Mechanical Properties and Shape Memory Behavior of Ti-Mo-Ga Alloys

Materials Transactions, Vol. 45, No. 4 (24) pp. 19 to 195 Special Issue on Frontiers of Smart Biomaterials #24 The Japan Institute of Metals Mechanical Properties and Shape Memory Behavior of Ti-Mo-Ga Alloys Hee Young Kim 1, Yoshinori Ohmatsu 1, Jae Il Kim 1, Hideki Hosoda 2 and Shuichi Miyazaki 1; * 1 Institute of Materials Science, University of Tsukuba, Tsukuba 35-8573, Japan 2 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-853, Japan Mechanical properties and shape memory behavior of Ti-Mo-Ga alloys were investigated in order to develop Ni-free biomedical shape memory alloys. The Ti-Mo-Ga alloys were fabricated by arc melting method. The ingots were cold-rolled up to 95% reduction in thickness. The cold-rolled specimens were heat treated in the temperature range 673 1273 K for 6 s 3.6 ks. The martensitic transformation temperature decreased with increase in Mo and Ga content. The maximum shape recovery strain was obtained in a solution treated Ti-6 at%mo-3 at%ga alloy. Mechanical properties and shape memory behavior strongly depend on heat treatment condition in the Ti-6 at%mo-3 at%ga. Premature failure was observed in specimens heat treated in the temperature range 673 773 K. Ultimate tensile strength decreased and fracture strain increased with increasing heat treatment temperature. Shape memory effect was obtained in specimens heat treated in the temperature range 173 1273 K. The shape memory effect was due to the stress induced martensitic transformation yielding tensile deformation and the reverse transformation upon heating after unloading. The martensitic transformation start temperature increased and the yield stress decreased with increasing heat treatment temperature and time. Stable superelastic behavior was obtained in a Ti-7 at%mo-4 at%ga alloy at room temperature by cyclic tensile tests. The recovery strain exceeding 4% was achieved in the pre-strained Ti-7 at%mo-4 at%ga alloy. (Received November 28, 23; Accepted January 7, 24) Keywords: shape memory alloy, superelasticity, biomaterial, smart material, titanium based alloy 1. Introduction Ti-Ni shape memory alloys have been used successfully as biomedical materials owing to their superior shape memory property and superelasticity. 1 3) However, the Ni-hypersensitivity and toxicity of Ni has been pointed out in Ti-Ni alloys. 4) Although the Ti-Ni alloys have been successfully applied for many medical products, the development of Nifree shape memory alloys is on the other hand strongly required in order to pursue absolute safety. The -type Ti alloys reveal a martensitic transformation from (disordered BCC) to two metastable martensite structures, either hexagonal martensite ( ) or orthorhombic martensite ( ), by quenching. The martensite structure changes from to above a critical alloying content. 5 7) Transformation strains from the to is accommodated primarily by internal twinning. The reversion of to is related to the shape memory effect in -type Ti alloys. Although the shape memory effect has been reported in Ti- Mo alloys, 8 11) Ti-Nb alloys 12 14) and Ti-V alloys, 15) the fundamental understanding is still insufficient in Ti-based shape memory alloys. In this study, mechanical properties and shape memory behavior of Ti-Mo-Ga alloys were investigated in order to develop Ni-free biomedical shape memory alloys. The effects of heat treatment temperature and time on mechanical properties and shape memory behavior were investigated by tensile testing at various temperatures. 2. Experimental Procedures The Ti-(5 7)at%Mo-(1 8)at%Ga alloys were prepared by an Ar arc melting method. Hereafter, Ti-xat%Mo-yat%Ga is abbreviated to Ti-xMo-yGa. The ingots were cold-rolled up to 95% reduction in thickness. The cold-rolled sheets were cleaned with ethanol, wrapped in Ti foil and encapsulated in *Corresponding author, E-mail: miyazaki@ims.tsukuba.ac.jp quartz tubes under a 25 torr partial pressure of high-purity Ar, and then heat treated in the temperature range 673 1273 K for 6 s-3.6 ks. The sheets were quenched into water by breaking the quartz tubes. Specimens for the mechanical tests were cut by an electro-discharge machine. The damaged surface was removed by mechanical polishing and eletropolishing. Bending tests were carried out at room temperature where the specimens were deformed in a round shape and heated up to approximately 5 K. Tensile tests were carried out under a strain rate of 1:67 1 4 s 1 at various temperatures. The gage length of specimens was 2 mm. XRD measurements were conducted at room temperature with CuK to determine the constituent phases. Si was used as a reference. 3. Results and Discussion The shape memory effect of Ti-Mo-Ga alloys was investigated by bending tests. Thin plates were bent at room temperature and then heated up to 5 K. Before bending the plates were solution treated at 1273 K for 3.6 ks. The results are summarized in Fig. 1. Open circles denote that shape Ti 2 4 2 6 Mo Content (%) 8 Mo 1 4 6 8 1 Ga Content (%) Fig. 1 Composition dependence of shape memory effect at room temperature. : shape memory effect, : superelastic behavior. Ga

Mechanical Properties and Shape Memory Behavior of Ti-Mo-Ga Alloys 191 Yield Stress, σy / MPa 12 1 8 6 4 2 1 5Ga 8Ga 2 4 6 8 1 8 6 4 2 memory effect was observed on heating after bending. Superelastic behavior was observed at composition marked by solid triangle. Neither shape memory effect nor superelastic behavior was observed at compositions marked by. Shape memory effect was obtained in Ti-5Mo-(2 4)Ga, Ti- 6Mo-(1 3)Ga and Ti-7Mo-(1 2)Ga alloys. And superelastic behavior was obtained in Ti-5Mo-(5 8)Ga, Ti-6Mo-(4 5)Ga and Ti-7Mo-4Ga at room temperature. Figure 1 indicates that the martensitic transformation temperature decreased with increasing Mo and Ga contents. This means that the shape memory effect or superelastic behavior is achievable at body temperature by controlling Mo and Ga contents. Figure 2 shows the stress-strain curves of Ti-6Mo-Ga alloys obtained by tensile tests at room temperature after the heat treatment at 1273 K for 3.6 ks. The elongation decreased with increasing Ga content. The Ti-6Mo-3Ga alloy exhibits double yielding including strain plateau with a low work hardening rate. The yield stress was evaluated using these stress-strain curves of Ti-6Mo-Ga alloys as shown in Fig. 2 as a function of Ga content. A minimum in yield stress was observed around 3% Ga, and then the yield stress increased rapidly between 3% and 5% Ga. It is supposed that the martensitic transformation start temperature (Ms) of Ti- 6Mo-3Ga is close to room temperature. Figure 3 shows the stress-strain curves obtained at room temperature for the Ti-6Mo-3Ga alloy after annealing in the temperature range 673 1273 K for 3.6 ks. Premature failure was observed in the specimen annealed in the temperature 3Ga 4Ga 1Ga 2 4 6 8 1 Ga Content (%) Fig. 2 Stress-strain curves and yield stress obtained at room temperature in the Ti-6Mo-(1 8)Ga alloys heat-treated at 1273 K for 3.6 ks. 1 8 6 4 2 773K 873K 673K 1273K 973K 1173K 173K 1273K 1173K 2 4 6 8 1 16 18 2 22 24 26 Fig. 3 Effect of heat treatment temperature on stress-strain curve of the Ti- 6Mo-3Ga alloy. Fracture Strain, εf (%) UTS, σuts / MPa 11 9 7 5 ( ) ( ) 3 6 7 8 9 1 11 12 13 3 25 2 15 1 5 Heat treatment temperature, T / K 6 7 8 9 1 11 12 13 Heat treatment temperature, T / K Fig. 4 Effect of heat treatment temperature on UTS and fracture strain of the Ti-6Mo-3Ga alloy. range 673 773 K. The specimen annealed at 873 K fractured immediately after yielding. The specimens annealed in the temperature range 173 1273 K exhibited a strain plateau with a low work hardening rate. Also a shape memory effect was obtained in the specimens annealed in the temperature range 173 1273 K. Figure 4 shows the ultimate tensile strength (UTS) and fracture strain evaluated using the stressstrain curves obtained in Fig. 3. The UTS decreased with increasing annealing temperature followed by saturation at 173 K. The fracture strain steeply increased with increasing annealing temperature from 873 to 1173 K. Figure 5 shows the X-ray diffraction patterns obtained at

192 H. Y. Kim, Y. Ohmatsu, J. I. Kim, H. Hosoda and S. Miyazaki at 298K α" (111) β (11) β(11) Intensity (a.u.) α (1) α (1) α (2) α (11) β (11) α (2) β (11) α (11) β (11) α (11) α (1) α (2) 673K 773K 873K 973K Intensity (a.u.) α (111) β(11) α (111) β(11) β (11) (c) α" (2) β (11) α" (111) 173K 34 36 38 4 42 2 θ α" (2) α" (111) 1273K 34 36 38 4 42 2 θ Fig. 5 X-ray diffraction patterns obtained at 298 K in the Ti-6Mo-3Ga alloy heat-treated in the temperature range 673 1273 K for 3.6 ks. room temperature for the Ti-6Mo-3Ga alloy after annealing in the temperature range 673 1273 K for 3.6 ks. The broad peaks corresponding to and were observed after annealing at 673 K. This indicates that the deformed structure and the stress induced remained after annealing at 673 K. Figure 5 indicates that the specimens consist of and after annealing in the temperature range 773 973 K. The peak corresponding (11) of became sharp after annealing above 873 K. In addition, it has been known that the! phase is formed during annealing in the temperature range 373 773 K. 16) It is supposed that the deformed structure and the formation of! phase resulted in the premature failure after annealing in the temperature range 673 773 K as shown in Fig. 3. The peaks corresponding to disappeared after annealing at 173 K. Thus, the = transformation temperature is between 973 K and 173 K for Ti-6Mo-3Ga. This result corresponds to the stress-strain curves that show the steep increase of a fracture strain after the annealing above 173 K. It is noted that the peaks from were detected after annealing in the temperature range 173 1273 K. This means that the Ms temperatures are higher than 298 K for the specimens annealed in the temperature range 173 1273 K. The strain plateau was observed during tensile tests at room temperature in the specimens annealed in the temperature range 173 1273 K. This strain plateau is associated with the formation of stress induced martensites or reorientation of pre-existing martensites. Figure 6 shows the XRD patterns obtained from undeformed, deformed and Fig. 6 X-ray diffraction patterns obtained from undeformed, deformed and shape recovered (c) condition in the Ti-6Mo-3Ga alloy solution treated at 1273 K for 3.6 ks. shape recovered specimens in the solution treated condition of 1273 K for 3.6 ks. The diffraction profile from the undeformed specimen exhibits the parent phase. Peaks corresponding to the martensite were also observed. After the deformation at room temperature, the intensity of (11) line of decreased and the reflections from increased. After heating up to 5 K, the reflections from disappeared and intensity of (11) line of increased. These results indicate that the shape memory effect in this case was due to mainly the stress induced martensitic transformation and partially the rearrangement of martensite variants during tensile deformation followed by the reverse transformation during heating. Figure 7 shows the stress-strain curves obtained at room temperature for the Ti-6Mo-3Ga alloy after annealing at 173 K for 6 s 3.6 ks. The UTS and fracture stain were plotted as a function of annealing time in Fig. 8. Fracture 1 8 6 4 2 2 4 As-rolled 6s 12s 6s 1.8ks 3.6ks 6 8 1 12 14 16 18 2 22 Fig. 7 Stress-strain curves obtained at room temperature for the Ti-6Mo- 3Ga alloy annealed at 173 K for 6 s 3.6 ks.

Mechanical Properties and Shape Memory Behavior of Ti-Mo-Ga Alloys 193 UTS, σuts / MPa 1 9 8 7 6 5 4 25 UTS Fracture strain 2 15 1 5 1 1 1 Fracture Strain, εf (%) Heat treatment time, t / s Fig. 8 Effect of heat treatment time on UTS and fracture strain of the Ti-6Mo-3Ga alloy. 6 4 2 6 4 2 6 4 2 236K 295K 6 4 2 27K 348K 6 4 2 6 4 2 326K 365K 6 4 2 6 4 2 6 4 2 6 4 236K 295K 6 4 2 6 4 2 27K 328K 348K 6 4 2 365K 382K Fig. 1 Stress-strain curves obtained at various temperatures for the Ti- 6Mo-3Ga alloy annealed at 173 K for 12 s. Yield Stress, σy / MPa 5 4 3 2 1 2 25 3 35 4 Temperature, T / K 173K/12s 1273K/3.6ks 2 1 2 3 Fig. 9 Stress-strain curves obtained at various temperatures for the Ti- 6Mo-3Ga alloy annealed at 1273 K for 3.6 ks. strain increased to 8 1% by annealing at 173 K for 3 12 s due to recovery of cold-worked microstructure. The UTS decreased and the fracture strain increased remarkably after annealing for 1.8 ks because of recrystallization and grain growth. Figures 9 and 1 show the series of stress-strain curves for the Ti-6Mo-3Ga alloys which were annealed at 1273 K for 3.6 ks and annealed at 173 K for 12 s, respectively. The residual strain after unloading was recovered by heating at about 5 K: broken lines with arrows indicate the shape recovery by heating. The critical stresses for apparent yield are plotted in Fig. 11. The stress-strain curves may be divided mainly into two temperature regions according to the temperature dependence of the critical stress. In the region T > Tm, where Tm indicates the temperature showing the Fig. 11 Temperature dependence of yield stress for the Ti-6Mo-3Ga alloy annealed at 173 K for 12 s and 1273 K for 3.6 ks. minimum of the critical stress, the critical stress increases with increasing temperature. The parent phase is more stable at higher temperatures, and thus higher stress is required for inducing the martensitic transformation, which is in accordance with the Clausius-Clapeyron relationship. In the region T < Tm, the critical stress decreases with increasing temperature. This is because the stress for the rearrangement of martensite variants increases with decreasing temperature. The minima of the critical stress are observed at 295 K and 328 K in the specimens annealed at 173 K for 12 s and at 1273 K for 3.6 ks, respectively. This indicates that the Ms is lower in the specimen annealed at 173 K for 12 s, because of higher density of dislocations suppressing the martensitic transformation. As shown in Figs. 9 and 1, in the vicinity of the minima, the maximum shape recovery strain about 2.5% was obtained. The recovery strain decreased with increasing and decreasing temperature from the minimum stress point. In the region T > Tm, the critical stress to induce the martensite increases with increasing temperature, while the

194 H. Y. Kim, Y. Ohmatsu, J. I. Kim, H. Hosoda and S. Miyazaki 8 6 4 2 1st 2nd 3rd 4th 5th 6th 7th 173K/12s at RT Plastic Strain (%) 6 5 4 3 2 173K/12s 1273K/3.6ks 8 6 4 2 2 4 6 8 1 1st 2nd 3rd 2 4 6 8 1 critical stress for slip decrease with increasing temperature. Slip occurs if the stress level for slip becomes lower than the stress to induce the martensite. Thus, the strain by slip increases with increasing temperature, causing the recoverable strain to decrease. The recoverable strain also decreases with decreasing temperature because the stress for the rearrangement of martensite variants increases above the critical stress for slip. Figure 12 shows stress-strain curves of the Ti-6Mo-3Ga specimens annealed at 173 K and 1273 K: they were obtained from cyclic deformation at room temperature to various strains followed by heating to 5 K for each cycle. Both specimens show the almost perfect shape memory effect at the first cycle. The shape recovery strain was about 2.5% for both cases. At the second cycle, residual strain was also recovered completely by heating in the specimen annealed at 173 K, whereas only.2% of recovery strain was obtained due to the permanent deformation by slip in the specimen annealed at 1273 K. It obviously indicates that the critical stress to induce the martensite is higher when the specimen has been annealed at 173 K. The remained plastic strain is plotted as a function of applied stress in Fig. 13. The critical stress for slip was defined as a stress inducing.5% plastic strain. Thus, the critical stresses for slip were obtained as 46 MPa and 66 MPa for the specimens annealed at 1273 K and 173 K, respectively. As a result, a higher critical stress for slip resulted in the larger shape recovery strain and more stable shape memory effect when annealed at 173 K for 12 s. In order to obtain superelasticity at body temperature, the 4th 5th 1273K/3.6ks at RT Fig. 12 Stress-strain curves obtained by cyclic loading-unloading tensile tests for the Ti-6Mo-3Ga alloy annealed at 173 K for 12 s and 1273 K for 3.6 ks. 1 2 3 4 5 6 7 8 Fig. 13 Remained plastic strain after cyclic loading-unloading tensile tests for the Ti-6Mo-3Ga alloy annealed at 173 K for 12 s and 1273 K for 3.6 ks. 8 6 4 2 6 4 2 1st 2nd 4th 5th 8 1th 2th 5th 1th Fig. 14 Change in stress-strain curves during cyclic tensile tests for the Ti- 7Mo-4Ga alloy annealed at 173 K for 12 s. reverse martensitic transformation finish temperature (Af) should be below body temperature. The Af temperature decreased with increasing Mo and Ga content. Ti-7Mo-4Ga was selected using the result of bending test as shown in Fig. 1. Cyclic tensile tests by repeated loading and unloading were carried out at room temperature by using a specimen annealed at 173 K for 12 s. The results are shown in Fig. 14. At the first cycle, the stress-strain curve exhibited incomplete superelastic behavior with a small amount of remained strain. After the 4th cycle, the complete superelastic behavior with the recovery strain of about 4% was obtained. The curves show that the critical stress for inducing martensite decreases with increasing cycle number. It is also noted that the stable superelastic curve was obtained after the 1th cycle. It is concluded that Ti-Mo-Ga alloys are promising for the biomedical shape memory and superelastic alloys. Further research on the effect of thermo-mechanical treatment on mechanical properties is needed to enhance the shape memory and superelastic properties of Ti-Mo-Ga alloy. 2%

Mechanical Properties and Shape Memory Behavior of Ti-Mo-Ga Alloys 195 4. Conclusion (1) The martensitic transformation temperature decreased with increasing Mo and Ga content. The shape memory effect and superelastic behavior can be obtained at body temperature by controlling Mo and Ga contents. The martensitic transformation start (Ms) temperature of Ti-6Mo-3Ga is close to room temperature. (2) The work-hardened structure and the formation of! phase resulted in the premature failure in the Ti-6Mo-3Ga alloy which was annealed in the temperature range 673 773 K. Ultimate tensile strength decreased and fracture strain increased with increasing heat treatment temperature due to recrystallization of cold-worked structure. Shape memory effect was obtained in the specimens heat-treated in the temperature range 173 1273 K. The shape memory effect was due to the stress induced martensitic transformation during tensile deformation and the following reverse transformation during heating. (3) The critical stress for slip decreased and Ms temperature increased with increasing heat treatment temperature and with decreasing heat treatment time. The higher critical stress for slip resulted in the larger shape recovery strain and more stable shape memory effect in the Ti-6Mo-3Ga alloy annealed at 173 K for 12 s. (4) The superelastic behavior with the recovery strain of 4% was obtained in Ti-7Mo-4Ga alloy. Ti-Mo-Ga alloys can be prospect biomedical shape memory and superelastic alloys at room temperature. Acknowledgments Center of Excellence Program and the Grants-in-Aid for Fundamental Scientific Research(Kiban A(1999 21), Kiban A(22 24)) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. REFERENCES 1) Y. Oshida and S. Miyazaki: Corros. Eng. 4 (1991) 19 125. 2) S. Miyazaki: Engineering Aspects of Shape Memory Alloys, Ed. by T. W. Duerig et al., (Butterworth-Heineman, 199) pp. 452 469. 3) S. Miyazaki and K. Otsuka: ISIJ Int., 29 (1989) 353 377. 4) S. Shabalovskaya, J. Cunnick, J. Anderegg, B. Harmon and R. Sachdeva: Proc. First Inter. Conf. Shape Memory and Superelastic Technologies, (SMST, 1994) pp 29 215. 5) R. Davis, H. M. Flower and D. R. F. West: Acta Metall. 27 (1979) 141 152. 6) T. Ahmed and H. J. Rack: J. Mater. Sci. 31 (1996) 4267 4276. 7) R. Davis, H. M. Flower and D. R. F. West: J. Mater. Sci. 31 (1979) 712 722. 8) W. F. Ho, C. P. Ju and J. H. Chern Lin: Biomaterials 2 (1999) 2115 2122. 9) T. Grosdidier and M. J. Philippe: Mater. Sci. Eng. A291 (2) 218 223. 1) H. Hosoda, Y. Ohmatsu and S. Miyazaki: Trans. MRS-J., 26 (21) 235 237. 11) H. Hosoda, N. Hosoda and S. Miyazaki: Trans. MRS-J., 26 (21) 243 246. 12) C. Baker: Metal Sci. J. 5 (1971) 92 1. 13) K. Nitta, S. Watanabe, N. Masahashi, H. Hosoda and S. Hanada: Structural Biomaterials for the 21st Century, (TMS, 21) pp. 25 34. 14) H. Hosoda, Y. Fukui, T. Inamura, K. Wakashima and S. Miyazaki and K. Inoue: Mater. Sci. Forum 426 432 (23) 3121 3125. 15) T. W. Duerig, J. Albrecht, D. Richter and P. Fischer: Acta Metall. 3 (1982) 2161 2172. 16) Y. Ohmatsu, H. Hosoda and S. Miyazaki: 4th Pacific Rim Inter. Conf. Adv. Mater. Proc., (The Japan Inst. Metals, 21) pp. 1627 1629. This work was partially supported by the 21 Century