NONFERROUS METALS AND ALLOYS

SCANDIUM-ALLOYED ALUMINUM ALLOYS

V. I. Elagin, V. V. Zakharov, and T. D. Rostova

DDC 669.715'793

Alloying with scandium has a strong and varied influence on the structure and properties of aluminum and aluminum alloys [1-12]. The character of influence of addition of Sc is close to that of widely used additions of such transition metals as Mn, Cr, and particularly Zr [13]. However, the action of Sc additions isstronger and appears more effective.

Scandium is a strong modifier of the as-cast grain structure and with addition of it to aluminum alloys in an amount exceeding a certain eriticai content the maximum possible refinement of the as-cast grains for the specific casting conditions occurs. Addition of scandium makes it possible to obtain continuously cast billets of aluminum alloys with a nondendritic temperature [8].

In comparison with other antirecrystallization additions scandium increases to the maximum degree the recrystallization temperature of worked aluminum alloy semifinished pro- ducts. With correct selection of the alloy composition, especially the Sc content, and also of the production parameters for production of semifinished products their recrystalliza- tion temperature becomes higher than the solidus temperature. During heating of such semi- finished products above the solidus temperature the aluminum matrix changes from the solid unrecrystallized state to the liquid without the recrystallization stage. As the result of the strong antirecrystallization action of Sc production of thin sheet (less than 1 mm thick) in cold rolling with high degrees of deformation (up to 85% total reduction) becomes possible. Such sheets have a completely unrecrystallized structure after heating for harden- ing or annealing.

The nature of the influence of Sc on the structure and properties of aluminum alloys has been discussed in sufficient detail in the literature [4-8]. The essence of it is that the AI3Sc phase formed in AI-Sc alloys has a crystalline lattice possessing practically complete dimensional and structural agreement with the crystalline lattice of AI. This feature provides the strongest modifying action of the primary AI3Sc particles in crystal- lization of the molten metal and ease in homogeneous origin of secondary particles in de- composition of the supersaturated solid solution of Sc in AI. The latter promotes the fact that secondary AI3Sc particles originate simultaneously in many areas of the aluminum matrix at once without passing through metastable stages and, what is most important, in very dis- persed form. The degree of dispersion of the particles also causes increased resistance of the alloys to recrystallization and significant strengthening of the aluminum matrix.

The basic practical value of addition of Sc to aluminum alloys is the possibility of an additional increase in the strength properties of wrought semifinished products as the result of preservation of the unrecrystallized structure, refinement of the subgrain struc- ture in the unrecrystallized matrix, and the direct hardening action of the dispersed particles containing Sc. An analysis of the data of [i] and of the results of our investi- gations showed that with addition of up to %0.4% Sc to aluminum and aluminum alloys each 0.1% Sc causes an average increase in o t of 50 N/mm 2. This significantly exceeds the strengthening action of all alloy element additions (Mg, Cu, Zn, Si, Mn, Cr, etc.) to com- mercial aluminum alloys known and used in practice.

The purpose of this work was determination of the basic metallurgical principles of alloying of aluminum alloys with scandium. The article summarizes the results of our many years' of investigations in this area and the literature data of other authors has also been used.

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. i, pp. 24-28, January, 1992.

0026-0673/92/0102-0037512.50 © 1992 Plenum Publishing Corporation 37

TABLE i

Scandium content, %

0 0,1 0,2 0,3 0,4 0,6

° t

90 lO0 180 240 270 300

I NI~ 2

o0.2

70 80

160 220 255 285

~,%

41,3 39,3 17,8 15,3. 16,0 14,8

t,°C ~00

500

ooo

300:

200, ~0

550

O50

550

2501

Fig. i.

b %.

z l

\ i \ ' I \

~0 z I03 10 ~ I05 fO6T, sec a

x i

i

¢0 102 ~03 lO* ~c, see

b

C-curves of the start of decom- position of the supersaturated solid solu- tions in 92 mm diam. billets of binary aluminum alloys drawn on the basis of a 5%

change in electrical conductivity (a~)

a: ,) 0.4% Sc; A) 0.6% Mn; ,) 0.3% Zr: b: o) 0.1% Sc; A) 0.2% So; a) 0.3% Sc; +) 0.~% Sc; x) 0.6% Sc.

Selection of the Limits of Scandium Content in Aluminum Alloys

One of the basic question related to alloying of aluminum alloys with scandium is its optimum content. The Sc content must be selected so that under conditions of crystalliza- tion corresponding to continuous casting of aluminum alloy ingots a large portion of the Sc is found in solid solution. In subsequent production heatings the solid solution contain- ing Sc decomposes with formation of secondary particles of AI3Sc of the optimum degree of dispersion providing a sharp increase in recrystallization temperature and strengthening of the alloy. The other, smaller portion of Sc must be precipitated in crystallization in the form of primary AI3Sc particles and modify the as-cast grain structure in the ingot or in the weld joint.

To solve this question let us consider the AI-Sc phase diagram. With aluminum scandium forms a eutetic-type diagram with limited solubility [4-6]. The maximum equilibrium solu- bility of Sc in AI is 0.35-0.40%. With cooling rates in solidification corresponding to continuous casting of ingots an anomalously supersaturated solid solution of Sc (up to 0.6%) in AI is formed. In connection with this the maximum or close to maximum hardening effect in wrought semifinished products obtained from continuously cast billets of binary aluminum alloys may be obtained with a content of about 0.6% Sc (Table i).

With an increase in Sc content from 0 to 0.6% the strength properties increase signi- ficantly. It should be noted that the increase in strength properties occurs with damping. In connection with this with a content of >0.6% Sc the increase in strength properties will probably be insignificant.

38

T i.p~ sec v, d , sec -I

fO 5 v~ lO-Z

r~.n / fO -J

I02 , ~ i / fO-°

\ 10 " " - ~ fO -5

OJ 0,2 0,3 0,~ 0,~ O,6So,°/o

The r e l a t i o n s h i p o f the i n - Fig. 2. cubation period ~i.p preceding de- composition of the solid solution and of the rate of decomposition v d at 450°C in binary AI-Sc alloys to scandium content.

H 100

80

60

4,0

fO

/

~0

/k

~--~--+~ ~,+

.x~X" " E - - " ~--x

fO 2 fO 3 I0 ~' f05 106 t', sec

Fig° 3. Change in microhardness of 92-mm-diameter ingots of binary AI-Sc alloys with different scandium contents during an isothermal hold at 300°C: ×) 0.1% Sc; +) 0.2% Sc; o) 0.3% Sc; o) 0.4% Sc; A) 0.6% Sc.

An increase in strength properties with an increase in Sc content all the way up to 0.6% is possible only with obsevation of certain conditions. It is necessary to strictly control the temperature-time parameters of homogenization and heating for working by pressure and heat treatment. This is necessary to provide the optimum degree of decomposition of the supersaturated solid solution of Sc in AI, which is unstable in comparison with the solid solutions of other transition metals in aluminum. As may be seen from Fig. la, the incuba- tion period preceding decomposition of the solid solution is three to four orders of magni- tude shorter in AI-Sc alloys than inAI-Mn and AI-Zr system alloys and the rate of decomposi- tion characterizing the reduction in solid solution content in aluminum is five to six orders of magnitude higher in AI-Sc alloys than in AI-Mn and AI-Zr alloys [7]. in this case the stability of the solid solution of Sc in A1 drops sharply with an increase in Sc content (Fig. ib). The incubation period preceding decomposition of the solid solution decreases and the rate of decomposition increases (Fig. 2).

The rate of coagulation of the decomposition of the solid solution of Sc in A1 (second- ary particles of AI3Sc) is also very high, several orders of magnitude higher than for the other transition metals. The rate of coagulation of secondary particles increases with an increase in original Sc content in the solid solution. It is possible to determine the rate of coagulation of the particles from the rate of softening of binary AI-Sc alloys during an isothermal hold at increased temperature. Figure 3 shows curves of the change

39

t, 96'

65o % 1 / A

350 " "---.........

25O 10 102 103 tO* 105 10 s ~, sec

F i g . 4- C - c u r v e s o f t h e s t a r t and f i n i s h o f d e - c o m p o s i t i o n o f t h e s o l i d s o l u t i o n i n 92 ram d i a m . b i l l e t s o f A 1 - 0 . 4 % Sc ( o , , ) and A 1 - 0 . 4 % S c - 0 . 1 5 % Ar (&, A) alloys drawn on the basis of the change in electrical conductivity: white symbols) start

of decomposition (A--I=5%], black symbols) finish , i p

I of decomposition (A--=6O%) p

H 100

50 fO 10 2

• c - ~ J

10 3 lO ~ /0 5 z', sec

Fig. 5. Relationship of the microhardness of billets of AI-0.4% Sc (D) and A1-0.4% Sc-0.15% Zr (o) alloys to annealing time at 350°C.

in microhardness of AI-Sc alloy ingots during isothermal annealing at 300°C. The character of change in microhardness of AI-Sc alloy ingots is determined by decomposition of the solid solution and the degree of dispersion of the decomposition products, secondary parti- cles of AI3Sc. The descending branches of the curves of change in microhardness are deter- mined by the rate of coagulation of the secondary particles of AI3Sc. The higher the Sc content, the faster the rate of softening and, consequently, the more intensely the process of coagulation of the particles occurs. After a long hold even at 300°C the advantage in hardness of alloys with a higher Sc content is partially lost.

Therefore in selection of the optimum Sc content it is necessary to take the following factors into consideration: i) in complex alloys the limiting solubility of Sc in A1 de- creases; 2) the solid solution of scandium in aluminum is unstable; 3) AI3Sc particles are prone towardcoagulation. In addition, it must be taken into consideration that under production conditions billets and semifinished products are subjected to long high-tempera- ture heatings, as the result of which practically complete decomposition of the supersatu- rated solid solution of Sc in A1 and significant coagulation of the decomposition products occur. In connection with the above for commercial aluminum alloys on 0.6% Sc content is not the optimum. The desirable Sc content limits in various aluminum alloys may be assumed to be 0.1-0.5%.

Alloying of Aluminum Alloys Containing Scandium with Zirconium. As accumulated ex- perience shows, Sc must be added to aluminum alloys together with Zr. The necessity of addition of Zr is related to the fact that it significantly (up to 50%) dissolves in AI3Sc phase [12, 14]. The Al3(Scl-xZrx) phase formed preserves all of the positive qualities of AI3Sc mentioned above and acquires new useful properties.

40

Fig. 6. Microstructure of AI-0.4% Sc (a) and AI-0.4% Sc-0.15% Zr (b) alloy billets after an isothermal hold at 450°C for 17.5 h. 22,000x

The same as particles of AIjSc phase, primary particles of Al3(Sc1_xZr x) are active centers of crystallization of grains of the aluminum solid solution and are capable of significantly refining the grain structure of aluminum alloys. In the presence of Zr a nondendritic structure is formed in continuously cast billets with a content of 0.2% Sc and more while in alloys without Zr a nondendritic structure is observed only with hyper- eutectic contents of Sc, that is, with more than 0.5% Sc.

During decomposition of the supersaturated solid solution of Sc and Zr in A1 formed in crystallization of the molten metal secondary particles of Al3(Sc~_xZr x) phase are pre- cipitated. These particles, as for particles of AI3Sc phase, are dispersed and cause a strong antirecrystallization and strengthening effect. However, together with this, in contrast to AI3Sc phase particles, Al3(Sc1_xZr x) intermetallide particles are significantly less prone to coagulation in high temperature heating and therefore the antirecrystalliza- tion and strengthening effect is preserved in formation of them to a greater degree.

Figure 4 shows C-curves of the start and finish of decomposition of the supersatu- rated solid solution in billets of AI-0.4% Sc and AI-0.4% Sc-0.15 Zr Alloys. Addition of Zr has little influence on the initial stages of the change in concentration of the aluminum solid solution but significantly accelerates this process subsequently. The curves of the change in microhardness of billets of these alloys during an isothermal hold at 350°C also indicates that softening of the alloy with addition of Zr occurs significantly more slowly (Fig. 5). This is related to the lower rate of coagulation of the Al3(Sc1_xZrx ) phase particles than of AI3Sc phase particles (Fig. 6). It should be noted that in billets of AI-0.4% Sc-0.15% Zr together with fine Al3(Scl_xZr x) phase particles coarser particles identified as AI3Sc were observed at the boundaries of the dendritic cells. Formation of them is apparently the result of enrichment of the billet dendritic cell boundaries with scandium and depletion of them of zirconium as the result of the different character of dendritic segregation of these elements during crystallization.

Therefore addition of Zr to aluminum alloys containing Sc makes it possible to use the very strong modifying action of Sc with lower contents of it (starting with about 0.2%) while without Zr the modifying effect of Sc appears only with hypereutectic contents (more than ~0.5%). In addition Zr makes it possible to increase the temperature of production heating and the time of it without decreasing the positive action of Sc.

Apparently Al3(Scl_xZrx) is an Al~Sc-base substitutional solid solution in which Sc is replaced by Zr, which is close to it in nature. In this case the type of lattice is preserved and the lattice parameter changes little. Zr may replace up to 50 wt.% of the Sc. Therefore Al3(Sc1_xZrx ) is a phase of variable composition and depending upon the quantity of Zr dissolved in it the properties of this phase change, particularly the thermal sta- bility of the particles of the phase and their tendency toward coagulation in high-tempera- ture heating. In this case, as experiments show, Al3(Sc1_xZrx ) phase particles with a maximum quantity of zirconium dissolved in them possess the maximum tendency toward coagu- lation. An analysis of the curves of the relationship of the microhardness of AI-Sc-Zr alloy ingots to the legnth of hold at 400°C shows (Fig. 7) that the optimum composition is the alloy with the same ratio of scandium to zirconium (AI-0.75% Sc-0.75% Zr). From the AI--Sc--Zr phase diagram[14] it has been established that this alloy is located in the

+ Al3(Sc1_xZrx) phase area at the boundary with the ~ + Al3(Scl_xZr x) + AI3Zr ternary phase area. In this case 50% of the Sc atoms in Al3(Scl_xZrx) phase must be replaced with Zr atoms.

41

H

5O

30 ! +

0 i i

" b - - .

"×..~ . + - - + - -

10 # ~05 "£,, sec

Fig. 7. Microhardness of AI-Sc-Zr system alloy billets with different ratios of the scandium and zirconium contents with the same total of them in relation to annealing time at 400°C: x) 1.5 Sc; o) 1.0% Sc + 0.5% Zr; 4) 0.75% Sc + 0.75% Zr; ,) 0.5% Sc + 1.0% Zr; A) 0.25% Sc + 1.25% Zr; +) 1.5% Zr.

H

70

55

60

55

50

#5

~0

50

Z5

0 10 I02 ~0 ~ fO ~" ~05 10 ~ "C, s e c

Fig. 8. Relationship of the microhardness of AI- 0.2% Sc alloy billets with additions of transition metals to hold time at 450°C: 4) without addi- tions; o) 0.25% Zr; ~ 0.15% Hf; ,) 0.08 Zr + 0.08% Hf; D) 0.05% Ti; [] 0.03% V; m) 0.025% Ti + 0.015% V; A) 0.15% Zr 0.03% Ti + 0.02% V; +) 0.015% Zr + 0.15% Co.

Consequently to preserve and even strengthen the positive qualities of'Sc the ratio between the Sc and Zr contents in commercial aluminum alloys must be i:i.

As experience shows, with a content of more than 0.10-0.15% Zr in high- and medium- alloy aluminum alloys coarse AIsSc primary intermetallides are formed, especially in cast- ing large billets. Taking this into consideration, the Sc content must be the same as of Zr, 0.10-0.15%. However, probably in this case not all of the potential possibilities of scandium as an alloying component will be used. Taking this into consideration the Sc content may be increased to 0.15-0.30%.

With decrease in Sc content from 0.4 to 0.2% its role as a strengthener decreases significantly (Fig. 8). In this case the Zr addition plays the role not only of a sta- bilizer (Fig. 6) but also of a strengthener. It more than doubles the strengthening effect as the result of increasing the supersaturation of the original solid solution and of the corresponding increase in the degree of dispersion of the products of its decomposition.

42

TABLE 2

Recrystallization temperature T r (°C) Silicon of sheet with a thickness of, content, %

I

0 0,2 0,3 0,4

3 2

610/620 610/620 580/620 570/590 420/600 450/575 400/540 400/450

61o/620 560/580 480/560 400/450

Explanation. The first figure is the tem- perature of the start of recrystallization and the second of the finish of recrystal- iization.

Consequently in alloying of alloys with comparatively small quantity of scandium (~0.2%) the role of Zr increases significantly. In this case alloying of aluminum alloys with only scandium without zirconium loses practical sense as the result of the small positive effect.

Alloyin~ of Aluminum Alloys Containing Scandium with the Transition

Metals Mn, Ti, Cr, V, and Hf

Manganese does not react with scandium and does not form intermetallic compounds with it. As in other industrial aluminum alloys, addition of Mn to aluminum alloys alloyed with Sc promotes an increase in strength properties and an improvement of the corrosion resistance of the alloys. For this purpose 0.2-0.5% Mn must be added to aluminum alloys containing Sc. Alloying with manganese in larger quantities is undesirable as the result of the reduction in solubility of Sc and formation of intermetallides.

In some cases the manganese content is specified such as if it is necessary to obtain high superplastic properties, thermal conductivity, etc.

Titanium, as for zirconium, is capable of dissolving in AI~Sc phase, replacing Sc atoms. However, in contrast to Zr the solubility of Ti in AI3Sc phase is significantly less. Like Zr Ti does not strengthen the modifying effect of Sc but decreases the critical Sc content starting with which its modifying action appears. Ti may be added to aluminum alloys alloyed with Sc as a complex modifier together with Zr in a quantity of 0.02-0.06%.

In addition, addition of Ti has a detremental effect on strengthening indices in de- composition of the supersaturated solid solution of Sc in AI, accelerating somewhat the softening processes (Fig. 8).

Chromium. Addition of Cr to aluminum alloys decreases somewhat the strengthening and antirecrystallization effects from addition of Sc. This was established in investigation of binary AI-Sc alloys and of AI-Zn-Mg-Sc system commercial alloys. In addition, addition of Cr to AI-Zn-Mg-Sc system alloys decreases their plasticity and fracture toughness~ strengthens their tendency toward exfoliation corrosion, and has a detremental effect on weldability.

Vanadium. A small addition of V to AI-Sc alloys decreases the strengthening effect in decomposition of the supersaturated solid solution (Fig. 8) and reduces the antire- crystallization effect related to allying with scandium. The presence of Zr neutralizes the negative action of V.

Hafnium. Addition of 0.15% Hf to AI-0.2% Sc alloy strengthens the strengthening effect in decomposition of the supersaturated solid solution (Fig. 8) and increases somewhat the recrystallization temperature of this alloy. Apparently Hf is a positive addition to aluminum alloys containing Sc but to obtain a significant effect it must be added in a larger quantity than was done in the experiments conducted.

43

Interaction of Scandium with the Primary Alloying Elements Used in Commercial

Aluminum Alloys

Of all the primary alloying elements normally used in commercial aluminum alloys (Mg, Zn, Cu, Sc, Li) Mg, Zn, and Li do not form chemical compounds with Sc. In connection with this it is desirable to add scandium primarily to aluminum alloys in which the primary alloy elements are Mg, Zn, and Li. The question of the desirability of addition of Sc to wrought aluminum alloys in which Cu and Si are used as primary alloy elements requires additional study.

With Sc copper forms W phase (AI, Cu, Sc) [15]. This phase is formed in crystallization of the molten metal and does not dissolve in subsequent heating. As lhe result of this SC and Cu in the composition of W phase do not participate in strengthening the alloy and thereby reduce the strength properties of the alloys. In addition W phase particles increase the volume share of excess phases in the structure of the alloys, as the result of which their plasticity, impact strength, and fracture toughness decrease. For example, an attempt to improve DI6 (Al-~u-Mg system) and type 1201 (AI-Cu system) alloys by addition to them of 0.4% Sc was not crowned with success. The strength properties, the relative elongation, and the fracture toughness of the alloys dropped as the result of formation of W phase particles.

However, as an analysis of the AI-Cu-Sc diagram shows [15], with contents of less than 1.5% Cu and 0.2% Sc W phase is not formed. In connection with this if in the alloy the Cu content does not exceed this level a positive effect may be expected from addition of Sc.

Silicon decreases the strengthening effect in decomposition of the supersaturated solid solution of Sc in A1 formed in crystallization of the molten metal and also sharply reduces the recrystallization temperature of aluminum alloy semifinished products contain- ing Sc (Table 2). This occurs for two reasons. First, with addition and an increase in the Si content there is a change in the character of decomposition of the solid solution from continuous to interrupted. The products of interrupted decomposition are coarser and the density of their distribution in a unit of volume of the alloy is significantly less. Second, Si forms with Sca chemical compound [16].

The Si content in aluminum alloys alloyed with Sc must not exceed 0.15%.

Iron. Fe impurity does not form chemical compounds with Sc, does not change the character of decomposition of the solid solution of Sc in AI, and therefore does not de- crease the positive action of Sc on the structure and properties of aluminum alloys. The Fe content in aluminum alloys alloyed with Sc must be limited guided by recommendations for commercial aluminum alloys without scandium.

Conclusion. The basic principles of alloying of aluminum alloys with scandium are:

it is desirable to add scandium to aluminum alloys in a quantity from 0.i to 0.3% together with zirconium (0.05-0.15%), which strengthens the positive influence of scandium on the structure and properties of alloys;

the greatest effect (that is, the positive influence on mechanical properties and other characteristics) from addition of scandium together with zirconium is observed for alloys not containing alloy elements combining scandium in insoluble phases, specifically the AI-Mg, AI-Zn-Mg, and AI-Mg-Li systems;

with a limited copper content alloying with scandium together with zirconium of AI- Zn-Mg-Cu and AI-Cu-Li system alloys is possible.

At present on the basis of or taking into considerationthe principles developed of alloying of aluminum alloys with scandium commercial aluminum alloys based on the AI-Mg- Sc-Sr (01570, 01571, 01523, 01505), AI-Zn-Mg-Sc-Zr (01970, 01975), and AI-Li--Mg-Sc-Zr (10421, 01423) systems have been developed.

1. 2.

LITERATURE CITED

Patent 3771669 USA (1971). M. E. Drits, N. I. Turkina, E. S. Kanader, et al., Rare-Earth Metals in Nonferrous Alloys [in Russian], Nauka, Moscow (1975).

44

o

4.

5. 6.

7.

8. 9

i0

ii

12

13

14

15

16

M. E. Drits, L. S. Toropova, and Yu. G. Bykov, Metalloved. Term. Obrab. Met., No. i0, 35-37 (1980). M. E. Drits, S. G. Pavlenko, Yu. G. Bykov, et al., Doki. Akad. Nauk SSSR, Met., 25__~7, No. 2, 353-356 (1981).

V. M. Panovko and Yu. G. Bykov, Fiz.-Khim. Obrab. Mater., No. 2, 76-79 (1982). M. E. Drits, L. B. Ber, Yu. G. Bykov, et al., Fiz. Met. Metalloved., 57, No. 6, 1172- 1179 (1984). V. I. Dobatkin, M. E. Drits, L. S. Toropova, et al., The Metallurgy of Light Alloys [in Russian], Nauka, Moscow (1983), pp. 214-219. V. I. Elagin, V. V. Zakharov, and T. D. Rostova, Tsvet. Met., No, 12, 96-99 (1982). V. I. Elagin, V. V. Zakharov, and T. D. Rostova, Metalloved. Term. Obrab. Met., No. 7, 57-60 (1983). V. I. Elagin, V. V. Zakharov, T. D. Rostova, et al., Izv. Akad. Nauk SSSR, Met., No. 4, 180-183 (1983). V. I. Elagin, V. V. Zakharov, and T. D. Rostova, Tekhnol. Leg. Splav., No. 4, 5-11 (1984). V. I. Elagin, V. V. Zakharov, S. G. Pavlenko, and T. D. Rostova, Fiz. Met. Metalloved., 60, No. i, 97-i00 (1985). V. I. Elagin, Alloying of Wrought Aluminum Alloys with Transition metals [in Russian], Metallurgiya, Moscow (1975). A. N. Kamardinkin, T. V. Dobatkina, and T. D. Rostova, Izv. Akad. Nauk SSSR, Met., No. 2, 214-216 (1991). M. L. Kharakterova and T. V. Dobatkina, Izv. Akad. Nauk SSSR, Met., No. 6, 180-182 (1988). L. S. Toropova, T. V. Dobatkina, and M. L. Kharakterova, The Metallurgy of Light Alloys. Form Materials of Papers of the Seminar in Memory of S. M. Voronov [in Russian], Vsesoyuz. Inst. Leg. Splay., Moscow (1985), pp. 54-58.

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