ISIJ Inte~national, Vol. 36 (1996), No. 3, pp. 255-262

Microstructural Features

Reaction Synthesis

of TiAl, Base OompoundsFormed by

M. SUJATA.S. BHARGAVAand S. SANGALDepartment of Materials and Metallurgical Engineering. Indian Institute of Technology.

(Received on July 10. 1995, accepted in final form on November29. 1995)

kanpur. Kanpur 208 O16, India.

Microstructural evolution in TiAl, and TiAl, base alloys formed by the reaction synthesis between liquid

Ai and solid Ti or Ti alloys has been studied by carrying out the tests on reaction couples isothermallyreacted between 973 and 1373Kfor different time intervals. It has been found that the reaction productforms by the exothermic chemical reaction and while TiAl, is the only reaction product formed up to thereaction temperature of 1273 K, TiAl, as weli as Ti*Al=, form whenthe reaction is carried out at 1373 K, In

all cases the microstructure of the reaction layer comprises titanium aluminide particles dispersed in an A]rich matrix. However, morphological features of titanium aluminide particles been shown to be widelydiffering with the reaction conditions. In pure Ti-AI reaction couples an increase in the reaction temperaturefrom 973 to 1173K ieads to the formation of particles having a bi-modal distribution, At the reactiontemperature of 1173 Ka large numberof particles formed contain several micro-cracks, In contrast to TiAl.formed in the case of pure Ti-AI couples, the morphology of the TiAl, based compoundscontaining VorMo, Zr and Si has been found to be significantly different. Instead of being equiaxed, the particles of TiAl*based compoundshave a continuously changing morphologyacross the reaction layer. The particles awayfrom the reaction interface, in general, are not equiaxed, and have sharp edges and manyof them havesevere internal micro-cracks.

KEYWORDS:reaction synthesis; exothermic chemical reaction; TiAI,; Ti*Al..; thermal cu~rents, particle

morphology; particle size; micro-cracking in TiAl,.

l. Introduction

For producing near-net shape parts of titaniumaluminides the reaction of liquid aluminum with solid

titanium plays an important role in developing several

advanced P/M technologies of current interest such asreactive sintering (RS), reactive hot pressing (RHP),reactlve hot isostatic pressing (RHIP) and reactive shockinduced explosive formation (RSIEF),1 -7) Basically, all

these techniques belong to the single generic group ofreaction synthesis based P/Mprocessing involving withthe preparation of powder compacts/blends from ele-

mental Ti and Al powders which are subsequentlysintered at a temperature higher than that of the meltingpoint of Al. Thepresence of liquid Al within the compactduring sintering initiates the chemical reaction betweensolid Ti and liquid Al and leads to the synthesis oftitanium aluminide(s) by thermal explosion. The dif-

ference in individual processes arises due to the

presence or absence of an external stress field which

mayalso be concurrently applied during sintering. Thenature of the stress field, i.e. whether it has only ahydrostatic componentor a deviatoric componentaswell, also creates the difference between these different

processes.Amongvarious intermetallic compoundsfound in the

binary Ti-AI phase diagram8) (Fig. l), only alpha-2

(Ti3Al), gamma(TiA1) and titanium trialuminide (TiA13)have been identified as futuristic low-denslty materialsfor high temperature applications and, therefore, aconsiderable amountof research work has currently beencarried out to understand their physical metallurgy andprocessability characteristics.9'10) Relevant propertiesneeded for their usage at high temperatures have beensurnmarized in Table I .

Fromthe data shownin the tableit becomesclear that amongthese compoundsTiA13 hasthe lowest density, the highest microhardness (and hencepossibly the strength) and the best oxidation resistance.

Aconsiderable interest, therefore, exists in the processlngof TiA13 by reaction synthesis.

Interest in studying the reaction between Ti and Al,

however, is not newand the kinetics of formation ofthe reaction product by both solid-state reaction of Tiwith All6- 19) as well as its reaction with liquid A120-23)have been reported in the literature. Though specific

reasons for its formation have not been discussed, it

is generally reported that TiA13 is the only Ti-A1intermetallic compoundformed during the reactionbetweenTi and Al under the studied reaction conditions(the highest reaction temperature being 1173 K),

However,the presence of Ti. Ti3Al, TiAl andTiA12 alongwith TiA13 has also been reported under certainconditions of heat treatment of the product formed as aconsequenceof reaction between Ti and Al.16,17)

255 @1996 ISIJ

18000

1700

1600

1500

_ 1400.O

- 1300

~1200

~~ 1100"~E 1000"* 900

800

700

600

5000

Ti

Atomic10 20 30 40 50

ISIJ International, Vol.

Percent ALuminium60 70 80

36 (1996), No. 3

90 100

1670 'C

(pTi )

882~CC(Ti)

~\~ /

,/

Ti3At !,,,

ll

ll

//

-1285'CTiAt

//"I125~!

l I~rl l!

l 16il'klrL ~l~i

'h- -,lllf' a',1 ,~ u]

4'a'll1, LI' o :',-, ~'fj

L uIf ~:'Ll'

:'+'If(f' ulI,

If

if

h~FT'f]

IAL2

L

-TiAt3

665'C

CCTiAt3 (AL)

10 20 30 40 50 60 70 80 90 100ALWeight Percent ALuminium

Table l. Properties of Ti3Al, TiAl, and TiA13 intermetallic compounds.

Fig. 1.

The Ti-AI phase diagram

PropertyTi3Al based

o(2 alloys

TiA1 base*/ alloys

TiA13base alloys

Ref.

Crystal structure D019 Llo D022 11)

Modeof deformation Slip of,

on

(i) (OOOl), (ii) (lOIO), (iii) (0221)

aSlip of ~~ 1210>+c' >on

(i) (121 1), (ii) (0221)

Slip of l0> on {1 1l}

Slip of l> on {1 1l}

1Slip of ~ 12>

Twinning of (1 11) [1 12]

Slip of [1 lO] on {OOl} at

l l)

Critical ordering

temperature ('C)l 100 1460 1350 1l)

Melting point (*C) l 600 l 460 l 350 l 1)

Density (g/cm3) 4. 1~L7 3.7-3.9 3.4 11)

Yield strength (MPa) 700990 400-650 11)

Tensile strength (MPa) 8001 100 450-700 l l)

Young's modulus (GPa) 120145 l60- 176 192* 11), 12)

Vickers microhardness(kg/mm2)

180350 180~50 465-670 13), 14)

Oxidation resistance ('C) 650 800 l OOO 13), 15)

* L1, based Tl,=Al*,Fe*

Figure 2 shows the SEMmicrograph of a carefully

reaction synthesized TiA13 sample prepared in ourlaboratory.24) It can be seen that it consists of well-

sintered particles of TiA13. It can be easily visualized

that such a sample can be further transformed to afull density material by the conventional P/M techniques.

Therefore, the interest in the subject from the point ofview of developing the P/M based technologies (as

revealed by Fig. 2) Iies in studying the effect of reaction

conditions on the evolution of (i) the particle size and

C 1996 ISIJ 256

its distribution, (ii) the shapes of particles and their

distribution, (iii) the porosity and the nature of its

distribution and (iv) the presence of other equilibrium

or non-equilibrium phases in the end- or intermediate-

product. It is to be noted that while the first three ofthe abovementioned objectives have not beengiven anyattention in the previous studies, the last objective is

important becausefirstly it maybe desirable to carry outthe reaction synthesis at temperatures higher than I 173K(the highest ternperature used in studies carried out so

ISIJ International, Vol. 36 (1996), No. 3

far) and secondly the presence of Ti, Ti3Al, TiAl or TiA12 were isothermaily reacted under a flowing high-purityalong with TiA13 as reported to be forming under certain argon in a specially designed horizontal tubular muffieconditionsl6,17) is expected to influence the properties of furnace at temperatures ranging between973 and 1373Kthe end-product. for time intervals ranging between 0.9 and 7.2ks. The

The present work was undertaken to study the effect muffie had a cold zone which wasmadeto project out-of reaction temperature on microstructural evolution in side the furnace and the zone was maintained at roomsynthesized TiA13 or TiA13 base alloys madeby reacting temperature. Prior to reaction synthesis, couples wereeither pure Ti, or Ti-Al-Mo-Zr-Si and TiAl-V alloys kept in the cold zone of the muffie and after attainingwith super-purity A1. In order to maintain the re- the set reaction temperature they were pushed into theproducibility of results, the experiments were carried reaction zone, reacted for the desired time interval andout using reaction couples madefrom titanium/titanium pulled out into the cold zone where they were cooled toalloys and Al. It is implied that the findings of the results the room temperature under flowing argon. Reactedof these fundamental experiments should be applicable couples were subsequently cut along their longitudinalon aluminide formation during reaction synthesis in the axes and the reaction lay.ers formed under differentabsenceof any concurrently applied external stress field. reaction conditions were subjected to X-ray diffractionThe results presented here deal with qualitative aspects analysis, microstructural analysis by optical microscopyof variation in the particle size, particle shape, and the and SEM(fitted with EDAXand X-ray mappingphases formed during reaction synthesis as a function of facilities) and the analysis by the EPMA.the reaction temperature.

3. Results and Discussion2. Experimental Procedure

3.1. Reaction Product and the Nature of ReactionPure tltanium, Ti-A1-Mo-Zr-Si, and Ti-AlV (all Since an excess amount of aluminium was present

in the form of rolled bars) supplied by Midhani Ltd., during all reaction conditions, the reaction layer (RL)India and super-purity solid aluminium supplied by formed in differently reacted samples consisted of alu-Semi Element Inc., USAwere the starting materialS minide particles dispersed in an Al rich matrix. Theused for the preparation of reaction couples. Chemlcal effect of reaction temperature on the thickness of thecompositions of pure Ti and Ti alloys have been given reaction layer formed in reaction couples prepared fromin Table 2. Reaction couples were prepared from cy- pure Tl and reacted for 3.6ks has been shownin Tablelindrical samples of 20mmlength and 20mmdiameter 3. It can be seen that an Increase in the reaction tem-cut from pure Ti and Ti alloy bars. Ablind hole of8mm perature resulted in the Increase in the reaction layerdiameter and about 10mmheight was dril]ed in each thickness. Similar results were obtained for other reac-of the samples along their cylindrica] axis. After clean- tion intervals and thelr nature was found to be simllaring internal surfaces of the blind holes with (1) acetone to that reported in previous investigatlons.2023)and (ii) HF-HN03solution they were fitted with solid As shown in Fig. 3. X-ray diffraction analysis ofcylindrical super-purity Al piece. Couples thus prepared phases present in the reaction layer and regions adjacent

to it revealed the presence of only Ti, A] and TiA13 in

~ t~t': ~. I couples reacted up to 1173K and Ti, A], TiA13 and\ ~~) r_

\ '

,'S~•-,

•,

Implying the formation of only TiA13 at lower reaction...•

.~'

,.

'i~::; '".

;'

r-

^~*

~ Ti9A123 In pure T1-AI reaction couples reacted at 1373K!'"' J~-:;~~' .

'l'( temperatures and that of TiA13 and Ti9A123 at higher"-'~'

'

_'~~

~'

tq~:r;~,~

=._reaction temperatures. Compositional analysis of parti-

', ~1~ f~ ,"= ,=

.\ cles present in the reaction layers and formed up to the

, ,

,

,

••:,•:_'~:: ~'

also confirmed that TiA13 was the only reaction product

+

4 ',• ' t.,=L ,~

~~,_ ,='• reaction temperatures of 1273 K, as given by EDAX,~ .\..~\l

! ~rl f ! '•

'•

/ formed at lower reaction temperatures. Similar observa-tions were madein the case of reaction couples made

~~

b~:~~ ~.~ \from T1-Al-MoZr-Si and Ti-Al-V and Al respec-

~.

~i ~~~;,."I;

. _

,~( ..•• ~ ~ H~H10;mj;tively. These observations are in agreement with thefindings of earlier workers whostudied the reaction of

Fig. 2. SEMview of arrangement of partides in a carefully solid titanium and liquid aluminium up to the reactionreaction synthesized TiA13 sample, synthesized at

l 073K for 14.4ks, temperature of I 173 K.20-23)

Table 2. Chemical compositions of titanium alloys used for preparing reaction couples.

Composition (wt"/o)

AlloyAl V Mo Zr Si Fe o H N Ti

Pure Ti

TiA1-VTiA1-Mo-ZrSi

6. l6.5

4.2

3.5

0.2

l .6 0.25

0.2

0.25

0.25

0.15O, 15O, 15

0.0 lO.Ol

0.01

0,05

0,05

0,05

bal

bal

bal

257 O1996 ISIJ

ISIJ International, Vol. 36 (1996), No. 3

Chemical analysis of aluminide particles in the two-phase microstructures of reaction couples madefromAl and Ti-Al-V and Ti-Al-Mo-Zr-Si alloys respec-tively were conducted by EPMAand also by EDAX.Theseanalyses showedthat the al]oying elements, viz. Vin the case of TiA1Valioy and Mo, Zr and Si in the

case of TiAlMoZrSi alloy were completely dissolvedin TiA]3 compoundsgiving rise to their compositions cor-responding to Ti-62.8Al-1.6V and Ti-62.8A11.4Mo-0.7Zr-0.1Si (all compositions in wto/o). This agreeswith the studies of Abdel-Hamid25)on crystallization ofalumlnide compoundsfrom Al-TI melts containing someadditlons of Mo, V, and Zr. In view of efforts beingcurrently made In developing ductile TiA13 basecompoundsby ailoying them wlth transition metals ofGroups IV-V (Zr, Nb, V etc.),12) these results showthat their formation can be attempted by carrying out thereaction synthesis between A1 and Ti alloys of suitable

compositions.Whl]e not enough reliable thermodynamic data is

currently available on different Ti-AI intermetallic

Table 3. Effect of reaction temperature on reaction layer

thickness In couples prepared from pure titanium

and reacted for 3.6 ks.

Reaction Reaction

temperature layer thickness

(K) (mm)

973

l 073

1173

l 373

0.302

l .088

l.810

l .900

phases such as Tl5Alll, TiA12 and Ti9A123, the

thermodynamic data on Ti3Al, TiAl and TiA13 hasrecently been published by Kattner et al.26) Accordingto Kattner et al., the free energy of formation of TiA13has been found to be the lowest amongTi3Al, TiAl andTiA13. Therefore, the formation of TiA13 at reaction

temperatures 1373K in the present as well as earlier

investigations20 - 23) can be explained.Figure 4 shows the SEMview of the reaction inter-

face in pure Ti-AI reaction couple reacted at I 073K for

3.6ks. As shownin Figs. 4(a) and 4(b), X-ray mappingfor Al on Ti region adjacent to the Ti/AI reaction inter-

face showedits insignificant/negligible enrichment withAl. Similarly, the Al matrix surrounding aluminide par-ticles was found to contain very little Ti (not exceedjng

2ato/, at any location) indicating that no significant

enrichment of Al matrix with Ti occurred during thereaction. Similar observations were made on othercouples reacted under other conditions. Also, as shownin Fig. 4(a), a thin and more or less continuoussingle-phase layer of TiA13 was found to be presentadjacent to the Ti/AI interface in all the cases.

Noenrichment of Ti with A1 near the reaction inter-

face suggests that under reaction synthesis conditionsstudied in the present investigation, the diffusion of A1in Ti does not play any role in aluminide formation.Under these conditions the formation of the reactionproduct at temperatures 1273K can be visualized tobe occurring by the following chemical reaction;

Ti(s)+3Al(1)-TiA13 AH298=-8.75kcallmol

and owing to the exothermic nature of the chemicalreaction28) a considerable amount of exothermic heat

C::-

d)LC,

;h

O1

CQ,

,:

H

co(C)

=(,,

rv)

e'

H~~:\o

(oo (N_cl' ~(,)S'~;_ llH

l(b)

-or~o-fl ~D~:

(a) ~c\-or~lfr'

~

o ~$1flr~t

h,.,r~'~;~f-a)~

- o(1) o~ F f~'

l rH

c')

.1;

ocl]

H .-~

cl'

o~:

oQ,,1 't)

(v) o,N o

(v'

~i ~a)a:'

h,-*fl(v'~oo- ~orh' 't)o::c~' Nr~'

'~';: ,v'~ ~~ ,g)

.- H~~la:'

oo'Ir)

o ~loNrl

r)o

c~

o

o

na

o

(C'

OOfr,

~f

~ONN

o

o

H

o

o ro -o'~ oc:o ~: o Hr'N

:~ -~1-

.g) ao 2;co

~~ Coo a)::o r~'h- 'r'~fl ~JI JP~, (~t a)

:: a)h ~1l _

ooo F-c~' ~o~~f o ~ or)

~ oNo~ o ;:

l l~

oo o~

~oo oo

~l

85 80 70 65 50 45 40 3560 55752e (degree)

Flg. 3. X-ray diffraction patterns from reaction layers of couples reaction synthesized for 3.6ks at (a) 973K,(b) I 173K and (c) 1373 K.

@1996 ISIJ 258

ISIJ International, Vol.

can be generated at the reaction Interface.

Formation of Ti Al besides that of TiA13, at the9 23,reaction temperature of 1373Kcan be explained in termsof the higher exothermic heat generated at higher reaction

temperatures. It has beenshownthat as the temperatureof reaction increases the exothermic heat liberated maybe sufficient to melt the freshly formed reaction productat the reaction temperature of 1373 K.27) Themechanism

Fig. 4.

l~~~-f.i~,=~ :19u~~

:

SEMmicrographs of pure Ti-AI couple reacted at

l 073K for 3,6 ks showing (a) Reaction Layer (RL)/Tiinterface and (b) X-ray dot mappingof Al on Fig. 4(a).

Notice that almost no diffusion of Al occurs in solid

titanium,

~RLThickness ~T= T1

t = tl

~1)

aJ:h oo er)

J~

~ Oc>~(~~~)~)e~ O0~~~O~O~~~)~

C> b'~~2()" /

~~)

~i:

~:'

~J

(o)

Reaction Interfoce

~ ,=,, ~~8:'~O~~~)o~~o ~'* *=* C~"'~~~~a~0~P'~)OS~f

~ • ,~'OG Lj'~~/ (*)

J

36 (1996), No. 3

of formation of Ti9A123 has been discussed in detailselsewhere.27) Essentially it forms due to the incipient

melting of freshly formed TiA13. Themolten TiA13 wetssolid titanium and forms Ti5All I and TiA12 intermetallic

compoundsas intermediate products. The formation ofTi9A123 can be explained in terms of the formation ofthese two compounds.

3.2. Microstructures of the Reaction ProductIn all cases of pure Ti-AI couples, the reaction layer

(RL) comprlsed a dispersion of near]y equiaxed particles

of titanium aluminide in an Al rich matrix. Schematicdiagrams of microstructures of reaction layers formedin pure Ti-AI reaction couples at different reaction

temperatures (T1' T2, and T3) and times (tl and t2),

where T3> T2> Ti and tl > t2, have been shownin Fig.5. Optical microstructures of mid-regions of reactionlayers formed at reaction temperatures of 973, I 073,

l 173 and 1373K respectively and reacted for 3.6ks in

pure Ti/AI system have been shown in Fig. 6. It wasobserved that an increase in the reaction temperaturefrom 973 to I 173K Iead to the formation of particles

having a bi-modal distribution [Figs. 6(b) and 6(c)].

Further, increase in the reaction temperature beyond1073K showeda marked increase in the particles size

[Figs. 6(c) and 6(d)]. Moreover, a large number ofparticles formed at reaction temperatures of 1173Kcontained several micro-cracks which were propagatedin them, as discussed later, during the course of their

formation.Optical mlcrographs of mid-regions of reactlon lay-

ers formed in Ti alloy-AI couples reacted at I073K for7.2 ks have beenshownin Fig. 7. In contrast to the mor-phology of unalloyed TiA13 formed in the case of pureTi-AI couples, the morphology of the TiA13 based com-pounds containing V or Mo. Zr and Si was found tobe significantly different. Instead of being equiaxed, theparticles of TiA13 based compoundsin both cases had acontinuously changing morphology across the width ofthe reaction layer. As shownin Figs. 7(a) and 7(b), theparticles awayfrom the Ti/AI reaction interface were, in

h-RLThi*k~*== --H+=T OO o'

=': ,O~aCj6\'~!~"

-

'O,(~.,;:o~);~~;.O'

'; 'O4~(')

*~ D"OOo,C~:(},~)Reoction Interfoce

T=T'OCO/n~i;(Dr(~i~C]~~~

~.i ~fi

t=t

"

(~_~~;~,

O~C)_+ (oL):-~)_ ~~:S~j~\~;~~;if

;~e):~)'~(d

)

~ (:O

~-'=~0O;O

h•-- RL Thickness =--H H--RLThickness --HFig. 5. Aschematic diagram of the microstructural evolution in the reaction layer at different reaction temperatures

Tl' T2 and T3 and time tl and t2 (TI T2 T3 and tl

259 O1996 ISIJ

ISIJ International, Vol. 36 (1996), No. 3

F-H50~~

Fig. 6. Optical microstructures of reaction layers formed at

v'arious temperatures in pure Ti/AI system synthesizedfor 3.6 ks at (a) 973 K, (b) I073 K, (c) I 173Kand (d)

1373 K.

general not equiaxed, had sharp edgesandmanyof themhad severe internal micro-cracks similar to those foundIn particles of TiA13 formed by reaction synthesis at

1173 Kin pure Ti/AI couples. Onthe other hand, I.6 wto/o

V containing TiA13 particles present near the reactioninterface [Fig. 7(c)] possessed round corners andsmoother surfaces and had moreequiaxed morphology.Also, the extent of cracking In them was found to belesser than in those which were awayfrom the reactionInterface. In contrast to the l.6wto/o V containingparticles, the extent of such changes in the morphologyof 1.4wto/oMoO.7wto/oZr-0,lwtoloSi containing TiA13particles was found to be less [Fig. 7(d)].

Thoughthe results of the quantitative metallographicanalysis are not available at present, other generalobservations on microstructures of reaction layers

formed in dlfferent reaction couples were (i) the volumefraction of aluminide particles in the structure of thereactlon layer varied with distance from the reactioninterface going through a maximum,(li) the distance

at which the maxlmumpopulation of the aluminidepartlc]es occurred was found tQ be characterlstlc ofspecific reaction conditions (temperature as well as time)

and whether or not the alioying elements were presentin Ti, (iii) the degree of particfe surface smoothnessdecreasedand the extent of micro-cracking increased withthe distance from the reaction interface and (iv) the size

O1996 ISIJ 260

Fig. 7. Optical microstructures ofTiA13 based alloys synthe-si7~ed at I073K for 7.2 ks; (a, c) TiA13 alloy containingl.6wt"/. V, (b,d) TiA13 alloy containing 1.4wt'/. Mo,0.7wt"/* Zr and 0,1 wt'/. Si; (a, b) segment of thereaction layer awayfrom the reaction interface, (c, d)

segment of the reaction layer near the reactioninterface.

of the equiaxed particles increased with the distance fromthe reactlon interface. These general micrQstructuralfeatures of the reaction layer can be understood in termsof the physical phenomenawhlch occur during reactionbetween solid titanium and liquid aluminium and havebeen briefly described below.

As shownby our restrlts, the non-enrichment of solid

titanlum with aluminium 'at the reaction interface impliesthat no dlffusion of Al in solid Ti occurs prior to theformation of TiA13 and the formation of the reactionproduct can be attrlbuted entirely to the chemicalreaction between so]id Ti and liquid A1. Oncethe thin

layer of titanium alumlnide forms by the chemical reac-tion, the direct contact between Ti and Al is stopped.

Underthese conditions further growth of thin TIA13 Iayerrequires diffusion of either Ti towards Al or that of Altowards Ti through the lattice of the compoundiayer.

It has been reported that the diffusivity of Al in TiA13is considerably higher than that of Till) and, therefore,it can be assumedthat the further growth of TiA13 Iayer

occurs predominantly by the diffusion of A] towards Tirather than that of Ti towards Al. Since the growth oftitanium aluminide layer is controlled by the diffusion

of Al towards the reaction interface, its thickness after

IS]J International, Vol.

a given reaction time increases with increasing reaction

temperature.Densities of Ti and TiA13, however, differ significantly

and thus the TiA13 Iayer formed and grown over solid

titanium tries to expand. Thus, the aluminide layer getssubjected to acomplexstate of stress, the degree of whichincreases with its thickness and the rate of its growth.However, since TiA13 has only a limited ductility,27,30)

after reaching a critical thickness its layer can undergocracking and fissuring under the state of stresses

developed due to its tendency to expand, Under suchconditions the layer fragments, gets detached from thesolid titanium and, in turn, exposes fresh solid surface

over which the chemical reaction betweensolid titaniumand liquid aluminium can start again.

Since the chemical reaction leading to the formationof TiA13 is exothermic in nature, the heat released atthe reaction interface increases the temperature of thereaction interface, the reaction product as well as theliquid aluminium in contact with the reaction product.Since the temperature of the liquid aluminium awayfromthe reaction interface is maintained at the set reaction

temperature and is lower than the reaction interface

temperature a thermal gradient is expected to be es-tablished ahead of the reaction interface. It can beshownthat by treating this situation as that of a naturalconvection driven flow in a liquid which is in contactwith a hotter plate, thermal currents will be establishedin liquid Al which is immediately ahead of the reactioninterface.31) As a result, an unsteady-state heat transfer

takes place from the reaction interface to the liquid

alurninium and, as shownin Fig. 8, circulating loops ofmetal flow are established with the fiow of liquid

aluminium being in the upwarddirection in the vicinity

of the reaction interface.32) Underthese thermal currentsthe reaction product, which is in the form of separatedand fragmented TiA13 Iayer, is transported away fromthe reaction zone. Since someof the separated particles

maystill contain cracks and fissures, such big particles

are, therefore, generally found to be present not in thevicinity of the reaction interface but awayfrom it [Fig.

6]. However, during their transportation by thermalcurrents they mayfurther get fragmented into smallerparticles.

Since now onwards the system consists of titaniumaluminide particles immersed in liquid aluminium, its

behaviour is similar to that undergoing persistent

liquid-phase sintering (PLPS), which consists of stagesof (1) dissolution, (2) reprecipitation and (3) Ostwaldripening. 33) In the present system the dissolution becomesselective at micro-cracks, sharp edges and corners. Thedissolution of material at micro-cracks becomes re-

sponsible for their further fragmentation while that atsharp edgesandcorners causes particle smootheningandrounding. In this mannerthe presence of micro-cracksin several particles towards the Al end of the reactionlayer and their non-equiaxed morphology constitute theproof of their drifting awayfrom the reaction interface[Figs. 7(a) and 7(b)]. In view of the similarity of the

present system with that undergoing PLPSthe bimodalparticle distribution, as shown in Fig. 6(b), can be

261

36 (1996), No. 3

(a)

:,

O

EQ,

h

Thln TiA[3

Tlt: 1523 - 1623 X

(b) [Rawers et a[]

Ti orTi Alloy

__~_~=~

Fig. 8. Aschematic representation of(a) thermal gradient and(b) thermal currents set up in front of the reactioninterface during reaction synthesis of titanium,aluminide.

thought to be arising due to the phenomenonof re-precipitatiori, Mathematical analysis of the convectiveheat andmasstransfer in a liquid having thermal currentssimilar to those established in the present system showsthat the liquid aluminium velocity component in the

upward direction, i.e, parallel to the reaction interface,

varies with distance from the reaction interface andgoes through a maximum.Theposition of the maximais

mainly influenced by (i) the thermal gradients establishedin the liquid metal and (ii) the viscosi,ty of the fluid. Inthe present case it will also be infiuenced by the volumefraction of the particles dispersed and their shape andsize distribution which, in turn, are functions of thereaction temperature, reaction time andalloying elementspresent because the fragmentation, dissolution andreprecipitation behaviour of the aluminide formed will

also be influenced its chemistry. The general trend offinding the volume fraction of aluminide particles goingthrough a maximacan be understood in terms of theseconslderations.32)

The mechanismsoperating during reactlon synthesis

betweenTi alloys and liquid Al remain same. Hol;vever,

as discussed above due to the presence of the alloying

elements in the reaction product formed, the degree ofcracking, dissolution, reprecipitation and particle coars-ening becomedifferent.

4. Summaryand Conclusions

Formation of titanium aluminide by reaction synthesis

betweenliquid aluminium andsolid titanium is associated

with a complex set of physical phenomena.Results ofthe present investigation showthat

@1996 ISIJ

ISIJ International. Vol. 36 (1996), No. 3

(1) Between 973 and 1373K as the reaction tem-peratures, the synthesis of titanium aluminide occursby the exothermic chemlcal reaction. Onthe other hand,the diffusion plays its role during the growth of thereaction product.

(2) As long as an excess amountofliquid aluminiumis present, while TiA13 is the only phase which forms upto the reaction temperature of 1273 K, TiA13 andTi9A]23form whenthe reaction is carried out at the temperaturesof 1373 K. Alloying elements whenpresent in solld Ti,

as in the TiAI-Mo-Zr-Si and Ti-A1-V alloys re-spectively, get dissolved in the reaction product.

(3) Microstructures of partially reacted couples showthat the reaction layer, in general, consists of fine

aluminide particles dispersed in a matrix of unreactedAl. Whi]e at lower reaction temperatures of 973 and1073K the reaction layer consists of fine monosizedorbimodal particles, at higher reaction temperatures it

conslsts of coarse particles which generally possess cracksand fissures.

(4) The distribution of titanium aluminide particles

in the reaction layer is not uniform across its thicknessand the volume fraction of particles in all the cases goesthrough a maximumwhich occurs at a specific distance

from the reaction interface. This specific distance is afunction of the reaction conditions as well as the presenceof the alloying elements in Ti alloys.

(5) Particles formed in couples madefrom Ti alloys

show similar microstructural features. However, they

possess, in general, morenumberof cracks and fissures.

A Iarger numberof them has also been found to havesharper edges.

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5)

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