-AIF^ [47]
FIGURE 4.1
-5 2 -
4 .3 Melt Activities and Structure
4 .3 .1 A c tiv it ie s in M o lten S a lts
The concentration o f a com ponent in a solution may not indicate the amount w hich is available for reaction i.e. its effective concentration or activity. In this case, w e w ish to determine the melt activities o f KF and A1F3. Although it is highly improbable that entities such as 'K F or 'A1F3' exist in the m olten flux, an activity for these entities may be assigned. The flux behaves as though it contained this apparent concentration o f fluorides. The advantage o f this approach is that no knowledge o f the structure o f the flux or metal phase is required.
4 .3 .2 T ech n iq u es f o r th e D e te rm in a tio n o f A c tiv it ie s in M o lte n S a lts
A number o f techniques exist for the determination o f activities in molten salts and they include the follow ing [18].
(a) C ryoscopy - the depression o f freezing point o f one salt by the addition o f another has been used to determ ine therm odynam ic inform ation on m olten salt mixtures.
(b) Vapour Pressure M easurement - the activity o f aA o f a component A in a liquid mixture may be calculated from vapour pressure measurements.
aA = PA/PA°
where PA = partial pressure o f A above the mixture PA° = partial pressure o f A above pure A.
(c) Solubility Measurements - the activities in binary melts may be determined from m etal-m olten salt equilibria i f the activities in the m etal phase are known and the necessary therm odynam ic inform ation are availab le. M ore sp ec ifica lly , the equilibrium:
i 3(NaF) + Al(j) -> (A1F3) + 3 N a
(where the brackets represent constituents d issolved in the flux and the underline indicates elem ents d issolved in the aluminium) has been studied to derive activity
-5 3 -
values o f NaF and A1F3 in the melt [50].
(d) Concentration C ells - measurements o f the E.M .F. o f concentration cells where the com position o f the electrolyte in one o f the cell compartments is variable can g ive activity values for ionic species in the melt.
D ew ing has used this technique to determine activities in the system s LiF-A lF3 [49] and NaF-AlF3 [50].
(e) Calorim etry - as described below in section 4 .3 .3 , m elt activities can be calculated from calorimetric data.
4 . 3 . 3 D e te r m in a tio n o f M F -A IF 3 M e lt A c tiv i t ie s f r o m C a lo r im e tr ic D a ta
H ong and K leppa [48] have reported calorim etric m easurem ents at 1025°C for mixtures o f LiF, NaF and KF with solid A1F3. A plot o f AHMS.L i.e. the enthalpy for the reaction M F(j) + A1F3(S), at 1025°C against N a i f 3> the m ole fraction o f A1F3, is given in Figure 4.2. The enthalpies o f m ixing becom e more negative as the size o f the alkali ion increases i.e. as the basic character o f the alkali fluoride becom es stronger.
U sing a concentration cell technique D ew ing has determined the integral and partial free energies for the mixtures LiF(i) + A1F3(S at 800°C [49] and NaF(i) + A1F3^ at 1020 and 1080°C [50]. B y the method o f intercepts the partial enthalpies for each com ponent o f the MF-A1F3 system s m ay be calculated from the data o f H ong and Kleppa. These values are given in Table 4.1. If w e assum e that the enthalpies are independent o f temperature then w e may directly compare AHMF, the partial enthalpy o f MF, with AGMp, the partial free energy o f MF, and hence obtain the partial entropy o f M F, ASMF. Plots o f AHm f and AGMF against N a i f for the liquid m ixtures LiF- A1F3 and N aF-A lF3 are g iven in Figure 4.3(a). Plots o f ASMF against N^1F3 f ° r the liquid mixtures L iF-AlF3 and NaF-AlF3 at 800 and 1020°C respectively, are given in Figure 4.3(b).
A ssum ing that A H MF and A H a i f 3 and A S MF and A S a i f 3 are independent o f temperature for the L iF-AlF3 and NaF-AlF3 mixtures, it is possible to calculate AGMF and A G a i p 3 and hence the m elt activities, aMF and a ^ lF y at any temperature. Isoactivity lines for the com ponents MF(j) and A1F3(S) are given in Figure 4 .4 for the
8
6 -
4-
M 2 "AH S-L(Kcal/mole)
A LiF - AIF3 A
♦ NaF - AIF3 a a
□ KF-AIF3
A ♦♦
A a a a a a A
AAA* A
AA A
#
□
-2 -
0 ♦♦ ♦ ♦ ♦-4 - □ □ □
0.0~|------------ r0.1 0.2 “ I—
0.3t------------r
0.4 0.5 0.6
N (AlEj)
iv/i-OI
Figure 4.2 A h for the reaction MF + AIF3 (S) at 1298K
Kcof
mole
-5 5 -
(
LiF.NoF LiF.NoF
N2(AIF3)(a)
N 2 ( A I F 3 )(b)
Partial enthalpies for the systems: LiF(1)~AlF3(s)
NaF(1)-AlF3(g)KF(i )-A1F3 ( s )Partial free energies for the systems: LiF(1)-AlF3(s) at 800°CNaF^1^-AlF3(s) at 1020°C
Partial entropies for the systems: FiF(l)~A1F3(s)
NaF(l)~A1F3(s)
FIGURE 4.3
-5 6 -
Np n A1F3 AHLiFKcal/mole
ASLiF cal/mole K
a h A1F3Kcal/mole
a s A1F3 cal/mole K
0 .75 0 .2 5 -2 .895 -0 .75 12.726 17.50 .7 0 0 .3 0 -4.225 -1 .2 16.262 18.70 .65 0 .3 5 -5 .619 -1 .75 19.435 20.10 .6 0 0 .4 0 -6 .458 -1 .75 20 .799 2 0 .0
N NaF N a i f 3 AHNaFKcal/mole
A% a F cal/mole K
a h A1F3Kcal/mole
a s A 1F3 cal/mole K
0 .7 0 0 .3 0 -6 .740 -2 .4 8 .7 94 19.40 .6 5 0 .3 5 -9.795 -3 .4 14.877 2 1 .20 .6 0 0 .4 0 -12.015 -3 .4 18.322 21.10 .55 0 .4 5 -13 .000 -2 .2 19.446 19.20 .5 1 4 0 .4 8 6 -12 .600 -0 .5 19.250 17.7
N KF N A1F3 a h KFKcal/mole
a s KF cal/mole K
a h A1F3Kcal/mole
a s A1F3 cal/mole K
0 .7 0 0 .3 0 -7 .100 -2 .4 2 .8 0 0 19.40 .65 0 .3 5 -9 .700 -3 .4 6 .5 00 2 1 .20 .6 0 0 .4 0 -10 .700 -3 .4 8 .8 0 0 21.10 .55 0 .4 5 -13 .300 -2 .2 12.200 19.2
PARTIAL ENTHALPY A N D ENTROPY DATA FOR LiF-AlF3, NaF-AlF3 A N D KF-A1F3 LIQUID-SOLID MIXTURES
T A B L E 4 .1
-5 7 -
system L iF (i)-A lF 3(S) and in Figure 4.5 for the system N a F ^ -A lF g ^ . The values obtained should be consistent with the phase diagrams for these systems. Consider the m elt com position NjqaF = 0-514, N a i f 3 = 0.486. For this com position A H /y F 3 - 19.25 K cal/m ole and ASa i f 3 = 17.7 cal/m ole K. AGa i f 3 = 0 at 815°C for this m elt com position. This corresponds to A1F3 at unit activity i.e. pure, solid A1F3. This situation arises inside the tw o phase L + A1F3(S) region. Inspection o f the N aF-A lF3 phase diagram - see Figure 4.5 - shows that the liquidus line is at approximately 815°C at this com position. Consider next the melt com position Njqap = 0 .65, N ^ i f 3 =0.35. For this com position AHjqap = -9.795 K cal/m ole, ASjqap = -3 .4 cal/m ole K, AHa i f 3 = 14.88 cal/m ole, ASa i f 3 = 21.2 cal/m ole K. This particular com position enters the L + N a3A lF 6(s) phase system at 925°C . For this temperature w e m ay calculate the m elt activities to be ajqap = 0 .09 and a A lF 3 = 0 .012 . H ence, the equilibrium constant, K, for the reaction at 925°C:
3(NaF) + (A1F3) -» N a3A lF 6(s)
K = aN a3A lF 6 = 1.14 x 105a3NaF aA lF3
This value is consistent with other thermodynamic data obtained for this reaction at 925°C . H ence, by using the enthalpy data o f Hong and Kleppa, in conjunction with the free energy data o f D ew ing, it is possible to calculate m elt activities in the system s LiF-AlF3 and NaF-AlF3.
Although H ong and K leppa have determ ined enthalpies o f m ixing in the KF-A1F3 system, no free energies o f m ixing have been reported for this system. Thus, although partial molar enthalpies may be obtained for this system , partial molar entropies may not be derived. It is, therefore, necessary to estimate the partial molar entropies. It w ill be assumed that the partial entropies in the system KF-A1F3 are equal to those in the system NaF-AlF3. The partial enthalpies in the system KF-A1F3 m ay be obtained by the method o f intercepts and are given in Table 4.1.
A ssum ing that the partial m olar enthalpies and entropies are independent o f temperature, the partial molar free energy, and hence the activity, o f a component m ay be determined at any temperature. U sing this approach the isoactivity lines shown in Figure 4.6 for the KF-A1F3 system, have been calculated.
L iF ^ j iso -ac tiv ity lines on the LiF-AlFj phase diagram
F I G U R E 4 . 4 ( a )
l iF M o l * / . A | F 3
A1Fj s j iso -ac tiv ity lines on the LiF-AlFj phase diagram
F I G U R E 4 . 4 ( b )
KF^j iso -ac tiv ity lines on the KF-AIF phase diagram
F I G U R E 4 . 6 ( d )
AIT 3 s j iso -ac tiv ity lines on the KT-A 1F phase diagram
F I CURE 4 . 6 ( b )
-60-
-6 1 -
The values obtained should be consistent with the KF-A1F3 phase diagram. Consider, for example, the follow ing equilibrium:
3(KF) + (A1F3) K3A1F6(s)
The equilibrium constant K for this reaction is as follows:
K = a K3A1F6 a3KFaAlF3
Janaf [40] gives data for the free energy o f formation o f KF(i), A 1F3(S) and K3A 1F6(S). H ence, the equilibrium constant, K, for this reaction m ay be calcu lated at all temperatures. This value should be equal to that calculated from the above expression for K using m elt activ ities along the K3A1F6 liquidus. C onsider the fo llow in g temperatures. At 980°C the value o f K, calculated from the m elt activities, is 1.7 x 10"6 as compared with the Janaf value o f 1.44 x 10" 6 . A t 940°C the value o f K, calculated from the m elt activities, is 6.9 x 10“7 as compared with the Janaf value o f1.1 x 10"6 ; at 870°C the value o f K, calculated from the m elt activities is 4 .5 x 10 "c.f. Janaf value 8 .8 x 10"7; at 840°C K = 4.5 x 10“8 c.f. Janaf value 8.9 x 10"8 . In view o f the assumptions made in calculating the m elt activities and the likely errors in the Janaf data the correlation between the two is extremely good.
4 .3 .4 S t r u c t u r a l C o n s t i t u e n t s o f M F - A I F 3 M e l t s
In attempting to determine the structure o f KF-AIF3 m elts, it is instructive to consider first the structure o f N aF-A lF3 m elts. It is now w ell established that m olten sodium cryolite, N a3A lF6, is com pletely ionized into sodium and hexafluoroalum inate ions, Na+ and A1F63\ It is also w ell established that the hexafluoroaluminate ion dissociates further. Howard [51] determ ined that the major vapour phase over sodium cryolite melts was NaAlF4, suggesting the presence o f the tetrafluoroaluminate ion, A1F4~. By comparing the measured liquidus curve with the theoretical liquidus curve calculated by the integrated cryosopic equation for various d issociation m odels, G rjotheim [52] concluded that the dissociation scheme:
(A1F63-) -> (A1F4") + 2 (F")
-6 2 -
produced the best fit and that the dissociation constant, a , for Na3A lF6 was about 0.3. In other words, for one m ole o f A1F63' ions initially the resultant solution w ill contain 0.7 m oles A1F63% 0.3 m oles A1F4" and 0 .6 m oles F" ions. Several other workers have reached the sam e conclusion [53, 54] and Raman spectroscopy [55] indicates the presence o f A1F63' and A1F4" ions in the m elt. It seem s fairly w ell established, therefore, that the principal anions o f Na3A lF 6 m elts are A1F63-, A1F4~ and F~. The only cation is Na+ .
The data o f H ong and Kleppa [48] also lend support to these conclusions for MF- A1F3 m elts in general. For exam ple, a plot o f the liquid-liquid enthalpy interaction parameters ^M(X,M = AHML_L/N MpNy\ ip 3) against N a i f 3 shows a m inim um for each o f the system s L iF-A lF3, N aF-A lF3 and KF-A1F3. For L iF-AlF3, the m inim um is quite shallow . H ow ever, for N aF-A lF3 and KF-A1F3, the m inim a are w ell defined and occur at values o f N ^ i f 3 w hich fall between 0 .2 and 0.3. This is suggestive o f the formation o f A1F63\ A lso , for the system KF-A1F3 a ’shoulder' in the plot o f against N / j f 3 *s seen near N a if 3 “ 0*5, indicative o f the importance o f A1F4_.
Consideration o f Figure 4 .3 a lso suggests the formation o f the A1F63- ion. ASMF starts out from N ^ jp 3 = 0 with small positive values, consistent with random mixing. H owever, since the partial entropy curves change sign near N ^ p ~ 0.15 - 0 .20 and show m inim a near N a i f 3 ~ 0 .35 - 0 .40, a high degree o f local order clearly exists in this concentration range. The s-shaped forms o f the partial entropy curves are centered on N / j p 3 ~ 0.25, which indicates that this is the com position o f maximum local order.
4 . 4 D e co m p o sit io n P r e ssu r e o f K F-A 1F3 M elts
4 . 4 . 1 V a p o u r S p e c ie s
U sing a m ass spectrometer and a m olecular velocity selector, it has been shown that the saturated vapour over the KF-A1F3 system contains the m olecular species KA1F4 and (KA1F4)2 [56]. The vapour pressures o f KA1F4 and (KA1F4)2, obtained with the aid o f a ve locity selector, made it possib le to estim ate the relative content o f the (KA1F4)2 dimer in the vapour to be 2-3% in the temperature range 412 to 465°C . The heat o f dissociation o f (KA1F4)2 into tw o m onom eric species was determ ined to be38.6 K cal/m ole. In other words, the relative content o f the dimer in the vapour is
-6 3 -
likely to decrease with increasing temperature.
4 .4 .2 P r e v io u s W o r k o n D e c o m p o s it io n P r e s s u r e s o f K F -A IF 3 M e lts
Thom pson and Goad [57] have used a thermogravimetric technique to m easure the partial pressure o f KAlF4(g) over KF-A1F3 m elts at temperatures up to 815°C. KA1F4 partial pressure isobars on the KF-A1F3 phase diagram from the work o f T hom pson and Goad are illustrated in Figure 4.7. Since KA1F4 is essentially the only contributor to the vapour phase, these isobars may also be regarded as total equilibrium pressure isobars. A ll o f the curves pass through a minimum at the com position associated w ith pure KAIF4 . In the two phase regions, the isobars are, in keeping with the phase rule, horizontal.
Thompson and Goad [57] used their data to calculate m elt activities assuming that the m elt w as a regular solution. This assumption means that the entropy change is equal to the value for an ideal solution i.e. A S ^ f = -R lnN MF and A S a i f 3 = -R ln N A lF 3- H ow ever, the work o f H ong and K leppa [48] has show n that the partial m olar entropies for MF-AIF3 solutions are far from being ideal. H ence, the assumption o f regular solution behaviour is not a valid one.
Thom pson and Goad calculated isoactivity lines on the KF-A1F3 phase diagram as illustrated in Figure 4.8. The activity values o f KF(S) are so sm all that they cannot be distinguished from the lower horizontal axis.
H owever, in v iew o f the fact that Thompson and Goad assum ed that the behaviour o f the solution was regular, and in v iew o f the excellent correlation o f the activity values calculated from the work o f Hong and Kleppa with the Janaf values for the free energy o f form ation o f K3A1F6(S), the values calculated from the latter work are to be preferred.
4 . 4 . 3 E x p e r im e n ta l P ro g ra m m e a n d R e su lts
The decom position pressures o f a 50 mole% A1F3 and a 45 mole% A1F3 m elt over a temperature range 692-833°C were measured using the transportation technique, as outlined in Chapter 2. The condensate from decom position pressure studies o f KF- AIF3 m elts using the transportation technique has been collected and exam ined by X - ray analysis. KA1F4 was seen to be the only phase present, in accordance with the
-6 4 -
KAIF^ partial pressure isobars on the
KF-A1F^ phase diagram [57]
FIGURE 4.7
FIGURE 4.8
Activities in the KF-A1F^ system at
600°C [57]
-6 5 -
findings o f K olosov's m ass spectrometry work [56]. The results obtained for the 45 mole% A1F3 melt are given in Table 4.2. A plot o f lnp vs. 1/T for this system is given in Figure 4.9. The results obtained for the 50 mole% A1F3 melt are given in Table 4.3. A plot o f lnp vs. 1/T for this system is given in Table 4.3.
M easurem ent o f the slope and intercept o f the lines o f best fit for the data g ive the enthalpy and entropy o f vaporisation for the 45 mole% A1F3 and 50 mole% A1F3 m elts. They are as fo llow s.
45 mole% AIF3
This work:
A H y = 33.0 Kcal/m ole A SV = 19.0 cal/m ole K
50 mole% AIF3
This work:
A H y = 30.6 K cal/m ole A S y = 18.0 cal/m ole K
4 . 4 . 4 D i s c u s s i o n
The vapour pressures measured during the course o f this work are som ewhat higher than those measured by Thom pson and Goad [57]. In both cases the m easurements were made in flow rate regim es w hich gave saturated vapour pressures. It is likely then that a systematic error exists in either this work or that o f Thompson and Goad.
Since the KF-AIF3 melt is not regular and since the free energy for the reactionKF + A1F3 —» KAIF4 is not known, it is not possible to derive activity values fromthe vapour pressure data.
4 . 5 E q u ilib r iu m R e a ctio n s in th e A I/K F-A 1F3 sy stem
Potassium in aluminium w ill be generated by the follow ing equilibrium:
Thom pson and Goad:
AHV = 33.7 Kcal/m ole ASV = 1 9 .1 cal/m ole K
Thompson and Goad:
A H y = 31.3 K cal/m ole A SV = 17.2 cal/m ole K
THE DECOMPOSITION PRESSURE OF A 0.45 A1F - 0.55 KF MELT(VAPOUR SPECIES ASSUM ED TO BE K A 1$)
R U N Am/g np /m oles VAr/dm 5 VAl/m lm in ' i^r/m oles t/°C P p /mmHg (1 /T )x 104 K ln(P/atm)
1 96 692 0.38 10.36 -7 .602 0.0661 4.65 x 10~4 9.88 82 0.412 735 0.86 9 .92 -6 .783 0.0727 5.12 x 10 ~4 11.88 99 0.495 735 0.79 9 .9 2 -6.874 0.0711 5.01 x 1 0 ' 4 11.20 93 0 .4 66 735 0.82 9 .9 2 -6 .835 0.0711 5.01 x 10~4 10.90 91 0 .454 735 0.84 9 .92 -6.816 0 .136 9.58 x 10 ~4 9 .9 4 83 0.414 783 1.76 9.47 -6.077 0.136 9.58 x 1 0 - 4 10.93 91 0.455 783 1.60 9.47 -6 .168 0.2698 1.900 x 10 ~3 10.04 81 0.418 831 3.45 9 .06 -5 .399 0.289 2.035 x 10 - 3 10.72 84 0.447 831 3.46 9 .0 6 -5 .3910 0 .2994 2.108 x 10 "3 11.80 100 0.492 831 3.26 9 .06 -5.45
TABLE 4.2
-6
6-
DECOMPOSITION PRESSURE OF KA1F4 (x}
(VAPOUR SPECIES ASSUM ED TO BE KAII^ )
R U N Am/g iy /m oles VAl/dm3 \^ r/m l min- n^j/moles t/°C Pjr/mmHg (l/T ) x 104K ln(P/atm)
1 0.0135 9.5 x 10 "5 10.85 91 0.452 600 0.16 11.45 -8 .482 0.0183 1.29 x 10 "4 11.20 95 0.467 612 0.21 11.30 -8 .2 03 0.0203 1.43 x 10 "4 10.90 87 0.454 628 0.24 11.10 -8 .074 0 .0 2 1.41 x 10 ~4 10.00 83 0.418 628 0.26 11.10 -7 .995 0.0328 2.31 x 10 ~4 10.00 88 0.418 650 0.42 10.83 -7 .506 0 .0289 2.04 x 10 - 4 10.00 92 0.418 651 0.37 10.82 -7 .647 0.041 2.89 x 10 "4 8.50 80 0.354 680 0.62 10.49 -7.118 0.0565 3.98 x 10 ' 4 9 .3 2 85 0.388 700 0.78 10.28 -6.889 0.0896 6.31 x 10 ~4 12.50 101 0.521 700 0.92 10.28 -6 .7210 0.0789 5.56 x 10 ~4 8 .60 91 0.358 720 1.18 10.07 -6.4711 0.1379 9.71 x 10 ~4 10.30 86 0.429 741 1.72 9 .8 6 -6 .09
TABLE 4.3
-67-
- 6 8 -
ln(P/atm)
ln(P/atm)
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3 (KF) + A la) -» 3 K + (A1F3)
where the parentheses indicate that a com ponent is part o f the flux phase and the underline indicates solution in the metal. U sin g the Janaf values the equilibrium constant, K, for the above reaction m ay be calculated at any temperature. In the production o f A l-T i-B alloys the flux com position is typically N a i f 3 = 0 .45 , N j q ? =0 . 55 and the temperature 750°C . At this temperature and com position a^\ip3 = 0 .025 , aKF = 6 .4 x lO 3 and K = 5.7 x 10 8. K is given by the equation:
K = a3K aM F3 a3K F aA l
H ence, a g = 8.4 x 10‘5
Potassium and alum inium are only partially m iscib le in the liquid state and the m onotectic point is at less than 0.05 wt% K, 660°C [69]. Since the level o f potassium in solution in aluminium is sm all then it w ill be assumed that Henry's law is obeyed1. e. that the activity o f potassium, aj£, is equal to its concentration, N&, m ultiplied by its activity coefficient in aluminium, fj£.
i.e. aK = fK N K
A t the m onotectic point pure liquid potassium is in equilibrium with pure liquid aluminium. In other words aj£ = 1 when Nj^ = 0 .05 wt%. Therefore, w hen the concentration o f potassium is expressed in wt%, fj£ = 20.
In this case aj£ = 8.4 x 10~5. Hence, Nj£ the concentration o f potassium in aluminium in equilibrium with a KF-A1F3 eutectic melt at 750°C is 4 .2 x 10"6 wt% or 4.2 ppm.
4 .6 Conclusions and Industrial Implications
The decom position pressure o f a 0.45 A1F3 - 0.55 KF melt at the temperature o f master alloy production is approximately 0 .002 atmospheres. In other words, no appreciable loss o f flux w ill occur due to evaporation during the production o f A l-Ti-B alloys.
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The equilibrium potassium level in aluminium due to metal-flux interaction is less than 5 ppm. On addition to the alloy to be grain refined this value w ill be lowered to less than 0.01 ppm. In other words, the potassium level in the final alloy due to the master alloy is much less than that (~ 10 ppm) w hich causes deleterious effects such as hot cracking.
5. INTERFACIAL PHENOMENA
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5. IN T E R F A C IA L P H E N O M E N A
5 .1 In tro d u c tio n
5 .1 .1 I n te r fa c ia l P h e n o m en a a n d M e ta llu rg ic a l P ro c e sse s
W hen tw o non-m iscible liquids are in contact, an interfacial tension always exists at the tw o phase boundary, which differs in magnitude from the respective surface tensions o f the individual liquids. Although interfacial phenomena have no effect on the equilibria between metal, slag and gas, they m ay exert profound effects on the rates o f reactions which occur across interfaces involving these phases.
Non-reacting surface active solutes may tend to keep reactants which are less surface active out o f the interface and so retard reaction. They m ay also retard mass transfer and hence reaction, by im peding surface renewal. H ow ever, surface active solutes which enter into reactions m ay speed up surface renewal and accelerate reaction by causing turbulence in the interfacial region.
Interfacial e ffects w ill also affect the separation o f the m etal and slag. A low interfacial tension w ill favour the formation o f sm all droplets and so im prove em ulsification betw een two liquids - a property also prom oted by high v iscosity . G ood em ulsification results in a better refining capacity, since a relatively large interfacial area is provided for the interchange o f components between metal and slag. H owever, good em ulsification can also cause problems in separating the tw o phases at the end o f the process. Selection o f a slag based on its surface properties usually has to be a com prom ise between good refining rate (favoured by a low interfacial tension) and m inim um entrapment o f slag in metal (favoured by a high interfacial tension).
5 .1 .2 In te r fa c ia l P h e n o m en a a n d th e P ro d u c tio n o f A l-T i-B A llo y s
Problems occur industrially with the separation o f the A l-T i-B alloy from the KF- A1F3 flux layer. T w o factors could lead to difficulties in separating these two phases: (a) L ow interfacial tension, or more specifically high work o f adhesion (see Section 5.2 .1), (b) H igh v iscosity o f the flux. H owever, the flux KF-A1F3 is known to be sufficiently fluid at the temperature o f production that v iscosity is unlikely to be a contributory factor to this problem. Rather, it is likely to be a problem o f interfacial
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tension .
The problem o f m etal-flux separation in the Al-Ti-B/KF-A1F3 system is exacerbated by the fact that the density difference between the metal and flux is expected to be small. Although no data exist for the density o f KF-A1F3 melts it is likely that they w ill be similar to data for the density o f NaF-AlF3 melts. It is known that the density difference between aluminium and a typical NaF-AlF3 m elt is only small [18].
Consider, first, the reactions taking place at the interface. Figure 5.1 show s the species present in the m etal phase and in the neighbouring flux, where I is used to represent trace impurities. Clearly, there are a number o f species and a number o f interfacial reactions. The purpose o f this section o f this project w as to try to isolate the effect o f each o f these species and reactions on interfacial phenomena encountered during the production o f Al-Ti-B alloys.
HBA1KI
TiF62-B F 4AIF4 ", A1F6 3 K +In+
TiB2 :TiAl3 :
METAL FL U X
THE Al-Ti-B/KF-A1F3 INTERFACEFIGURE 5.1
5 .2 In ter fa c ia l E n erg y
5 . 2 . 1 D e f i n i t i o n s a n d T h e o r y
(a) Interfacial Energy
The plane o f separation o f two phases is known as a surface or interface. For a given
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system at constant temperature and pressure, the increase in free energy per unit increase in surface area is the surface tension (surface energy) or interfacial tension (interfacial energy) a
a = 6 G 8 A S
T, P, 14
where
G = Free Energy A s = Surface Arean | = Number o f m oles o f the ith component.
A s defined, the interfacial energy, a , is applicable only to equilibrium situations i.e. ones for w hich temperature, pressure and com position are constant. In this case, the com position o f the flux and metal is constantly changing and the term interfacial energy is not strictly applicable.
H ow ever, it is universally accepted that Gibbs free energy change represents the driving force for chem ical reactions and diffusional processes. S ince interfacial energy is directly related to free energy by definition, it is considered acceptable to also relate interfacial energy changes during the course o f a reaction as representing the driving force to an equilibrium situation.
(b) A ngle o f Contact, 0
Consider a drop o f one phase, resting on a solid surface, im m ersed in a second im m iscible phase. 0 is the angle o f contact between the two tangent planes to the two surfaces measured through phase 1 - See Figure 5.2.
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DEFINTnON OF CONTACT ANGLE 0FIGURE 5 .2
(c) Shape o f Drop
In Figure 5.3 the potential energy o f the drop is made up of:
(i) Surface potential energy
(ii) Gravitational potential energy
The centre o f gravity is pulled dow n, in the first instance, as the decrease in gravitational potential energy exceed s the increase in surface potential energy. Equilibrium is attained when any further movement would increase the total potential energy.
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centre of gravity
TOO MUCH TOTAL P.E. TOO MUCHGRAVITATIONAL MINIMUM SURFACE P.E.P.E.
POSSIBLE SHAPES OF A LIQUID DROPFIGURE 5.3
The exact shape o f a m eridional section o f the drop is described by the Laplace equation [61].
(d) Work o f Adhesion, Work o f Cohesion, Spreading C oefficient
The work o f adhesion, W MS, is defined as follows:
w ms = Ts + Ym " Yms
Where
ys = Surface tension o f slagyM = surface tension o f metalYms = ^terfacial tension between metal and slag.
The tendency o f the slag to adhere to, and becom e entrapped in, the metal, increases with an increase in the difference between the work o f adhesion betw een the two phases, W MS, and the work o f cohesion o f the slag, W s ,
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W s = 2 ys
or in other words as the spreading coefficient, S, becom es more positive.
S = WMS - w s.
5 .2 .2 T e rn a ry In te r fa c ia l E n e rg y D ia g ra m s
Terms such as the work o f adhesion and the spreading coefficient g ive an indication o f how difficult it w ill be to separate the metal and slag. However, what happens if a third, minor phase is introduced into the slag-m etal system? W ill it be dispersed in the slag or m etal, or w ill it sit at the interface betw een the two major phases? The answer depends on the relative magnitudes o f the interfacial tensions present in such a system . The principles o f interfacial tensions in ternary system s, as developed by Conochie and Robertson [58] are outlined below.
5 .2 .2 .1 P r in c ip le s
Let phase 1 and phase 2 be continuous phases and phase 3 the minor dispersed phase. Consider a drop o f phase 3 resting at the interface between phases 1 and 2. D ifferences in density betw een the phases are ignored. Tw o contact angles are defined 0 and <J) (see Figure 5.4) and there are three interfacial energies, a 1/2, <71/3,a2/3‘
DIAGRAM OF DROP OF PHASE 3 AT PHASE 1/2 INTERFACEFIGURE 5.4
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The forces at point A are resolved in X and Y directions and at equilibrium sum to zero.
X : a 1/2 = ^ 3/2 cos 9 + a i/3 cos ♦ (5 .1 )
Y : a 3/2 sin 0 = O j/3 sin $ (5 .2 )
The interfacial energies are made dimensionless by defining:
X = o 1/3/X c (5 .3)
Y = 0 3/2/X a (5 .4 )
Z = o 1/2/ £ g (5 .5 )
W here Xcy — -t" -t-
and X + Y + Z = 1 (5 .6 )
Equations (5.1) and (5.2) can be rewritten as:
1 - X - Y = Y cos 0 + X cos <|> (5 .7 )
Y sin 0 = X sin <j) (5 .8)
Elimination o f <j> gives:
cos 0 = X + Y - X Y - Y 2 - 0.5 Y (X + Y - 1)
(5 .9 )
The values o f X and Y are bounded by the restrictions
-1 < cos 0 < 1 and -1 < cos <{> < 1
There are four limiting combinations o f 0 and <J>:
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(a) 0 = 0°, <J> = 0°
Phase 3 is spread as a thin film between phases 1 and 2 . Cos 0 = 1 in (5.7) gives:
X + Y = 1/2 = Z (5 .11 )
(b) <j> = 0°, 0 = 180°
Phase 3 is only in contact with phase 1, i.e. phase 3 is dispersed in phase 1.
Cos 0 = 1 , cos <|) = -1 in (5.7) gives:
Y = 1/2 (5 .12)
(c) 0 = 0°, <j) = 180°
Phase 3 is only in contact with phase 2, i.e. phase 3 is dispersed in phase 2.
Cos <|> = 1, cos 0 = -1 in (5.7) gives:
X = 1/2 (5 .13)
(d) <j) = 0
In this case, phase 3 is spaced equally between phases 1 and 2.
W hen <{> = 0 in (5.8)
X = Y (5 .14)
5 . 2 . 2 2 T e r n a r y I n t e r f a c i a l E n e r g y D i a g r a m s
The four boundaries given by equations (5 .11), (5 .12), (5 .13) and (5.14) are plotted on a ternary interfacial energy diagram shown in Figure 5.5. This approach is useful in that if the three interfacial energies are known, then the configuration that the three phases adopt may be predicted.
In A l-T i-B master a lloy manufacture the metal and flux constitute the tw o major
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Generalised ternary interfacial energy diagram (after Conochie and Robertson [58])
FIGURE 5.5
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continuous phases, w ith T iA l3 and T iB 2 as minor phases. W ill the T iA l3 and T iB 2 phases be d ispersed in the m etal or in the flux? C learly, w e require the form er situation. Sim ilarly, in A l-B m aster alloy manufacture w e require the boride phase to be dispersed in the m etal rather than the flux. B y investigating the interfacial tensions in this system and by representing these values on a ternary interfacial energy diagram it is possib le to predict where the m inor phase w ill reside under given conditions.
5 .2 .3 In flu e n c e o f M a ss T ra n sfe r u pon In te r fa c ia l P h en o m en a
The adhesion betw een tw o liquids m ay w ell be increased and the interfacial tension decreased, w hen a chem ical reaction takes p lace betw een them. Take, for exam ple, the interface betw een an aqueous solution and toluene (or benzene). During m ass transfer o f a third constituent from one phase to the other e .g . ethanol, or acetone, transferred from toluene towards water, or acetic acid transferred in either direction there is a drastic low ering o f interfacial tension [59 ]. It is suggested that this is because the bonds associated w ith form ation o f activated com p lexes, by w hich reaction m ust proceed, extend across the interface. In other w ords, the interface ceases to ex ist. There are also m any exam ples o f such phenom ena in steelm aking. These phenom ena have been review ed by Ribould and Lucas [60].
W hen such a lloy-slag system s are brought into contact the fo llow in g sequence o f events usually occurs:
(i) Progressive low ering o f effective interfacial tension.
(ii) Com plete disappearance o f interfacial forces.
(iii) R ecovery o f high interfacial tension values.
A typical interfacial tension versus tim e o f reaction graph is given in Figure 5 .6 . If this explanation o f low ering o f interfacial tension in term s o f intense m ass transfer at the interface is true, then w hy is an initial low ering o f interfacial tension seen? Surely m ass transfer is at its m ost in tense at the start o f reaction. In the case o f the Fe- A l/C a 0 -S i0 2-A l20 3 system investigated by Ribould and Lucas [60] this in itial period la sts for 10 m inutes fo llo w ed by a ten m inute p eriod w hen in terfacial forces disappear.
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PLOT OF a V S. t FOR A TYPICAL REACTION INVOLVING M ASS TRANSFER AT THE SLAG-M ETAL INTERFACE [60]
FIG URE 5 .6
A n alternative explanation m ay be that w hen the m etal and slag reach a certain com position, then the interfacial forces disappear.
5 .2 .4 T e c h n iq u e s o f M e a su re m e n t
T he sessile drop technique for the m easurem ent o f the surface tension o f a liquid involves the observation o f a liquid drop resting on a horizontal solid surface under a gaseous atm osphere. B y studying the shape o f the drop and, if the density o f the liquid is know n, by applying the Laplace equation the surface tension o f the liquid m ay be determined.
W hen m easuring interfacial tensions between m etals and slags, the gas is replaced by the slag. Various techniques have been developed for the m easurem ent o f interfacial tensions betw een m etals and slags [61] e .g . the liquid len s technique, drop w eight, drop pressure. H ow ever, the m ost reliable and w idely used technique is that o f X - radiography o f a m etal drop. It is essen tia lly a m odification o f the sess ile drop technique used for the determ ination o f the surface tension o f liquids. X radiation is used to photograph the profile o f a sessile drop o f m etal im m ersed in liquid slag. Figure 5.7 is a schem atic diagram o f the typical experim ental arrangement.
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X RADIOGRAPHY TECHNIQUE TO DETERM INE INTERFACIALTENSIO N [64]
FIG URE 5 .7
In addition to the obvious disadvantages w ith respect to high capital cost equipm ent and the p oten tia l radiation hazards, th is m ethod p o ssesse s severa l tech n ica l lim itations. The crucible m aterial m ust be chem ically com patible w ith both the slag and the m etal. Both the crucible and the slag m ust be sufficiently transparent to X radiation to allow reasonable photographic tim es. The exposure tim e for satisfactory photography is relatively long, w ith the possib ility o f blurred im ages resulting from any m ovem ent o f the m etal drop during the exposure period, due to either m echanical vibration or change in surface properties. N evertheless, the technique has been used by a variety o f workers [62 , 6 3 , 64] perform ing investigations in both ferrous and non-ferrous system s. In particular, the m ethod has been used by Utigard and Toguri [64] for the investigation o f interfacial tensions in the system A l/N aF-A lF3. The main problem that they encountered w as that alum inium absorbs on ly slightly m ore X rays than cryolite. T o obtain good contrast im ages o f the alum inium drop, the am ount o f construction materials that the X rays m ust pass through, in addition to the A l/cryohte cell, w as reduced to a m inim um . Even so a 15 m inute exposure tim e w as necessary.
5 .2 .5 E x p e r i m e n t a l M e t h o d
The objective o f this section o f the project was to investigate interfacial phenom ena during the production o f A l-T i-B m aster a lloys from fluoride salts. T he X -
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radiography technique w as not suitable for tw o m ain reasons. F irstly, there is the capital cost o f such a technique. Secondly, as pointed out in 5 .2 .4 , long exposure tim es (approx. 15 m inutes) are required. In v iew o f the fact that the k in etics o f reaction in this system are relatively fast (< 15 m inutes) the X-radiography technique is unsuitable.
A technique sim ilar in principle to the X-radiography technique w as used. Rather than v iew in g the m etal drop in situ , it w as quenched to room tem perature. The crucib le and its contents w ere then section ed and the shape o f the m etal drop exam ined. R ibould and Lucas [60] and D ew ing and D esclaux [65] have both used this technique in association w ith other m ethods. Its basic assum ption is that the m etal drop does not change shape significantly during quenching.
The experim ental procedure w as as fo llow s. An alum inium pellet o f m ass 1.75g w as placed under a know n m ass o f KF:A1F3 flux, containing known additions o f K BF4 and K 2T iF 6. T he KF-A1F3 flu x w as o f eutectic com position i.e . 45 m ole % A1F3 flu x , 55 m ole % K F, M pt. 560°C . It w ill be denoted (KF-A1F3)E. The m etal and flu x w ere contained w ith in a graphite crucib le o f 2 .5 cm inner diam eter. For investigations in the corresponding sodium system an equim olar NaF:A1F3 flu x w as used w ith additions o f N a ^ iF g and NaBF4. The crucible and its contents w ere then raised into the hot zone o f a furnace. After holding at temperature for a know n tim e, the crucib le w as then w ithdraw n from the hot zon e o f the furnace to a room temperature region in approxim ately 15 s. The crucible and its contents w ere then sectioned and exam ined.
5.3 Metal-Flux Systems
5 .3 .1 In tr o d u c tio n
The overall aim o f this section o f the project was to investigate interfacial phenom ena in the production o f A l-T i-B alloys from A1 and K^TiFg and KBF4. The system is a com plicated one to analyse d irectly, in volving as it d oes a flu x con sistin g o f KF- A1F3, K^TiFg and K BF4 and a m etal containing the phases T iA l3 and T iB 2 w ith T i, B , K and im purity elem ents in solution . It w as d ecid ed , therefore, to first study sim pler system s e .g . A l-T i/K F-A 1F3 in an attem pt to d evelop understanding in a series o f le ss com p lex con d ition s. A lso , by m aking sim ilar stud ies u sin g the equivalent sodium salts and flux it w as hoped to be able to isolate the role o f the alkali m etal in the process. A dditionally, studies o f the Al-Zr/K F-A1F3 system have been
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perform ed in order to shed light on the role o f titanium in the Al-Ti/KF-A1F3 system .
It is important to note that the kinetics o f the interfacial reactions and phenom ena are as interesting and important as the equilibrium situation itself. For exam ple, A1 and K A 1F4 m ay form tw o d istinct layers at equilibrium . H ow ever, if an em ulsion betw een these tw o phases is form ed during reaction, then this em ulsion m ay be d ifficu lt to destroy and rem ain as ’m etastable’ state w ithin the tim escale o f the p rocess.
5 .3 .2 T h e A llN a F -A lF 3 S ystem
B efore considering interfacial phenom ena in the A1/KF-A1F3 system it is instructive to con sid er the A l/N aF -A lF 3 system . The interfacial properties o f the alum inium - sodium cryolite system play an im portant role in the H all-H eroult process for the p rod uction o f alum inium and have thus been the subject o f a num ber o f in vestigation s. D ew ing and D esclaux [65] and U tigard and Toguri [64] have both studied interfacial tensions in this system . Both sets o f workers have found that the interfacial tension is strongly dependent on the NaF:A lF3 ratio, a decreasing tension being seen w ith increasing cryolite ratio. D ew ing and D esclaux postulated that the effect o f the NaF:AlF3 ratio is due to the adsorption o f sodium atom s at the surface o f the alum inium . The sodium atoms are generated by the follow ing reaction:
3(N aF) + A l(i) -> (A1F3) + 3 N a
w here an underline indicates that the elem ent is in solution in the m etal and brackets indicate that the constituent is d issolved in the m olten salt. A pplication o f the G ibbs adsorption isotherm to the data suggests that at m olar ratios NaF: A1F3 above 2.8 the in terface is covered w ith a m onolayer o f sodium atom s. B y increasing the N aF content even m ore, the change in interfacial tension w ith increasing sodium activity b ecom es le ss pronounced. T his is reasonable, because a m onolayer has already form ed and additional in creases in sodium a ctiv ity on ly cau ses the sodium concentration at the interface to increase above the first outer layer.
In v iew o f the sim ilarity betw een the A1/KF-A1F3 and the A l/N aF-A lF3 system s it is reasonable to predict that the interfacial tension betw een alum inium and a potassium cryolite m elt w ill depend on the KF-A1F3 ratio. Potassium , w hich is expected to be surface active, is generated by the follow ing equilibrium :
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i
The influence of different additives on the interfacial
tension at the electrolyte/Al boundary at 1273K.
Electrolyte consists of 88 wt°o of NaF-AlF^ mixture and
12 wt% A1203 [66]
FIGURE 5.8
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3(K F) + A l(!) -> 3 K + (A1F3)
A flu x o f low KF-A1F3 ratio should then result in less adsorption o f potassium at the interface and thus a higher interfacial tension. The investigation o f this system is described in Section 5 .3 .3 .1 .
5 .3 .3 E x p e r im e n ta l P ro g ra m m e a n d R e su lts
S essile drop experim ents w ere perform ed, using the technique outlined in section5 .2 .5 , for the m etal-flux system s given below .
5 .3 .3 .1 A U K F -A IF 3
T w o KF-AIF3 flu xes, w ith com positions 42 m ole % A1F3 and 53 m ole % A1F3, w ere prepared by fu sin g the requisite quantities o f K F, A1F3 and a flu x o f eu tectic com position , (KF-A1F3)E. The tw o flux com positions prepared are the extrem es o f single phase liquid com position that can exist at 750°C .
S essile drop experim ents w ere then perform ed using 1.75g o f com m ercial purity alum inium under 9 .5 g o f flux at 750°C . There w as no detectable d ifference in the shape o f the alum inium drops prepared under the tw o different flu xes. In other w ords, the in terfacial tension betw een alum inium and the flu x w as seen to be approxim ately the sam e in both cases.
5 .3 .3 .2 A l/N a F -A lF 3
T w o N aF-A lF3 flu xes, w ith com positions m ole 40% A1F3 and m ole 48% AIF3, w ere prepared by fusing together the requisite quantities o f N a3A lF 6 and A1F3. The tw o flu x com positions prepared are the extrem es o f sin gle phase liquid com position that can exist at 750°C .
S essile drop experim ents w ere then perform ed using 1.75 g o f com m ercial purity alum inium under 9.5 g flux at 750°C . There w as no detectable difference in the shape o f the alum inium drops prepared under the tw o different fluxes. In other w ords, the interfacial tension betw een the alum inium and the flux as measured by this technique w as seen to be the sam e in both cases.
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5 .3 .3 .3 A U T U K F -A IF 3
690°C
A n A1 - 9 wt% T i alloy w as prepared by the addition o f 0 .78g K^TiFg in 9 .5g (K F-A1F3)e flu x to 1.75g A l. The results obtained after various reaction tim es at 690°C are given in Figures 5 .9 (i) to (v). The salt and flux m ixture m elts first and then the alum inium p ellet m elts from its surface inw ards. The salt reacts w ith the outer layer o f alum inium to form a layer o f T iA l3. This T iA l3 layer then appears to be w et by the sa lt-flu x m ixture, w hich penetrates the T iA l3 layer and pushes out the m etal from the centre o f the drop.
Figure 5 .9 (v i) illustrates the result obtained w hen the alum inium drop is in itia lly m elted under 9 .5 g (KF-A1F3) flux containing 1 wt% CaF2 and then 0 .7 8 g K^TiFg are added. The flu x d oes not push out the m etal in th is case su ggesting that the presence o f calcium has a marked effect on the w ettability o f T iA l3 by the m olten salt solution.
720°C
A l-T i alloys o f various com positions were prepared at 720°C by the reaction o f 1.75g A l w ith the required am ount o f K^TiFg under 9 .5 g (KF-A1F3)E flux. The results are illustrated in Figure 5 .1 0 (i) - (iv). It is seen that the interfacial tension decreases as the titanium concentration in the m etal increases. In Figure 5.10 (iv ) an A l- 15 wt% T i a lloy is illustrated. In this case, a m etal-flux em ulsion has form ed, indicating that the interfacial tension is either very low or zero.
F igures 5 .1 0 (v) and (v i) illustrate the situation w here an A l - 9 wt% T i a lloy is prepared by the sam e m ethod as above, but w ith 1 wt% CaF2 and M gF2 respectively in the flux. The interfacial tension was seen to be increased by these additions.
740°C
A series o f A l - 9 wt% T i alloys w ere prepared at 740°C by the reaction o f 1.75g A l w ith 0 .7 8 g K^TiFg under 9 .5 g (KF-A1F3)E flu x . T he results are illustrated in Figures 5.11 (i) - (v ). A progressive low ering o f interfacial tension as reaction proceeds causes the alum inium drop to em ulsify in the m olten salt.
Magnification x 2
(iii) t = 45s
t = 240s (vi) 7.5 wt% K2TiF6+1 wt% CaF?
t = 240s
FIGURE 5.9
The Reaction of A1 with 7.5 wt% K^TiF^ in (KF-AIF^)^ for various times at 690°C
Magnification x 2
(i) 2.7 wt% K^TiFg in(k f-a i f3)e
(ii) 7.5 wt% K^TiFg in
(k f-a i f3)e
(iii) 9 wt% K^TiFg in (iv) 12 wt% K^TiFg in
(k f-a i f3)e (k f-a i f3)e
(v) 7.5 wt% K^TiFg + 1 wt%CaF2
in (KF-A1F3)e(vi) 7.5 wt% K^TiFg + 1 wt% MgF
in (KF-A1F3)e
FIGURE 5.10
The Reaction of A1 with Various Flux Compositions for 15 minutes at 720°C
Magnification x 2
t = 120s (vi) 7.5 wt% K^TiFg + 1 wt% CaF?
t = 120s
FIGURE 5.11
The Reaction of A1 with 7.5 wt% K^jiF6 in (KF-A1F^)^ for various times at 740°C
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Figure 5.11 (v i) illustrates the result obtained w hen the sam e procedure is repeated except that 1 wt% CaF2 has been added to the flux. In this case, the drop rem ains intact.
79Q°C
A series o f A1 - 9 wt% T i a lloys w as prepared at 790°C by the reaction o f 1.75g A1 w ith 0 .7 8 g K2T iF 6 under 9 .5 g (KF-A1F3)E flu x. Increasing am ounts o f CaF2 w ere added to the flux. The results are illustrated in Figure 5 .12 . It is seen that a critical C aF2 lev e l in the flu x (0 .28 - 0 .2 9 wt% CaF2) prevents the em u lsification o f the drop.
840°C
A series o f A1 - 9 wt% T i a lloys w as prepared at 840°C by the reaction o f 1.75g A1 w ith 0 .78g K2T iF6 under 9 .5 g (KF-A1F3)E flux. Increasing am ounts o f CaF2 - up to 5 wt% in flux - w ere added to the flux. In each case the alum inium drop w as seen to em ulsify. Additions o f M gF2 w ere also ineffective in preventing the em ulsification o f the drop.
5 . 3 .3 .4 A U T H N a F -A I F 3
A l-T i a lloys w ere prepared by the reaction o f alum inium and N a2T iF 6 under a NaAlF4 flux. A number o f different alloy com positions w ere prepared at a number o f different tem peratures. The results obtained in all cases were sim ilar to those for the Al-Ti/KF-A1F3 system i.e . the interfacial tension was seen to decrease with increasing titanium concentration in the m etal, additions o f CaF2 and M gF2 w ere seen to increase the interfacial tension and temperature w as seen to have an im portant effect on the process.
5 .3 .3 .5 A U B I K F - A I F 3
A series o f A l-B a llo y s w ere prepared by the reaction o f 1 .7 5g A1 w ith the stoichiom etric quantity o f K BF4 in 9 .5 g (KF-A1F3)E at 740°C . It w as found that, for a given reaction tim e, the results were not dependent upon the predicted boron level in the m etal.
A series o f A l-B a lloys w ere then prepared by the addition o f 2g KBF4 to 1.75g A1
Magnification x 2
(i) 0 wt% CaF^ (i i) 0.24 wt% CaF^
(iii) 0.27 wt% CaF^ (iv) 0.28 wt% CaF^
(v) 0.24 wt% CaF^ (vi) 0.25 wt% MgF^
FIGURE 5.12
The Influence of Various CaF~ Addition Levels on the Reaction of A1 with 7.5 wt% K^TiFg in (KF-A1F^)^ for 15 minutes at 790°C
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under 9 .5 g o f (KF-A1F3)E flu x at 740°C . The results obtained after various tim es are illustrated in Figure 5 .13 . The interfacial tension is seen to decrease in itially (Figure 5 .1 3 (i)), achieve a m inim um value (Figures 5.13 (ii) and (iv )), after four m inutes o f reaction and then begin to increase again after 7 .5 m inutes (F igures 5 .13 (v) and (v i)).
The addition o f 1 wt% CaF2 to the in itial flu x w as seen to have no e ffect on the interfacial tension (Figure 5.13 (iii)).
The alum inium drops form ed in this sequence were exam ined m etallographically. N o second phase particles w ere seen, neither the A1B2 nor the A1B12 phase. C hem ical analysis confirm ed that on ly very low boron levels w ere present in the m etal and that all the boron rem aining was presented in the flux. X -ray analysis o f the flux revealed the presence o f A1B12.
A series o f A l-B alloys w as then prepared at 740°C w ith increasing concentrations o f KBF4 in the initial flux. T he follow ing specim ens w ere prepared.
A 1.75g A1 9 .5 g (KF-A1F3)e 0 .3 g K BF4
B 1.75g A1 9 .5 g (KF-A1F3)e 0 .7 5g K BF4
C 1.75g A1 9 .5 g(K F -A lF 3)E 0 .9 g K BF4
D 1.75g A1 9 .5 g (KF-A1F3)e 1.2g K BF4
T he results w ere as follow s:
A . A rim o f precipitate w as form ed around the surface o f the alum inium drop. This rim extended right around the m etal surface in contact w ith the flux and w as not only present on the upper surface o f the alum inium as w ould be expected from density differences. T his surface layer o f m etal and precipitate w as filed aw ay and X -ray analysis o f the filin gs carried out. The phase A1B12 w as identified as being present. N o evidence for the presence o f the A1B2 phase w as found.
B . A rim o f A1B12 w as present around the surface o f the alum inium drop. The A1B12 particles were agglom erated into one long 'stringer' w hich took the shape o f
Magnification x 2
(v) t = 7\ minutes (vi) t = 9 minutesFIGURE 5.13
The Reaction of A1 with 15 wt% KBF. in (K F - A 1 ) for various times at 740°C
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the surface o f the alum inium - see Figure 5.16(i).
C . N o precipitate w as observed in the m etal. Sam ples o f flux from c lo se to the interface and from the bulk w ere exam ined by X -ray analysis. The sam ple from the bulk o f the flux contained no A1B12 phase, w hereas the sam ple from the interface region contained the A1B12 phase.
D . N o precipitate w as observed in the m etal. X -ray analysis o f flux from c lo se to the interface and from the bulk, revealed that A1B12 w as present in the flux and uniform ly distributed throughout
The m etal and flux from repeat runs o f A -D w ere analysed chem ically. T he results are illustrated in Figure 5.14.
5 .3 .3 .6 A l - B I N a F - A l F 3
A number o f A l-B a lloys w ere prepared by the reaction o f A1 with N aBF4 under a N aA lF4 flux. The results obtained w ere sim ilar to those in the A1-B/KF-A1F3 system i.e . the interfacial tension decreased w ith increasing tim e o f reaction, reached a m inim um value and then increased again after further holding tim e. A dditionally, it was seen that at a critical NaBF4 concentration in N aA lF4 the A1B12 was dispersed in flux rather than the metal.
5 . 3 . 3 . 7 A l - T i - B I K F - A I F 3
A series o f A l-T i-B a lloys w as prepared by the reaction o f 1.75g A1 w ith 2g K BF4 and 2g K^TiFg under 9 .5 g o f (KF-A1F3)E flux at 740°C . Equal m asses o f K BF4 and K2T iF6 g ives the Ti:B stoichiom etry required for T iB 2 form ation. The progress o f the reaction with tim e is illustrated in Figures 5 .15 (i) - (iv).
The outer layer o f alum inium reacts with the salts to form a layer o f T iB 2. The salt- flux m ixture then appears to w et this T iB 2 and it penetrates the layer pushing out the m etal. H ow ever, as the reaction proceeds the m etal w ets the T iB 2 and b egins to clim b up the T iB2 layer (see Figure 5.15 (ii)).
The effect o f separate salt additions w as then investigated. Figure 5.16 (i) illustrates the result w hen 0 .7 5 g K BF4 is added to 1.75g A1 under 9 .5 g (KF-A1F3)E flu x at 740°C . A s described in 5 .3 .3 .4 this w ill result in the form ation o f a surface layer o f
3
wt% B
wt% KBIjj in Flux
Figure 5.14 Distribution of boron between metal and flux with
varying concentration of KBF4in initial flux.
-96-
Magnification x 2
t = 0.5 (i i) t = 20s
) t = 30s (iv) t = 240s
6.5 wt% Na^TiFg.S.S wt%
NaBF4 in NaAlF^, 740°C
t = 240s
(vi) 7 wt% K^ZrFg-6.5 wt% KBF^
in (KF-A1F3)e 740°C, t=240s
FIGURE 5.15The Reaction of A1 with 15 wt% KiTiF5 and 15 wt% KBF4 in (KF-AlFo)p
for various times at 740°C
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A 1B 12. Figure 5 .1 6 (ii) illustrates the result w hen 0 .7 5 g K^TiFg is added to th is system . A surface layer o f T iB 2 is form ed w hich is w et by the flu x. T he flu x penetrates this layer pushing out the metal from the centre o f the drop.
Figures 5 .16 (iii) - (vi) illustrate the effects o f separate salt additions on a scale larger than the sessile drop. Figure 5 .16 (iii) illustrates the result w hen lOg A1 is reacted w ith 3 .5 g K2T iF 6 at 740°C . Som e entrapm ent o f flu x in m etal is seen , but no em ulsification. W hen 3 .5 g KBF4 is then added to this system the result is show n in Figure 5 .16 (v). A gain , entrapment o f flux in m etal is seen, but no em ulsification. The sequence o f events w as then repeated in the opposite order i.e . KBF4 added and then K^TiFg. In Figure 5 .1 6 (iv) the result o f reacting lOg A1 w ith 3 .5 g KBF4 at 740°C is show n. N o entrapm ent o f flux in m etal is seen . W hen 3 .5g K^TiFg is then added to this system the result is a flux-m etal em ulsion show n in Figure 5 .16 (v i). Around each pocket o f flu x is seen a layer o f T iB 2.
The effect o f salt concentration in flux on the w etting o f T iB 2 w as then investigated. A n A l-T iB 2 a lloy w as prepared by first reacting 1 .75g A1 w ith 0 .7 5 g K BF4 and 0 .7 5 g IC^TiFg contained in 9 .5g (KF-A1F3)E flux. The result is illustrated in Figure5.17 (i). Then, an A l-T iB 2 a lloy o f the sam e com position w as prepared by reacting 1.75g A1 w ith 0 .7 5 g K B F4 and 0 .75g K2T iF 6 in 19g (KF-A1F3)E. The result is illustrated in Figure 5 .17 (ii). A t low er salt in flux concentrations the flux does not penetrate the TiB2 layer and push out the metal.
The effect o f flux com position on TiB2 agglom eration w as also investigated. lOg o f alum inium pow der w as physically m ixed w ith lg T iB 2 pow der and 5g (KF-A1F3)E flu x and m elted in a graphite crucible at 740°C . The resulting alloy w as exam ined m etallographically. The T iB 2 particles were seen not to be agglom erated as illustrated in Figure 5.17 (iii). lOg o f alum inium powder was then reacted w ith 3.5g KBF4 and 3.5g K^TiFg (so as to g iv e lg T iB 2) and 5g (KF-A1F3) E flux at 740°C in a graphite crucible. The resulting microstructure is illustrated in Figure 5 .17 (iv). In this case the T iB 2 particles are agglom erated and form long 'stringers'. E D A X analysis o f these stringers in the S .E .M . reveals the presence o f the elem ents T i, K, A1 and F. In other w ords, the T iB 2 particles are bonded by the KF-A1F3 flux.
5 .3 3 .8 A l-T i-B ! N a F -A lF 3
A n A l-T iB 2 alloy w as prepared by the reaction o f 1 .75g A1 w ith 0 .7g N aBF4 and 0 .7 g N a ^ iF g in 9 .5g N aA lF4 flux. A s w ith the A l-Ti-B/K F-A 1F3 system the flux is
Magnification x 2
(i) Al-6 wt% KBF4 in (KF-A1F3)E
t = 15 minutes(ii) Al-6 wt% KBF4 in (KF-A1F3)E
than 6 wt% K^TiFg in (KF-AlF3)r t = 15 minutes
(iii) 10g Al-3.5g K^TiFfi
t = 15 minutes(iv) 10g A1-3.5g KBF4
t = 15 minutes
(v) 10g Al-3.5g K ^ T i ( v i ) 10g Al-3.5 KGF4
than 3.5g KBF4 than 3.5g K^TiFgFIGURE 5.16
The Effect of Order of Addition of K^TiFg and KBF4 to A1 at 740°C
Magnification x 2
(i) Al-7 wt% KBF4 and 7 wt% K^TiF6 (ii) Al-3.5 wt% KBF^ and 3.5 wt%
in (KF-AIF^)^ K^TiF^ in (KF-AIF^)^
740°C t = 15 minutes 740°C t = 15 minutes
(iii) 10g Al-lg TiB2-5g (KF-A1F3)E
740°C t = 15 minutes
(iv) 10g A1-3.5g KBF4
-3.5g K2TiF6-5g(KF-AlF3)E
740°C t = 15 minutes
FIGURE 5.17
The Influence of K2TiF^ and KBF4 Level in the Flux
on the Interfacial Tension and Agglomeration of Borides
- 1 0 1 -
seen to w et the T iB 2 layer and push out the m etal as illustrated in Figure 5.15 (v).
S .3 .3 .9 A l-Z r -B /K F -A I F 3
A n A l-Z rB 2 a lloy w as prepared by the reaction o f 1.75g A1 w ith 0 .7 5 g K^ZrFg and 0 .7 g K BF4 under 9 .5 g (KF-A1F3)E flu x. A s for the A l-T i-B /K F-A 1F3 and A l-T i- B /N aF-A lF3 system s the flux is seen to w et the boride layer and push out the m etal as illustrated in Figure 5.15 (vi).
5 .3 .4 D isc u ss io n
5 .3 .4 .1 A U K F -A IF 3
Potassium , w hich is know n to be surface active in alum inium [6 6 ], is generated by the equilibrium:
3 (KF) + AIq -> (A1F3) + 3 K
Increasing the KF:A1F3 ratio, and hence the potassium activ ity , should therefore decrease the interfacial tension . A lthough this e ffect m ay occur it cou ld not be detected in the experim ents. It is not possib le to account for flux-m etal entrapment by changes in the KF:A1F3 flux ratio alone.
5 .3 .4 .2 A V N a F -A lF 3
T he interfacial tension in this system is know n to decrease with increasing NaF: A1F3 ratio [64, 65]. H ow ever, this effect could not be detected in these experim ents.
5 .3 .4 .3 A U T U K F -A IF 3
The results obtained at 690°C show that T iA l3 is dispersed not in the m etal, but in the salt-flux mixture. This can account for the results obtained at 720°C i.e . the low ering o f interfacial tension w ith increasing titanium content. A t a critical l^T O ^ level in the flu x a m etal-flux em ulsion is form ed, as show n in Figure 5 .1 0 (iv ). A t K^TiFg concentrations in the flux low er than this critical value the T iA l3 is dispersed in the m etal. T hese results m ay be illustrated on a ternary interfacial energy diagram as show n in Figure 5 .18(a). T he representation o f interfacial phenom ena in this system on the K F-AlF3-TiF4 ternary diagram is given in Figure 5.18(b).
- 1 0 2 -
Increasing Mg, Ca concentration in flux
Increasing K^TiF^ concentration in flux
1 = Flux
2 = Metal
3 = TiAl3
Ternary i n t e r f a c i a l energy d iagram f o r th e system
A l - T i A l 3- ( K F - A lF 3 )
FIGURE 3.18(a)
-1 0 3 -
A T i A 1^ d i s p e r s e d i n m e t a l B T i A l ^ d i s p e r s e d i n f l u x
Interfacial phenomena in the KF-AlF^-TiF^ system represented
on the ternary phase diagram
FIGURE 5.18(b)
-1 0 4 -
A ddition o f M g or Ca to the system is seen to increase the interfacial tension. M g and Ca are known to be surface active in alum inium [67]. It is postulated that M g and Ca are effective in preventing m etal-flux em ulsification by occupying surface sites in the alum inium and so inhibiting the A l-K 2TiF6 reaction. The rate o f form ation o f T iA l3 w ill be decreased and a build up o f T iA l3 at the surface w hich m ay cause m etal-flux em ulsification w ill be prevented. The results at 740°C and 790°C show that a certain critical lev e l o f M g, Ca in the flux is required to prevent the T iA l3 being dispersed in the flux. The lev el o f M g required is less than the corresponding calcium lev e l to prevent em ulsification. T he level o f M g, Ca required increases as the tem perature increases. A t 840°C , M g and Ca are no longer effective in preventing the break up o f the alum inium drop.
5 3 . 4 . 4 A l-T i/N a F -A lF 3
The results obtained for th is system seem to confirm the conclusions drawn for the Al-Ti/KF-A1F3 system i.e . that a Ti rich flux w ets T iA l3. A ternary interfacial energy diagram sim ilar to the one for the Al-Ti/KF-A1F3 system may be drawn up.
5 3 .4 .5 A U B /K F -A IF 3
The A1B12 phase can be dispersed in either the flux or m etal phase depending on the process conditions. A t high KBF4 lev els in flu x the A1B12 is dispersed in the flu x and at low er KBF4 lev e ls in the m etal. A t 740°C the critical lev e l o f K BF4 in flu x seem s to be 0 .9 g K BF4 in 9 .5 g (KF-A1F3)E i.e . approxim ately 9 wt% . T he system m ay be represented on a ternary interfacial energy diagram as show n in Figure 5.19(a). The representation o f interfacial phenom ena in this system on the KF-A1F3- BF3 ternary diagram is given in Figure 5.19(b).
M agnesium and calcium are not effective in increasing the interfacial tension. This is probably due to the formation o f m agnesium and calcium hexaborides.
The results obtained at 720°C g ive an interfacial tension vs. reaction tim e curve sim ilar to that in Figure 5 .6 . Can this low erin g o f the in terfacial tension to a m inim um and then its restoral to higher values be accounted for in terms o f the usual explanation o f intense m ass transfer at the interface? In v iew o f the fact that the chem ical reduction o f K BF4 by alum inium is lik ely to occur w ithin the first 3 0 s o f reaction [see 3 .5 .3 ] this should be the period w hen the in terfacial tension is at a
-1 0 5 -
Increasing KBF^ concentration in flux
------------------------------------------------ ►
1
23
= Flux
= Metal
= A1B 12
Ternary i n t e r f a c i a l energy d iagram f o r th e system
A1-A1B 2- (K F -A lF )
FIGURE 5.19(a)
-1 0 6 -
A A1B.J2 dispersed in metal
B A I B ^ rests at metal/flux interface
C A1B.J2 rests at flux/metal interface
D A1B^^ dispersed in flux
Interfacial phenomena in the KF-AIF^-BF^ system represented
on the ternary phase diagram
FIGURE 5.19(b)
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m inim um . The fact that the m inim um in the interfacial tension is not achieved until several m inutes after the m ost intense m ass transfer has fin ished suggests that the conventional explanation for the minimum is questionable in these system s.
5 .3 .4 .6 A l - B / N a F - A l F 3
The results obtained for the Al-B/NaF-A1F3 system are sim ilar to those obtained for the A I-B /K F-A IF3 system . It w ould , therefore, be p ossib le to draw up a ternary interfacial energy diagram for the A l-B /N aF-A 1F3 system sim ilar to that show n in Figure 5.19.
5 . 3 . 4 . 7 A l - T i - B I K F - A I F 3
T he resu lts obtained show that, depending on the process con d ition s, the T iB 2 particles can be w et by either the flux or the m etal. T iB 2 w ill be w et by the flux when it contains relatively high salt levels. H ence, the A l-T i-B/K F-A 1F3 system may be represented on a ternary interfacial energy diagram as show n in Figure 5.20.
The formation o f T iB 2 agglom erates in master alloys can be explained in terms o f this diagram . In the in itial stages o f a lloy form ation the le v e ls o f K BF4 and K^TiFg in flu x w ill be very high. T his, the T iB 2 particles form ed w ill be w et by the flux, so agglom erating the particles. W hen reaction is com plete the equilibrium situation w ould be for the T iB 2 particles to be dispersed in the m etal. H ow ever, they cannot 11 see" the alum inium because they are surrounded by a flu x layer and hence the agglom erates remain.
A voiding the form ation o f T iB2 agglom erates is o f particular im portance to the master alloy manufacturer. The form ation o f T iB 2 agglom erates not on ly adversely affects the grain refin in g perform ance o f a m aster a lloy [6 8 ], but it m ay a lso lead to deleterious e ffects in the alloy to be grain refined e .g . the surface properties m ay be affected. B lake and Sm ith [2] have suggested that the problem o f T iB 2 agglom eration m ay be im proved by the addition o f a suitably surface active com ponent.
A n alternative suggestion , based on the present work, is that the concentration o f K BF4 and K2T iF 6 in the flu x be kept low . The problem o f boride agglom eration is m ost lik ely to occur at the beginning o f the process w hen the pure salts are added to pure alum inium . T his could be avoided by having a layer o f KF-A1F3 flu x on the
-1 0 8 -
Increasing K2TiF^ and KBF^ concentration in flux
-------------------------------------------------------------------------- ►
1 = Flux
2 = Metal
3 = TiB2
Ternary i n t e r f a c i a l energy d iagram f o r th e system
A l- T iB 2_ (K F -A lF 3 )
FIGURE 5.20
-1 0 9 -
surface o f the alum inium and by then feed ing the salts onto the m elt at a relatively slow rate.
5 3 . 4 . 8 A l - T i - B / N a F - A l F 3
T he results obtained for the A l-T i-B /N aF -A lF 3 system are sim ilar to those obtained for the A l-T i-B /K F -A 1F 3 system . It w ould , therefore, be p ossib le to draw up a ternary interfacial energy diagram for the A l/T iB 2/N aF -A lF 3 system sim ilar to that show n in Figure 5.20.
5 3 . 4 . 9 A l - Z r - B I K F - A I F 3
The results obtained for the Al-Zr-B/KF-A1F3 system are sim ilar to those obtained for the A l-T i-B /K F -A 1F 3 and A l-T i-B /K F -A 1F 3 system s. It w ou ld , therefore, be p ossib le to draw up a ternary interfacial energy diagram sim ilar to that show n in Figure 5 .20 .
5.4 Conclusions and Industrial Implications
Interfacial tensions have been seen to play a critical role in the production process for A l-T i-B m aster alloys from fluoride salts. The interfacial tensions determ ine the ease o f m etal-flux separation, and hence m etal clean liness, and whether any third phases present e .g . T iA l3, T iB 2, A1B12 w ill be dispersed in the m etal or in the flu x . For optim um m etal cleanliness and ease o f m etal-flux separation a high interfacial tension is desirable at a ll stages o f the production process. I f the interfacial tension is low in itially and increases to a high value at the end o f the process then problem s m ay still be encountered sin ce an em ulsion m ay form in itia lly w hich m ay be d ifficu lt to alleviate.
The kinetics o f m etal-salt reactions are also o f im portance to interfacial phenom ena. Fast transfer o f T i and B to the m etal w ill result in the build up o f com pounds such as T iA l3, T iB 2 or A1B12 at the interface. H ere, they can be w et by the flu x and an em ulsion is lik ely to result. The k inetics o f transfer can be inhibited by having a low er tem perature, a low er concentration o f K BF4 or K2T iF 6 in the flu x or by form ing a barrier betw een the alum inium and salt. T his can be ach ieved by the presence o f surface active elem ents in the alum inium such as M g or Ca.
- 1 1 0 -
T he con d ition s required for optim um m etal-flux separation and for m inim um agglom eration o f T iB 2 particles may be summarised as follow s:
(a) Low temperature.
(b) Low concentrations o f K^TiFg and KBF4 in the flux.
(c) H igh levels o f surface active impurity elem ents such as M g or Ca.
6. ALLOY STRUCTURE
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6 . ALLOY STRUCTURE
6 .1 Introduction
M uch work [1-15] has been done in trying to elucidate a m echanism for grain refinement and relate alloy microstructure to grain refining action. There has been a good deal o f disagreem ent over these proposed m echanism s and even disagreement over the experim ental results them selves. The purpose o f this project w as not to establish a grain refining mechanism. To do so would require a great deal o f effort and expense, with no real prospect o f a solution. Rather, the aim o f this investigation is to attempt to optimise the production o f aluminium grain refining master alloys.
In the past, the primary means used to describe a grain refiner was the bulk chemistry o f the alloy. T w o alloys o f the same chem ical com position can, how ever, behave quite differently as grain refiners. M ore recently, it has becom e apparent that the microstructure o f the alloy is an important factor in determining its grain refining characteristics. The structure depends not only upon com position, but also on the process history o f the alloy. B y studying the effects o f process parameters on alloy structure, and by relating this to grain refining action, it should then be possib le to produce reliable grain refiners. This section o f the thesis w ill, therefore, consider the effect o f process history o f the alloy on its structure. In particular, the fo llow in g aspects o f production w ill be investigated: -
(a) The effect o f com position, temperature and coolin g rate on alloy, and in particular titanium aluminide, structure.
(b) The effect o f impurities on alloy structure.
(c) The effect o f post-fabrication heat treatment on alloy structure.
6 .2 AI-Ti Master Alloys
Consideration o f the A l-T i phase diagram, Figure 6.1, [69] indicates that commercial master a lloys, w hich contain 5 to 10 wt% T i, w ill be primarily a m ixture o f a aluminium and crystalline TiAl3. When the master alloy is added to the aluminium to be grain refined (a 0.01 to 0.03 wt% Ti addition being typical) the T iA l3 crystals w ill d isso lve when held in contact with the liquid alum inium for a sufficient tim e.
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T/°C
Wf % T i
Aluminium rich end of the Al-Ti phase diagram
FIGURE 6.1
- 1 1 3 -
Estimates o f this time are o f the order o f a few minutes [3].
The aluminide crystals can exhibit several different m orphologies and show a marked variation in size. Am berg, Backerud and Klang [4] have classified the tw o m ost com m on m orphologies as 'blocky' and 'flaky' or 'needle like' with a third, less com m on, m orphology being termed 'petal like'. T h ese three m orphologies are illustrated in Figure 6.2. Am berg et al. report that the particular T iA l3 m orphology assumed depends upon the temperature history o f the alloy, blocky aluminides being formed when the titanium addition is made at relatively low temperature (about 900°C or below ) and flaky aluminides when this addition is made at a higher temperature. Petal like particles are formed when the alloy is rapidly cooled.
A m berg et al. [4] found, as did M axw ell and H ellaw ell [5], that the grain refining response o f the alloy depends upon the m orphology o f the titanium alum inide. B locky particles tend to act fast, but their effect fades quickly. Flake and petal-like particles act more slow ly, but their grain refining effic ien cy im proves with tim e and lasts longer. The blocky structure is the m ost important com m ercially for aluminium producers w ho make grain refining additions in rod form to the transfer trough or launder. In this case, a fast acting grain refiner is required. W hen holding furnace additions are made, however, the longer lasting flake structure is preferred.
The crystallographic structure o f the three aluminide form s are similar. T hey are formed by growth on (100) planes which are stacked together in various forms. The flaky crystals expose predominantly (001) planes towards the m elt [57] and there is no low index plane in alum inium w hich fits esp ecia lly w ell to this plane o f the substrate. This is a possib le reason for the long term effectiveness o f such alloys. The blocky crystals on the other hand expose both (001) and (011) planes towards the melt. The (O il) plane in T iA l3 has an alm ost perfect match to the alum inium (021) plane. This good crystallographic match m akes nucleation and growth o f aluminium more likely in the presence o f blocky aluminides rather than flaky ones.
In addition to the nucleating potential o f TiAl3, the presence o f the peritectic reaction:
L + TlA13 — oc A l
at temperatures ab ove the m elting point o f pure alum inium has a great influence on the rate o f growth o f so lid aluminium nuclei. R eference should be m ade to the peritectic region o f the phase diagram - see Figure 6.1. Consider a T iA l3 particle in
'Blocky' aluminide phase (dark grey) with TiB^ particles delineating grain boundaries
5 0 pm
(ii) 'Flake' or 'Needle' likealuminide phase (dark grey) with TiB^ particles delineating grain boundaries
i---------------------15 0 pm
FIGURE 6.2: Typical Aluminide (TiA13 ) Morphologies in anAl-5 wt% Ti-1 wt% B Alloy
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contact with pure aluminium, or an alloy o f low titanium content. Shortly after the addition o f a master alloy, a diffusion field w ill be established in the vicinity o f the T iA l3 crystal. At the surface o f T iA l3 the titanium content in the liquid metal is 0 .15 wt% and the concentration in the bulk o f the metal phase is m uch lower. H owever, w hen the m etal is cooled , because titanium raises the m elting point o f the metal and because T iA l3 is an effective nucleant, solid aluminium w ill form at the surface o f the particle at temperatures above the m elting point o f the bulk metal. Once solid has formed, however, the T iA l3 particle becom es engulfed in the solid phase, and further growth becom es limited by the diffusion o f the titanium from T iA l3 through the shell o f solid aluminium. Normal dendritic growth cannot occur because the solid metal nucleus is still above the growth temperature o f the low titanium content bulk metal. The great effect o f this m echanism on inhibiting grain growth has been considered in detail by Backerud et al. [6].
M axw ell and H ellaw ell [5] have also shown in a theoretical treatment that the growth rate o f aluminium is proportional to:-
1/Xx i = M CA (K - 1)
where:-
Ca = composition at the peritectic K = distribution coefficientM = rate o f change o f melting point o f aluminium (° C/wt% Ti)
Thus, w e m ay say that the grow th factor 1/X is determ ined so le ly by the characteristics o f the phase diagram. W hen comparing the three peritectic system s (A l-T i, Al-Zr, Al-Cr), M axw ell and H ellaw ell found that their grain refining ability w as directly related to the growth factors.
X Ti = 0.018
xZr = 0.1
Decreasing grain
refining ability.
In accordance with this explanation, it is possible to predict that T iAl3 particles should be present in the centre o f aluminium grains. This has indeed been observed [7].
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A lso , etch ing has revealed a titanium rich halo around T iA l3 particles [5]. Presum ably these titanium rich areas are the non-dendritic slow grow ing, high titanium nuclei which have formed at temperatures above the melting point o f low Ti - aluminium. C ooling curves published by Arnberg et al. [4] and by G uzow ski et al.[6] support the hypothetical sequence o f nucleation and growth events described above. W hen no master alloy is present, an undercooling o f 1 to 2°C is observed, follow ed by recalescence and growth at a higher temperature. W ith a master alloy the undercooling disappears and instead the nucleation starts at a temperature slightly higher than the freezing point o f the alloy.
6 .3 Al - Zr alloys
Zirconium acts as a grain refiner in aluminium alloys, but since its efficiency is lower than that o f titanium, and is not enhanced by boron additions, it is not com m ercially used for this purpose. The aluminium rich end o f the Al-Zr phase diagram is similar to that o f the Al-Ti phase diagram [69]. The invariant reaction at the aluminium end is a peritectic by which the liquid containing 0.11 wt% Zr reacts w ith ZrAl3 to form a aluminium solid solutions. The invariant temperature is 660.5°C . The liquidus rises more steeply from the peritectic point in the case o f Al-Zr than Al-Ti.
Zirconium alum inide, ZrAl3, is known to exist in both the blocky and flaky forms[3].
6 .4 Al - Ti - B Master Alloys
It has been w ell known since the work o f Cibula [1] that the presence o f boron increases the e ff ic ie n cy and fade-tim e o f A l-T i m aster a lloys sign ifican tly . Essentially, four theories have been proposed to account for the effect o f boron:
(a) The solubility o f T iA l3 is decreased in the presence o f boron. H ence, its solution rate is slow ed or stopped [8].
(b) Boron somehow nucleates TiAl3 during solidification [9].
(c) Boron form s a titanium diboride phase w hich has a low solubility in aluminium and is an effective nucleant [1].
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(d) A m etastable boride phase (A l, T i) B 2 form s and acts to grain refine aluminium [10].
The first tw o hypotheses are related to the equilibrium phase diagram for the ternary A l-T i-B system . Sigworth [11] has calculated the ternary phase diagram from available thermodynamic data. His calculated phase relations show that:
(i) The ternary peritectic
L + T iA l3 (+ T iB2) —> (X A l (+ T iB 2)
is virtually identical to the binary peritectic. In other words, T iA l3 solubility is unaffected by the presence o f boron.
(ii) There is no thermodynamic reason w hy T iB 2 w ould nucleate T iA l3 during solidification.
H ence, theories (a) and (b) seem to be invalid. This leaves the boride and metastable boride theories to explain the beneficial effects o f boron in the grain refining o f aluminium. Cibula [1] w as the first to propose that T iB 2 was an effective nucleant having low solubility in aluminium. H ow ever, there are certain problem s with the boride theory. Firstly, w hen an A l-T i-B master alloy is exam ined it is com m on to find T iA l3 in the centre o f so lid aluminium grains, whereas the T iB 2 particles are pushed to the grain boundaries. If T iB 2 had the characteristics needed to act as an effective nucleant, w e should find T iB 2 in the centre o f aluminium grains. Instead, w e find T iB 2 segregated to gain boundaries w hich su ggests that T iB 2 is a poor nucleant. Secondly, com m ercial experience show s that master alloys having a 2 .2 Ti:B ratio, the stoichiometric value for T iB 2, are very poor grain refiners. Thirdly, i f the relatively insoluble boride is responsible for grain refinem ent how can the grain refining characteristics o f the master alloy be explained. Jones and Pearson [12] postulated that borides first disperse and then settle out o f the melt, so accounting for fade. H owever, Tanaka et al. [13], on the other hand, have not been able to account for fade in terms o f settling o f borides.
A number o f workers [7, 14] have observed a com plex (A l, T i)B 2 phase. This has been thought to be a m etastable boride, since it m ay disappear w ith long holding tim es. It has been proposed that the (A l, T i)B 2 phase assists the grain refinem ent
- 1 1 8 -
either by acting directly as a nucleant, or by nucleating TiAl3 crystals during cooling. Backerud [6] has proposed that the (A l, T i)B 2 phase assists grain refinm ent by nucleating T iAl3 through a series o f reactions:
T iB 2(s) + A1(i) CTi, A1)B2(s) + l i
T L + 3A l(l) -> T iA l3(s)
T iA l3^ + L —> oc-Al
M ore recently, work by G uzow ski, Sigworth and Sentner [7] has identified a new alum inide m orphology, the so called ’duplex’ crystals. These crystals are obtained by:-
(i) Producing aluminide crystals which contain boron in solution, (Ti, B)A13 and then
(ii) A geing these crystals for a sufficient tim e to precipitate out all, or at least som e, o f the boron in solution.
This results in a duplex structure o f (Ti, B)A13 and (A l,T i)B 2. They claim that these duplex crystals are extremely potent grain refining agents.
6 .5 The Effects of Process Parameters on Alloy Microstructure
6 .5 .1 T h eo ry
Previous work [4] has shown that the temperature o f titanium addition is a factor o f im portance in determ ining the m orphology o f the titanium alum inides formed. H owever, by consideration o f the aluminium-titanium phase diagram it would seem that not only should the temperature o f titanium addition be important, but also the am ount o f titanium added. This conclusion m ay be drawn from the probable explanation for the formation o f blocky and flaky aluminides.
The blocky particles have a high volum e to surface area ratio. This suggests that these particles have formed in the tw o phase liquid + solid region o f the phase diagram. In this region the strain energy betw een the alum inide particle and the alum inium , present as a liquid, is o f no relevance. In any nucleation and growth
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process there are tw o terms o f importance - the strain energy term and the surface energy term. A system w ill adjust itself so as to g ive the minimum value for the sum o f these tw o terms. In this case the strain energy term is unimportant and so the particle w ill grow so as to m inim ise the surface free energy term. H ence, low aspect ratio blocks w ill be formed. It m ay be predicted, therefore, that aluminide particles form ed in the liquid + solid tw o phase region o f the phase diagram w ill adopt a blocky morphology.
Inspection o f the Al-Ti phase diagram reveals that at relatively low temperatures m ost alloy com positions are in this two phase region. H ence, w e can explain the reported findings that blocks are formed at low titanium addition temperatures. H owever, this explanation show s that not only should the temperature o f titanium addition be important, but also the amount added. It should be the combination o f temperature and com position which is important.
B y a similar argument w e m ay account for the formation o f flaky particles. These have a low volum e to surface area ratio. This suggests that the particles have grown under strain energy control. A likely explanation w ould be that the flakes are formed in the solid phase by precipitation from a supersaturated solid solution o f Ti in Al. In the solid phase, the strain energy term w ill dom inate over the surface term, thus favouring a high aspect ratio flake. Thus, w e can expect that i f an alloy w as solidified from the liquid single phase region, then flaky particles w ould be present sin ce, i f the coo lin g rate is sufficiently fast, the liquid w ill so lid ify to g ive a supersaturated A l-T i so lid solution. A lloys w hich fa ll in the single phase liquid region are generally at high temperatures. However, it should be possible to produce flakes at low temperatures if the titanium concentration is low enough.
In accordance with this approach, w e m ay predict that cooling rate w ill also be important in determ ining the aluminide m orphology formed. A s an A l-T i liquid solution is cooled the equilibrium Ti concentration w ill decrease. If this cooling rate is slow , the rejected Ti w ill form TiAl3 in the liquid phase, so giving blocky particles. If, on the other hand, the cooling rate is fast there w ill not be sufficient time for TiAl3 to form in the liquid and so a flaky aluminide particle w ill result. A t intermediate cooling rates, it should be possible for both blocky and flaky aluminides to be present in the same specimen.
I f this explanation holds then introducing Ti in the form o f N a2T iF 6 rather than
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K 2T iF6 should have no e ffect on alloy microstructure. A similar explanation o f aluminide morphology should also hold true in the Al-Zr system.
6 .5 .2 A llo y S tru c tu re - E x p e r im e n ta l W o rk
In order to test the hypothesis concerning the influence o f process parameters on aluminide m orphology, a series o f binary A l-T i alloys were prepared. In accordance with the proposed explanation for blocky and flaky aluminides, alloys were prepared w hich w ould fall in both the single liquid phase region and the liquid + solid tw o phase region. A lloys with 1 wt% Ti, 2 wt% Ti and 7 wt% Ti were prepared by additions o f potassium fluorotitanate to aluminium in a graphite crucible under an argon atmosphere. The experim ents were then repeated using sodium fluorotitanate. 100% titanium transfer efficiency from the salt to the alloy w as assumed in both sets o f experim ents. The alloys produced were sectioned and exam ined in the optical microscope. The results o f the investigation are summarised in Table 6.1.
The experiments to produce the alloys were repeated at three different temperatures: 7 5 0 , 9 0 0 , 1000°C at various com positions - by the reaction o f K 2Z rF 6 and aluminium. The results o f these experiments are summarised in Table 6.2.
The effects o f post fabrication heat treatments on alloy structure were then studied. A series o f A l-5 wt% T i-0 .2 wt% B alloys were prepared at 750°C and then aged at 700°C for 1/2 hour, 2 hours and 30 hours. Such an ageing treatment should give, according to the patent issued by the Cabot Corporation [15], alloys in the unaged, optimum and overaged condition respectively. B y using an iodine-methanol solution the aluminium matrix w as etched away, leaving the aluminides in relief. The deep etched samples were then exam ined in the scanning electron m icroscope (S.E.M .). The resulting micrographs are given in Figure 6.3.
6 .5 .3 R e su lts a n d D isc u ss io n
The results obtained for both the A l-T i and Al-Zr system s seem to confirm the idea that blocky alum inides result from alloys processed in the two phase liquid + solid region and flaky alum inides from alloys processed in the single phase liquid region. The use o f Na2T iF 6 rather than K2T iF 6 w as seen to have no effect on the alloy structure.
(ii) Optimum Ageing.Aged for 2 hours - uniform dispersion of fine boride particles in aluminide surface
(iii) Over Aged Aluminide Aged for 30 hours - Agglomeration of boride particles
FIGURE 6.3: The Effect of Ageing at 700°C on Aluminide Morphologyin an Al-5wt% Ti-0.2 wt% B Alloy
Under-aged Aluminide Aged for | hour
( i )
Temperature o f K 2TiIg AdditionAlloy
Composition 750°C 900°C 1000°C
1% Zr Predominantly blocky Average block diameter d = 30 pmR akes at outer edge o f specimen
100% flakes Average flake length i = 400 pm
Flakes and petals - associated with high cooling rate
2% Zr Predominantly blocky d = 22 pm
50% blocks d = 45 pm 50% flakes at outer specimen edge i = 1000 pm
100% flakes i = 700 pm
7% Zr 100% blocks d = 15 pm
Furnace cool gives 100% blocks d = 25 pm Air cool gives a mixed flake + blocks morphology
Predominantly blocky some flakes d = 35 pm
TABLE 6.1
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Temperature o f K^Zrlg AdditionAlloy
Composition 750°C 900°C 1000°C
1% Zr Predominantlyblocky
Blocks and flakes Flakes only
2 % Zr Blocks only Blocks only Blocks and flakes
7% Zr Blocks only Blocks only Blocks only
TABLE 6.2
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6 .6 The Aluminium-Boron System
6 .6 .1 In tro d u c tio n
Alum inium-boron master a lloys are used industrially as additions to the aluminium intended for use as an electrical conductor. E lem ents which w ould remain in solid solution in the alum inum , and so increase its resistivity , are precipitated out as borides. Alum inium - 3 wt% B and A1 - 4 wt% B alloys are produced com m ercially for this purpose by the addition o f K BF4 to alum inium . Certain problem s are associated with this production route:
(a) The metal and flux are sometim es very difficult to separate (see Chapter 5).
(b) The non-equilibrium phase A1B12 is often present in such master alloys. This is highly undesirable, since A1B12 d issolves on ly very slow ly when added to aluminium.
It w as decided, therefore, to study the production o f A l-B master alloys. This should give information o f interest to the production o f A l-B alloys per se, but it m ay also give further insights into the production process o f A l-T i-B alloys.
6 .6 .2 P h a se E q u ilib r ia
It is agreed that a eutectic [7 0 ,7 1 ,7 2 ]
Liquid —> Solid + A1B2
exists at low boron contents, and that boron has a low solubility in the solid (< 1 0 0 ppm ). H ow ever, there is disagreem ent on the boron solubility in the liquid. The older data [70, 73], when extrapolated to 660°C, g ive a eutectic com position o f 0.09 wt% B . M ore recent studies [71] show the eutectic to be at 0 .0 22 wt% B and 6 5 9 .7°C , and g ive higher boron solubilities at temperatures greater than 900°C . Sigw orth [11] has review ed the available data and recalculated the A l-B phase diagram, w hich is given in Figure 6.4.
H ow then can the presence o f A1B12 in A1 - 3 wt% B and A1 - 4 wt% B alloys, processed at say 750°C, be explained in terms o f the A l-B phase diagram? Consider the free energy diagram for the A l-B system at 750°C . It w ill have the appearance o f
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the diagram shown in Figure 6.5. The solubility lim it C' o f the alum inium solid so lu tion , a , in equilibrium w ith the A1B12 phase is higher than that, C, for equilibrium with the A1B2 phase. Thus, a second liquidus line can be entered on the phase diagram below that for A1B2, to denote temperatures and com positions at which the liquid phase becom es supersaturated with respect to the precipitation o f the A1B12 phase.
From Figure 6.5 it m ay be seen that the partial free energy o f boron for the liquid- A1B2 situation is more negative than the partial free energy o f boron for the liquid-A1B12 situation.
i.e . A G g < AGba i b 2 a i b 12
H ence, RTln a g < RTln aa i b 2
a B < a Ba i b 2 A1B12
If the activity o f boron in solution in aluminium is above a certain critical valueB A1B 12 -then formation o f A1B12 can occur. Such a situation could arise i f the
boron concentration is loca lly very high e.g. at the m etal-salt interface. From the A1B12 metastable liquidus projection in Figure 6.4 , it is seen that only a very narrow band o f com position ex ists at 750°C where on ly A1B2 can be formed. Thus, in com m ercial A l-B master alloys the presence o f A1B12 is highly likely. The presence o f the A1B12 phase at relatively low boron concentrations has been described in5 3 .3 .5 . Even at the low est boron concentration investigated - 3 wt% - A1B12 w as seen to be the only boride phase present.
6.7 Conclusions and Industrial Implications
The grain refining perform ance o f an A l-T i-B m aster alloy is related to the alloy microstructure. The microstructure is, in turn, related to the process history o f the alloy. H ence, by careful control o f process parameters it should be possib le to produce efficient and reliable grain refiners. The critical process parameters appear to
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Aluminium rich end of the Al-B phase diagram
FIGURE 6.4
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Composition
Figure 6.5 Free energy - composition relationships in the system Al-B.
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be temperature, composition and holding time.
In order to produce A l-B master alloys w hich do not contain the deleterious A1B12 phase it is necessary to prevent a build up o f boron in aluminium. Boron build up is likely to occur at the metal-salt interface where reaction occurs. Its occurrence will be prevented by inhibiting the kinetics o f the metal-salt reaction. This could be achieved by, for exam ple, maintaining a low concentration o f KBF4 in the flux phase. Such a m ethod would have the additional advantage o f avoiding the problems o f m etal-flux separation outlined in Chapter 5.
7. CONCLUSIONS AND INDUSTRIAL IMPLICATIONS
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7. CONCLUSIONS AND INDUSTRIAL IMPLICATIONS
The aim o f this project has been to investigate the production route for Al-Ti-B master alloys from K BF4 and K^TiFg in order to optim ise the process with respect to (a) transfer o f Ti and B from the salts to the metal, and to (b) producing an efficient and reliable grain refiner free from flux inclusions and boride agglomerates. The main conclusions o f the work may be summarised as fo llow s.
(a) Salts
Potassium salts have low er decom position pressures than their sodium equivalents and hence the elem ental transfer efficiencies from salt to m etal are higher for the potassium salts. Simultaneous salt addition yields better transfer efficiencies than do single salt additions.
(b) Flux
The potassium level in the metal due to reaction with the KF-A1F3 flux is less than 5 ppm. During grain refinement this w ill be reduced by approximately 500 times i.e. to less than 0.01 ppm in the final alloy. This value is m uch lower than that known to be responsible for hot cracking in aluminium alloys (~ 10 ppm alkali metal). A ny higher levels o f potassium seen in the alloy are m ost likely to be due to entrapment o f flux, as outlined below.
(c) Interfacial Phenomena
Interfacial phenom ena have been seen to play an extrem ely important role in the process with respect to m etal-flux separation and boride agglomeration. It has been seen that for m inim um slag entrapm ent in the a lloy and m inim um boride agglomeration, a low concentration o f salt in flux is required. The concentration o f salt in flux w ill be at its highest value when the salt is fed directly onto the surface o f the aluminium. This situation can arise at tw o stages o f the production process. Firstly, at the beginning o f the process before any KF-A1F3 flux has formed. Secondly, i f the m elt is stirred by electromagnetic induction then aluminium surface m ay be exposed even w hen a significant amount o f KF-A1F3 flux has been formed. O ne solution to the first problem would be to m elt the aluminium under a layer o f KF-AIF3 flux and then feed the salts at a relatively slow rate onto the surface o f the
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flux layer. A solution to the second problem would be to stir the m elt less vigorously during the feeding stage. A lso, the elements M g, Ca inhibit the kinetics o f reduction o f K^TiFg and KBF4 and, by so doing, they can prevent m etal-flux em ulsification. The temperature o f the process also plays an important role in this regard. A relatively high level o f M g, Ca and a low temperature is necessary for optim um m etal-flux separation.
(d) A lloys
For A l-T i-B m aster a llo y s a relatively lo w temperature and high titanium concentration w ill produce the conditions necessary to yield an alloy which has a fast, effective grain refining action. A geing can be used to alter the structure o f the alloy and thereby influence grain refining action. For A l-B master alloys a high level o f boron build up at the surface o f the aluminium can result in the formation o f the nonequilibrium particle A1B12 which is deleterious for alloy properties.
In addition to these individual suggestions for processing conditions an overall p rocess change m ay be put forward, that o f subm erged pow der in jection . C onventionally, the salts K BF4 and IC^TiFg are poured onto the surface o f m olten alum inium . P ossib le alternatives to surface additions w ould be, for exam ple, plunging o f the salts into the molten bath, overpouring o f liquid metal on to the salts or submerged powder injection. O f these proposed alternatives the powder injection technique w ould appear to be the m ost promising. In such a process an inert gas is used as a carrier for powdered reactants to the liquid metal bath. The technique is becom ing increasingly important throughout process metallurgy. For instance, in steelm aking pow der injection is used in the Q .B.O .P. process for decarburisation (oxygen/tim e injection); for deoxidation (argon/aluminium); for desulphurisation (argon/CaO -CaF2-A l20 3); for dephosphorisation (argon/CaO-CaF2-C aC 2-F e20 3) and for alloying (argon/ferro alloy fines). Its use is not only restricted to steelmakinge.g . E lkem in Norway have been injecting silicon into aluminium for m any years [74]. The advantage o f powder injection is that, in general, it im proves process kinetics. Each powder particle is brought intimately into contact with the metal, so providing a much greater surface area for reaction, and also injection w ill agitate the m elt and thereby improve process kinetcs.
Felski et al. [75] have seen that, in general, powder with a lower density than the bath and reacting exoth erm ically can be injected m ore easily than high density endotherm ically reacting com pounds. K BF4 and K 2T iF 6 are thus favourable
- 1 3 1 -
candidate materials for injection into aluminium. A number o f advantages m ay be expected from the injection o f KBF4 and K2T iF6 into aluminium over conventional process routes. Firstly, excellent transfer efficiencies can be expected. Any volatiles form ed by the thermal dissociation o f the salts w ould have to rise through the m elt and so capture the boron or titanium by aluminium would be likely. Secondly, the interfacial problems encountered when the pure salts are added directly to aluminium are likely to be avoided. This is because a much larger area w ill be provided for reaction and also the agitation introduced by injection w ill m inim ise titanium and boron build up at the interface. Thirdly, during the production o f A l-B master alloys form ation o f A1B12 m ight be prevented since build up o f the boron leve l in local regions o f the m elt would be avoided by the agitation induced by injection.
8. REFERENCES
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A PPE N D IX
S O M E F U R T H E R C O N S I D E R A T I O N S O F I N T E R F A C I A L P H E N O M E N A I N T H E S Y S T E M A l / K F - A l F g - T i F ^ A T H I G H T E M P E R A T U R E S
D u r in g the course o f this study it becam e apparent that certain anom alies existed betw een the behaviour o f the A l / K F - A l F 3- T i F 4 system at temperatures b e lo w about 8 0 0 °C and above 8 0 0 °C . F ir s t ly , additions o f c alciu m and m agne sium w ere no lon ge r e ffe c tive in pre venting the break up o f the a lu m in iu m droplet at the higher tem peratures. S e c o n d ly, the a lu m in iu m m etal tended to w e t the graphite crucible rather than the flu x at the higher temperatures. Fu rth e r consideration o f this system together w ith tw o recent publications [ A l , A 2 ] , that have just com e to o u r attention, a llo w us to propose an explanation fo r these anomalies.
Z h u x ia n et al [ A l , A 2 ] have investigated the preparation o f A l - T i - B master alloys b y therm al reduction and electrolysis o f B 20 3 and T i 0 2 in cryolite-alum ina melts in the presence o f alum inium at 1 0 0 0 °C . A llo y s containing up to 4 w t% titanium and 2 w t% boron w ere fo rm e d , w ith titanium to boron w e ig h t ratios in the range fro m 3 to 10 . T h e w e ttin g o f the carbon crucible increased stro n g ly w ith increasing titan iu m contents in these alloys. In the case o f pure a lu m in iu m , the cryolite m elt w etted the graphite bottom o f the crucible m uch better than the alum inium d id , and the metal then had the shape o f a globule. W h e n T i 0 2 and/or B 2Q 3 w ere added to the m elt, the alloy fo rm e d w etted the bottom m uch better than the m elt d id . T h u s , the shape o f the allo y becam e concave d o w n w ard s, and in some cases it even clim bed up along the inner w a lls o f the crucible. Z h u x ia n et al [ A l , A 2 ] attribute this p h eno m en on to the fo rm a tio n o f T i B 2, a m aterial that can be used as a w ettable cathode in alu m in iu m electrolysis. H o w e v e r , the fact that this phenom enon has been observed in the A l - T i system alone, both in the present w o rk and in that b y Z h u x ia n et al, suggests that the w etting o f T i B 2 b y alum in iu m cannot be the explanation fo r this phenom enon. T h e fo rm a tio n o f T i C and its w etting b y a lum inium w o u ld seem to be able to e xplain the results obtained fo r the A l - T i system . T i C is th e rm o d yn a m ic a lly m ore stable than T i A l 3 at all tem peratures. H o w e v e r , it w o u ld seem that kinetic factors prevent the fo rm a tio n o f T i C at lo w tem peratures i.e . b e lo w a bout 8 0 0 °C . T h e w e ttin g o f graphite b y m etal is not observed at these tem peratures. A t higher tem peratures reaction between titanium in solution in alum inium and the graphite crucible w ill lead
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to the formation o f a layer o f TiC at the m etal-crucible interface. Liquid aluminium w ill then w et this T iC layer. This explanation can be used to understand the phenom ena observed in the A l-T i system at 790°C and 840°C [see 5 .3 3 .2 ] . For exam ple, in Figures 5 .12 (i) and (ii) the metal is seen to w et the crucible rather than the flux. M etallographic exam ination o f the droplet illustrated in Figure 5 .12 (ii) show s that at the metal-crucible interface fine particles are present which do not have the appearance o f T iA l3 particles. It is suggested that these are TiC particles. These fine particles are segregated at grain boundaries in the aluminium, rather like the segregation o f T iB2 in aluminium.
Industrially, m etal-crucible interaction w ill not occur. Thus, it is proposed that the presence o f calcium and magnesium in the flux w ill prevent flux-metal emulsification at high temperatures as w ell as at low temperatures.
References
A1 Q. Zhuxian, Z. M ingjie, Y . Yaxin, C. Zhenghan, Shenyang, K. G ijotheim , H. Kvande: Alum inium, 64, 606-609 (1988).
A 2 Q. Zhuxian, Y . Y axin , Z. M ingjie, Shenyang, K. Grjotheim, H. Kvande: Alum inium , 6 4 ,1 2 5 4 -1 2 5 7 (1988).
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M y thanks are due to the following:
1. Laporte Fluorides Ltd.
This project has been fully funded by Laporte Fluorides Ltd. I wish to express m y sincere gratitude to Dr. G eoff Hignett o f Laporte for providing a level o f interest and support that has proved invaluable.
2 . A nglo-B lackw ells Ltd.,Kawecki Billiton Metaalindustrie,London and Scandinavian Metallurgical Co. Ltd.
I w ould like to thank the above named com panies for providing sam ples and analytical services and also for a number o f useful d iscussions about the production and use o f aluminium grain refiners.
3 . Professor P. Grieveson
I am especially glad to have this opportunity o f acknowledging my indebtedness to Paul Grieveson for the numerous inform ative and stimulating discussions that w e have had throughout the course o f this work.
4 . Dr. Brian Terry, for supervising this project.
5 . To the technical staff o f the Department o f Materials, Imperial C ollege for their assistance and in particular to Mr. Alan W illis and Mr. Jim Wright.
6. M rs. G ill H opkins, for her secretarial serv ices and m any entertaining conversations.
7 . T o the past and present members o f the John Percy Group for their help and friendship. In particular, I w ould like to thank Dr. N. Raghunathan for his help.
8. Mrs. Ann Toum azou for typing this thesis and Mr. Stephen D avey for his assistance with a number o f the figures.
A C K N O W L E D G E M E N T S