Effect of temperature on tensile properties of TiAl base alloys

Scripta Metallurgica n Matcrialia, Vol. 32, No. 3, pp. 417-422,1995 Copyright 0 1994 ElsevierScienceLtd Rinted in the USA. All rights reserved 0956-716X/% $9.50 t .OO

EFFECT OF TEMPERATURE ON TENSILE PROPERTIES TiAl BASE ALLOYS

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K.Hashimoto, S.Kajiwara, T.Kikuchi and M.Nakamura National Research Institute for Metals 1-2-1 SENGEN, TSUKUBA-SHI IBARAKI 305, Japan (Received July $1994) (Revised September 9,1994)

Introduction 7 -TiAl(Ll ,,,fct) base alloys have been of much interest in recent years as lightweight structural materials for high temperature aero-space applications( 1). A major research effort has been directed towards improving room temperature ductility with the addition of alloying elements and/or microstructure control. Extensive studies have shown encouraging results; for example, the addition of V(2), Mn(3), Cr(4), MO(~), and Ni(6) is found to improve ductility of TiAl base alloys. In addition to the improvement of room temperature ductility, the increase of high temperature strength is also required (7). Generally, the ductility of the materials decreases with increasing strength. Then the well-balanced improvement in room temperature ductility and high temperature strength is required for TiAl base alloys. Lipsitt et al. reported that the elongation of a bii TiAl base alloy did not change in the temperature range of room temperature (RT)-870 K (8). However, Blackbum et al. reported in their patent that the elongation of a vanadium containing TiAl ternary alloy increased clearly with increasing temperature from RT to 470 K (9). Recently in the temperature range of 473-673K, the elongation increase was reported by Wunderlich et al. but they did not find clear evidence of the elongation increase at the temperature range of RT-47OK(lO). If the increase of elongation found by Blackbum et al. is confirmed in other studies of TiAl base alloys with or without other ternary elements, and if the mechanism of the elongation increase is elucidated, information on the well-balanced improvement for room temperature ductility and high temperature strength will be available. In this work, the temperature and microstructure dependence of tensile properties has been examined in the temperature range from RT to 770 K for binary and manganese containing ternary TiAl base alloys, and the elongation has been found to increase with increasing temperature from RT to 470 K in the alloys heat-treated at 1550 K. Some other results of temperature effects on the tensile properties are also presented. Experimental Procedure Ingots of Ti-5lat%Al and Ti-48Sat%Al-lat%Mn were prepared by vaccum arc remelting in Kobe Steel LTD. The alloys were homogenized for 24 h at 1470 K, HIP’ed for 2 h at 1470 K and at ISOMPa, and isothermally die-forged at 1420 K by a total reduction of 80%. Tensile specimens with a gauge section of 0.4x4x15 mm were machined from the forged ingots by spark erosion and ground. The binary and ternary alloy specimens were electropolished after annealing at 1470-1550 K for 2 h in vaccum. Tensile tests were performed at strain rates of 3x10-3 and 5x10-4 s-1 in argon with 4 nine purity and in the temperature range from room

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temperature to 770 K. Three specimens were used for one test condition, and an average value was adopted for strength and ductility. Microstructures and fracture surfaces of tensile specimens were observed using a scanning electron microscope. The microstructures of the binary and ternary alloys deformed up to 3% in tension were examined by transmission electron microscopy(TEM). Results Figure 1 shows the microstmctures of the alloys heat treated for 2 h at 1470 K and 1550 K. The alloys had a two phase structure with a grain size of 50 to 100 n m. The bright small particles (volume fraction: 2 to 4 %) of the T&Al phase exhibit a stringlike appearance(Fig.l(a),(c)). Bright needle or plate-like T&Al phase (volume fraction: 15 to 20 %) precipitates along TiAl grain boundaries(Fig. l(b)) an d/ or in grains interiors (Fig. l(d)). Figure 2 shows the tensile stress-strain curve of the ternary alloy tested at 670 K at a strain rate of 3x10-s s-1. At temperatures above 670 K, serrated flow is observed in the ternary alloy heat-treated at 1470 K or 1550 K. The serration was also observed above 570 K in the binary alloy, irrespective of the heat-treatments, Figure 3 shows the tensile properties versus testing temperature relationships. The elongation in the ternary alloy annealed at 1550 K increases markedly at 470 K, but the increase of elongation in the binary alloy with the same heat-treatment is observed only above 570 K(Fig.3(a)). In both alloys annealed at 1470 K an increase in elongation with temperature is observed at temperatures below 470 K, and becomes smaller above 570 K. The 0.2% offset stress decreases with increasing temperature, irrespective of the alloys and heat treatments (Fig.3@)). The temperature dependence of ultimate tensile strength(UTS) is slightly different from that of the 0.2% offset stress(Fig,3(c)); that is, no change of UT’S with temperature is observed except for the ternary alloy heat treated at 1550 K. UTS of the ternary alloy heat-treated at 1550 K increases with increasing temperature. The ternary alloy was also tested at a strain rate of 5x10-4 s-1 in the temperature range from room temperature to 470 K. No difference in elongation and the 0.2% offset stress between the lower and the higher strain rates was observed. Figure 4 compares the fracture surfaces of the specimens tensile tested at room temperature and at 570 K. The specimens were annealed at 1470 K. The intergranular fracture area in the specimen tested at 570 K is much larger than in that tested at room temperature, irrespective of the alloys. A similar trend was also observed for the specimens annealed at 1550 K. The intergranular fracture area increases with the increase of elongation owing to the rise in test temperature. Figure 5 shows the deformation twins in the binary specimens deformed to 1% plastic strain at room temperature. In the vicinity of twin intersections many a/2<110](001) dislocations were observed. Such dislocation emission due to a twin intersection has been proposed recently by Wardle etal. (11). The number of the dislocations in the specimen deformed at room temperature is greater than that in the specimen deformed at 570 K. This is a characteristic feature of the 1% strained specimens at room temperature. The other prominent feature in TEM observation on the deformed structure is that the density of twins in the specimen deformed to fracture at 570 K is extremely higher than that in the specimen fractured at room temperature, irrespective of the alloys and heat treatments. Discussion The room temperature elongation of both the binary and ternary alloys annealed at 1550 K for 2 h is higher than that of the alloys annealed at 1470 K for 2 h. That is, the room temperature elongation of those alloys clearly depends on their microstructures (Fig, 1). A prominent difference betweeen the microstructures is in the grain size of the TiAl phase ( 7 ) and the amount of the Ti,Al phase( a J. In a previous work (12) the Hall-Petch relation between the 0.2% offset stress and grain size in a TiAl base alloy held true in a grain size range of 20500 ,u m. But a relation between the elongation and the grain size was not recognized in this grain size range. That is, the elongation of the smaller grained specimen is not necessarily higher than that of the larger grained

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specimen. The grain size of the specimens used in this study are also within this grain size range. Therefore, it may be considered that the grain size effect on the room temperature elongation is small. The specimens showing a higher room temperature elongation contain a large amount of the a 2 phase. In the ternary and binary alloys heat-treated at 1550 K most of the 012 phase is plateliie in the 7 phase grains and cuboidal at the 7 grain boundaries, respectively. Vasudevan etal. proposed that the a 2 phase scavanges interstitial elements from the 7 phase, and increases the mobility of twinning dislocations. Hence the ductility enhancement may be brought by the increase of the a 2 phase(13). However, in this study, the serrated flow which could be caused by interstitial impurities or additive element (14) appears at the same temperature for both the specimens annealed at 1550 K and 1470 K. This fact suggests that the a 2 phase in the specimens annealed at 1550 K may not be effective for impurity absorption, Thus it is considered that the the a 2 phase as an impurity getter in the the 7 matrix is not so effective in promoting ductility. Recently, Wunderlich etal. studied the a 2/ 7 interphase boundaries using TEM, and concluded that 1/2<1 lo] screw dislocations on the interphase boundaries are initiated to move during plastic deformation and contribute to the plasticity (15). The a 2 phase plates in the specimens annealed at 1550 K may be coherent with the 7 matrix, becuase the a 2 plates with a Widmanstatten type structure precipitate with a { 111) habit plane in the 7 matrix. The cuboidal Q 2 phase in the binary ahoy annealed at 1550 K may be also coherent with the matrix. Thus the interfacial dislocations may play a significant role in the improvement of ductility. On the other hand, the change of elongation with increasing temperature also depends on both the microstructure and ahoy system. That is, the elongation increases clearly with test temperature from room temperature to 370 K in the alloys annealed at 1470 K for 2 h, while it slightly increases above 470 K in the alloys annealed at 1550 K. The distinctive increase of elongation at the vicinity of 370 K was reported in a vanadium containing TiAl base ahoy (9), and also in Ni,Al alloys(l6). In the latter alloys, an increase in elongation with temperature from room temperature to 570 K was explained by a suppression of an environmental effect with increasing temperature (17). As the room temperature elongation of the 7 single phase alloys was more sensitive to an environment than that of TiAl base alloys with a large amount of the a 2 grain (18),(19), and as the amount of the a 2 phase in the alloys annealed at 1470 K was significantly smaller than that in the alloys annealed at 1550 K, the elongation increase at the vicinity of 370 K may be mainly attributed to degradation in environmental enbrittlement with increasing temperature. In other words, the higher room temperature elongation of the specimens annealed at 1550 K may be due to lacking of the environmental enbrittlement in addition to the nucliation of interfacial dislocations. In the temperature range from 570 K to 770 K, the elongation increase of the ternary specimen annealed at 1550 K is particularly higher than that of the other specimens. The temperature dependence of the elongation cannot be explained only by the environmental effect. As the plate-like a 2 phase was observed much more in the specimens , the improvement of elongation may be caused by the thermal excitation of interfacial dislocations. The density of deformation twins and intersections between twins in the specimen tested at 570 K was much higher than that of the specimen tested at room temperature. But the density of a/2<1 lO](OOl) dislocations shown in Fig.5 is rather higher in the specimen deformed at room temperature, which indicates, that the critical stress for the generation of a/2<110](001) dislocations is reached at room temperature in this ahoy, and the fiction force acting on the a/2~110](001)dislocation at room temperature is higher than that at 570 K. This fact suggests that the friction force acting on the twinning partial dislocation may be reduced with increasing temperature(20) and/or the glide of a/2<1 lO](lOO) dislocations emitted from the intersections between twins may be facilitated with increasing temperature, for the cube glide with a high lattice fiction is generally observed at a high temperatures (11). Thus the propagation of twins might become easier and the amount of the deformation twins might increase with increasing temperature.

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The intergranular fracture ratio to transgranular fracture increased with increasing test temperature to 570 K(Fig.4). The increase of the ratio is accompanied by the increase of elongation. This fact suggests that the fracture stress of the 7 phase lattice increases with test temperature and becomes higher than or aproximately equal to the fracture stress of the grain boundary. Conclusion The room temperature elongation of the specimens with a microstructure consisting of the TiAl( 7 ) matrix and Ti,Al( a 2) phase is three times larger than that of the specimen with a small amount of the cr 2 particles in the TiAl matrix. This microstructure dependence of the room temperature elongation is considered to be mainly caused by the a 2/ 7 interphase boundary structure, i.e., misfit dislocations on the interphase boundary asist the room temperature deformation and partly caused by the degradation of environmental brittlement with a large amount of the a 2 phase. The elongation increases with increasing temperature from about 370 K in the specimens with a small amount of the a 2 particles, and from about 470 K in those with a large amount of the a 2 phase. The temperature dependence of the elongation in the former may be due to the degradiation of environmental brittlement, and the dependence in the latter due to the acceleration of twinning with increasing test temperature. REFERENCES 1. H.A.Lipsitt, MRS Symp.Proc., 39,351(1985). 2. S.H.Whang and Y.D.Hahn, MRS Symp.Proc., 133,687(1989). 3. K.Hashimoto, H.Doi, K.Kasahara, T.Tsujimoto, and T.Suzuki, JJapan InstMetals, 52, 816(1988). 4. T.Kawabata, T.Tamura, and O.Izumi, MRS Symp.Proc., 133,329(1989). 5. T.Maeda, M.Okada and Y.Shida, JIM, Spring Meeting, p.329, (1989). 6. S.H.Whang, J.Y.Kim, G.C.Chen, and Z.X.Li, Scripta.Metall.et Mater, 27, 699(1992). 7. K.Hashimoto,M.Nobuki, H.Doi, TKimura, T.Tsujimoto, and M.Nakamura, J.Jpn.Inst.Met., 57,898(1993) 8. D.Shechtman, M.J.Blackburn and H.A.Lipsitt, Metall.Trans.6A, 1373(1975). 9. M.J.Blackbum and M.P.Smith, U.S.Patent No.4,294, 615(1981). 10. W.Wunderlich, Th. Kremser, and G. Frommeyer, Z.Metallkde.81,802(1990). 11. S.Wardle, I.Phan, and G.Hug, PhilMAgA, 67,497(1993). 12. K.Hashimoto, M.Nobuki,H.Doi, M.Nakamura, T.Tsujimoto and T.&z&i, Proc.Intem.Symp.“Intermetallic Compounds” JIMIS-6, p.457, (1991). 13. V.K.Vasudevan, M.A.Stucke, S.A.Court andH.L.Fraser,Phil.Mag., 59,299(1989). 14. A.W.Mullendore and N.J.Grant, Structural Processes in CreepSpecial Repot No.70, The Iron and Steel Institute, London 44(1961). 15. W.Wunderlich, ThKremser and G.Frommeryer, ActaMet., 41, 1791(1993). 16. C.T.Liu, and Y.W.Kim, Scripta Metallet Mater. 27, 599(1992). 17. C.T.Liu, Scripta Metallet Mater., 27, 25(1992). 18. M.Nakamura, KHashimoto, and T.Tsujimoto, J.Mater.Res., 8, 68(1993). 19. M.Nakamura, NItho, KHashimoto, T.Tsujimoto, and T.Suzuki, Met.Trans.A 25,321(1994). 20. S.Farenc, A.Coujou and A.Couret, Phil.Mag., A.67, 127(1993).

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TENSILEPROPERTIESOFTiAl

Fig. 1. SEM backscattered electron images of Ti-5lat%Al ((a) and (b)) and Ti-49at%ALlat%Mn ((c) and (d)). (a) and (c) : ‘anealed at 1470 K for 2 h, (b) and (d) : anealted at 1550 K for 2 h.

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Fig.3. Variation in tensile elongation (a), proof stress (b) and ultimate tensile strength (c) with temperature for the binary and ternary alloys examined at 0

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Fig.2. Stress-strain curve for the ternary alloy tested at strain rate of 3x10-3 s-l Magnified curves shows serrated flow.

a strain rate of 3x 1Om3s- 1 l,O:binary alloys,~,O:temary alloy. Solid marks : anealed at 1470 K for 2 h. Open marks : anealed at 1550 K for 2 h.

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(a-1)

(4 (a-2)

(b-1)

(b) (b-2)

Fig.4. SEM fractographs of the binary (a) and ternary alloys (b) anealed at 1470 K for 2 h tested at room tempeature ((a-l) and (b-l)) and at 570 K ((a-2) and (b-2)).

Fig.5. TEM image of the deformation twins in the binary alloy anealed at 1470 K tested at room temperature. 112~1lO](OOl) dislocation(indicated by arrows in (a) and (b) are emitted from the intersection of deformation twins. The Burgers vector and slip plane of the dislocations were determined by contrast analysis and tilting experiment((b):edge on view).