Phase equilibria and microstructural control of gamma TiAl based alloys

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Intermetallics 6 (1998) 643-646 1998 Elsevier Science Limited

Printed in Great Britain. All rights reserved O966-9195/98/%see front matter

Phase equilibria and microstructural control of gamma TiAl based alloys M. Takeyama, * Y. Ohmura,t Makoto Kikuchi & T. Matsuo Department of Metalhugical Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan

(Received 3 April 1998; accepted 30 April 1998)

Phase equilibria among the B (bee or B2), (Y(hcp) and y (Llc) phases in Ti-Al-M ternary systems at elevated temperatures have been studied, where the M is j3 stabilizing element for pure titanium. The /3(82) + Q + y three-phase coexisting region exists at temperatures above 1473 K, and it moves towards a direction of high aluminum and high M concentrations with increasing temperature. The change in the phase equilibria by the addition M is associated with the cxw j?~ allotropic transformation temperature of pure titanium and can be thermodynamically interpreted in terms of the lattice stability ratios of M to Ti. The change in the phase equilibria results in new reaction pathways peculiar to the ternary systems, thereby opening more possibility for microstructure control of the gamma based alloys. The vertical section drawn based on these studies demonstrates that the y phase can be in equilibrium with the /?(BZ) phase at relatively low temperatures in the ternary systems and create a reaction pathway of o + ,!J(BZ)+ y. Upon cooling along the same pathway of ar -+ y as in the binary alloy, the addition of third element affects the transformation rate, and the massive y structure is found to be formed even under slow cooling rate. 0 1998 Elsevier Science Limited. All rights reserved Key words: A. titanium

aluminides, based on TiAl, B. phase diagram, phase stability, phase transformation, D. microstructure.

enhance the formation of high-temperature bee /Ititanium phase and thereby improved the roomtemperature ductility as well as high-temperature deformability. 617Even though the /3 phase is found to exist together with (Y(CQ)and y phases in those alloys, attention has been paid mainly to the role of /3 phase in the mechanical properties but not to the formation mechanism of the microstructure as well as the phase equilibria among /I, (Yand y phases in the multi-component alloy systems. Therefore, the microstructural formation in the alloys under development has in most cases been interpreted based on the binary phase diagram, since the amount of the additive elements in the developing alloys is not so large. However, the Ti-Al binary system is a special case of multi-component systems, so that it is at least necessary to understand correctly the phase equilibria and phase transformations to occur in Ti-Al-M ternary systems. Until recently, however, the phase relationships among the p, a! and y phases in Ti-Al-M ternary systems at elevated temperatures have not been well established, and little attempt has systematically been made on the change in the phase

1 INTRODUCTION Alloys based on y-TiAl have been received many attentions because of their potential applications to high-temperature structural components.’ In the currently well accepted Ti-Al binary system,2-5 the ordered fct y-TiAl phase with the L10 structure is in equilibrium with either disordered hexagonal w Ti phase or its ordered version of a2-Ti3Al phase with DOi9 structure at temperatures above and below the eutectoid reaction temperature Tk (1398 K), respectively, but should never be in equilibrium with disordered bee B-Ti phase. The phase relationships among the /3, Q and y phases in the binary system are such that the Q + y phase transformation occurring upon cooling is important for microstructure control of the alloys based on y-TiAl. Most of the alloys under development contain several minor elements, and the additions of vanadium and chromium to the binary Ti(45-48) at% Al alloys have been reported to *To whom correspondence should be addressed. 5734 2874; E-mail: [email protected] +Now with the Sony Corporation.

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equilibria among these phases with temperature. In this paper, we focus on the phase equilibria in TiAl-Nb ternary system and summarize the effect of the third element on phase equilibria in terms of thermodynamic consideration. Based on these phase equilibria in the ternary systems, we discuss some of the novel methods for microstructure control of the gamma alloys.

2 PHASE EQUILIBRIA IN Ti-AI-M TERNARY SYSTEMS

In case of the binary system, the y-TiAl phase can only be in equilibrium with a! phase at temperatures above T&. However, the y-TiAl phase can be in equilibrium with B(B2) phase when a certain amount of the third elements of M, which is the strong ,9 stabilizing element against a! for pure titanium, is added to the binary alloys. We have systematically studied the phase equilibria among j?, cx and y phases in several Ti-Al-M (V, Nb, Cr, MO) ternary systems at elevated temperatures and revealed that in any system the three-phase coexisting region exists at temperatures above 1473 K, higher than the (Yc, (~2 congruent temperature in the binary system Tt (1452 K), although the composition range at which the three-phase region appears depends on the kind of third element.8-10 We have also found that the region moves toward the direction of high aluminum and high M concentrations with increasing temperature.8-11 Figure 1 is the modified isothermal sections showing the three-phase coexisting regions at several temperatures in Ti-Al-Nb system, as a representative of Nb 30 50 A

30

40 50 Atomii: percent Al

1653 K 1605 K 1473 K

m -

60

Fig. 1. Isothermal section at 1473K and the change in the /?(BZ) + 01+ y three-phase tie triangle with temperature in TiAl-Nb ternary system.

et al. Table 1. Ratios of lattice stability between p/a and a/y phases of the third element M to that of Ti at 1473 K Element M v

Nb Cr MO

Lattice stability ratio ‘LyTi

6.58 11.82 3.87 10.08

A-Y M/T1

0.42 0.42 0.53 0.78

the ternary systems. The M is partitioned to B more than both cxand y in any system, as is seen in the tie lines between /3/a and B/r of the threephase region in Fig. 1. The M is prone to be partitioned to a? phase more than y phase as well, although niobium is partitioned evenly to both phases.‘O These facts indicate that the element M which stabilizes p phase against (Yphase also stabilizes the a! phase against y phase. The change in the phase equilibria among these phases in Ti-Al-M ternary systems is associated with the B + a transformation of pure titanium and can be interpreted quantitatively in terms of lattice stability ratio, A. The A for #? against a! in ternary Ti-Al-M systems, for example, is defined as follows: A&$ = A’G~“/A’G~~“, with two assumptions that (1) the y-TiAl has a disordered fee structure and (2) the extra energy term derived from the concentration of M in each phase can be ignored. The A”Gca refers to the difference in Gibbs free energy between bee (j?) structure and hcp (a) structure for pure M, and A”GtTn that for pure titanium. When the value of AtTU is larger than unity, the /3 phase field expands toward a direction of high Al concentration with the addition of M and vice versa. Table 1 summarizes the two values of A at 1473 K calculated using the thermodynamic database.12 The values of ALT;i for each element is nearly equal to 10 and at least one order of magnitude larger than those of AiyGi. These results indicate that the u phase field would not expand against y phase field with the addition of M, whereas the /l phase field greatly expands against the a! phase field. Then, the two two-phase fields of /? + a! and a! + y meet each other when a certain amount of M is added to binary Ti-Al alloys, resulting in the formation of the B + Q + y three-phase coexisting region. The reason the AS+ M/Ti has a large positive value for each element is that the value of A”GrU is fairly smaller than that of AOG&+ff,since the temperature to be interest is only 300K away from the += a! allotropic transformation temperature eiaB (1155 K) of pure Ti. When the temperature raises and the difference in

Phase equilibria and microstructural control of y TiAl based alloys

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temperature from the i’$f becomes large, the value of A”GB,-ff almost remains unchanged whereas that of A”GBTa increases, leading to a smaller s-2 This is responsible for the threevalue of A,,,,. phase coexisting region moving toward the direction of high aluminum and high M concentration as temperature increases.

3 MICROSTRUCTURE

CONTROL

The change in the phase equilibria by the addition of the third element creates new reaction pathways from the high-temperature CI phase upon cooling. This opens new possibility for microstructure control of the gamma alloys. Figure 2 shows two vertical sections at 1ONb and 48Al (at%) in the TiAl-Nb system, which are drawn based on Fig. 1 (all the compositions are hereafter given in at%). As shown in Fig. 2(a), the y phase becomes in equilibrium with p(B2) phase at a low temperature region whereas it is in equilibrium with cxphase at a high temperature region.9 Then, a new reaction pathway of cx-+ p(B2) + y, which are impossible to take place in binary alloys, can be available for the microstructure control when the alloy compositions are appropriately selected, i.e. Ti-44Al10Nb. The microstructure formed along this pathway is found to exhibit a lamellar structure consisting of B(B2) and y phases.‘0,t3 Addition of niobium to Ti48Al changes the reaction pathway from a! + CX~ + y to Q + y, as shown in Fig. 2(b). Along these pathways both Ti48Al and Ti--48Al-8Nb were heat treated in Q single phase region of 1693 K, followed by air cooling to room temperature, and the resulted microstructures are shown in Fig. 3. The binary alloy clearly shows the Widmanstatten-type lamellar structure as is reported previously,14,15 whereas the

Y p! 31600E i?.

a+y

E +1500-

; :

,400k

,

,

,

35

40

45

50

Al content (at%)

.:’

../

Y

, KG,, 550

2 Nb

I

I

I

,

4

6

8

10

content (at%)

Fig. 2. Vertical sections at (a) 10 at% Nb and (b) 48 at% Al in Ti-Al-Nb ternary system.

Fig. 3. Optical microstructures of (a) Ti-48Al and (b) Ti48AI-8Nb, heat treated at 1693K/1.8 ks, followed by air cooling to ambient temperature.

niobium containing alloy exhibits the massive y structure. These results imply that the addition of niobium would make the precipitation of the y plates in the Q matrix sluggish, thereby retaining the cxphase to lower temperature enough to cause the cr + y massive transformation to occur even under slow cooling rates. Since rapid quenching is needed to obtain the massive y structure in the binary alloy, lb17 the fact that the massive y structure can be obtained even under slower cooling rates by the addition of niobium suggests a possibility for the microstructure control using the quenching and annealing, just like a common heat treatment technique for mild steels. The direct quench/aging along these pathways is effective in controlling the microstructure. l8 Figure 4 shows the optical microstructures of the Ti--48Al8Nb directly quenched from the cz single phase region of 1693 K to the y single phase region of 1473 K and aged there for 10 and 300s followed by water quench. The specimen aged for 10 s exhibits the Widmanstatten-type lamellar structure, together with a feathery type structure (Fig. 4(a)). However, further aging changes the microstructure to complete equiaxed y grained structure with an average grain size of 20pm (Fig. 4(b)). Thus, the

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Research (07650811) and for Scientific Research on ‘Phase Transformations’ area the priority (09242209) from the Ministry of Education, Science and Culture, Japan. REFERENCES

Fig. 4. Optical microstructures of Ti-48Al-8Nb directly quenched from 1693 K/l .8 ks to 1473 K and aged there for (a) 10 s and (b) 300 s, followed by water quench.

Kim, Y. W., JOM, 1994,46(7), 30. McCullough, C., Valencia, J. J., Levi, C. G. and Mehrabian, R., Actu Met&, 1989,37, 1321. Hellwig, A., Inden, G. and Palm, M., Scripta Met&l. Mater., 1992, 27, 143. Perepezko, J. H. and Mishurda, J. C., in Titanium ‘92 Science and Technology, ed. F. H. Froes and I. Caplan. TMS, Warrendale, PA, 1993, p. 563. Petzow, G. and Effenberg, G., Ternary Alloys 4. VHC Verlagsgesellschaft, Stuttgart, 1993, p. 433. Shi, J. D., Pu, Z., Zhong, Z. and Zuo, D. Scripta Metall. Muter., 1992, 27, 1331. Masahashi, N., Mizuhara, Y., Matsuo, M., Hashimoto, K., Hanamura, T. and Fujii, H., in High Temperature Ordered Intermetallic Alloys 4. MRS Symp. Proc., Vol. 213, 1991, p. 795. 8. Nakamura, H., Takeyama, M., Yamabe, Y. and Kikuchi, M., Scripta Metall. Mater., 1993, 28, 997. 9. Nakamura, H., Takeyama, M., Wei, L., Yamabe, Y. and Kikuchi, M., in Intermetallic Compounds for High-Temperature Structural Applications. 3rd Japan Intl. SAMPE Symp. Proc., ed. M. Yamaguchi and H. Fukutomi, SAMPE, Japan, 1993, p. 1353. 10. Takeyama, M. and Kikuchi, M., Materia Japan, 35(10), 1058.

microstructure can widely and easily be controlled from fully lamellar structure to equiaxed structure by using the pathway of a! to y. It is interesting to note here that the feathery microstructure, which was reported by others in binary Ti-48Al alloy17-19 and has been believed to be associated with the massive transformation, is caused by coarsening of fine primary lamellae and has nothing to do with the massive transformation, since the massive start temperature (MS) of this alloy was identified to be around 1400 K, which is far below the aging temperature. The details on the microstructure evolution during direct quench/aging will soon be reported elsewhere. ACKNOWLEDGEMENTS

This research is supported by the research grant on ‘Research for the Future Program’ from Japan Society for the Promotion of Science (96R12301), and in parts by Grant-in-Aids for Scientific

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