Relation between Tensile Deformation Behavior and Microstructure in a Ti Ni Co Shape Memory Alloy

Full text

(1)Materials Transactions, Vol. 43, No. 5 (2002) pp. 834 to 839 Special Issue on Smart Materials-Fundamentals and Applications c 2002 The Japan Institute of Metals. Relation between Tensile Deformation Behavior and Microstructure in a Ti–Ni–Co Shape Memory Alloy Yoichi Kishi, Zenjiro Yajima and Ken’ichi Shimizu AMS R&D Center, Kanazawa Institute of Technology, Matto 924-0838, Japan Relation between tensile deformation behavior and microstructure in variously aged Ti–49.7 at%Ni–1.3 at%Co shape memory alloys has been investigated. Stress-strain curves for the alloys aged at 623 K and 723 K for 1.8 ks were of a work-hardening type, but those for the alloys aged at the two temperatures over 28.8 ks were nearly of a constant-stress type, as for binary Ti–51 at%Ni aged alloys. Lenticular precipitates were observed in both the 623 K and 723 K aged alloys, and the distance among the precipitates increased with increasing aging temperature and period, as well as the size of the precipitates. The lenticular precipitates were identified to be of the composition of Ti3 (Ni, Co)4 from an EDX analysis. Based on these observations, the constant-stress type stress-strain behavior for the alloys aged over 28.8 ks was attributed to some composition change accompanied with the aging progress by which Ti:(Ni + Co) composition ratio in the matrix of the Ti–Ni–Co alloys approached 1:1, as in the equi-atomic Ti–Ni binary alloys. (Received November 20, 2001; Accepted March 18, 2002) Keywords: shape memory alloy, stress-strain behavior, precipitate, transmission electron microscope, martensitic transformation, R-phase. Stress,. / MPa. 800 600 400 623K–1.8ks aged Ti–Ni–Co alloy B2 R tansformation start temp. = 305 K Test temp. = 320 K. 200 0. 0. 1. 2. 3. 4. 5. 1000 800 / MPa. Recently, a part of Ni atoms of equi-atomic Ti–Ni shape memory alloys was replaced by Co atoms, and Ti–Ni–Co ternary alloys have been developed in order to improve the superelastic behavior of the Ti–Ni alloys. Ms temperature and yield stress of the Ti–Ni–Co ternary alloys were reported1) to be lower and higher, respectively, than those of the Ti–Ni binary alloys. Therefore, the Ti–Ni–Co alloys are promising as an actuator material which is operated in near or below room temperature. However, mechanical properties of the ternary Ti–Ni–Co alloys have not been well understood, although those of the binary Ti–Ni and ternary Ti–Ni–Cu alloys have been reported somewhat in detail.2) The present authors previously investigated tensile and fatigue properties of variously aged Ti–Ni–Co alloys, which have different transformation temperatures, at 295 K and higher temperatures.3–5) Figure 1 is a result of the previous investigation, and shows stress-strain curves of the Ti–Ni–Co alloys aged at 623 K and 723 K for 1.8 ks,4, 5) from which critical stress for inducing B19 martensites, σM , of the 723 K aged alloy was known to be nearly equal to that of the 623 K aged alloy. However, elongation at tensile fracture point of the former alloy was about 2 times larger than that of the latter alloy. It is then necessary to clarify the relation between the stress-strain behavior and the aging treatment, and microstructures of the Ti–Ni–Co alloys aged at 623 and 723 K for various periods will be observed by a transmission electron microscopy method in the present investigation. Based on the observation, relationship between the composition of (Ni + Co) in the matrix and the aging period will be quantitatively evaluated, and then the effect of aging period on stressstrain behavior will be discussed.. 1000. Stress,. 1. Introduction. 600 400 723K–1.8ks aged Ti–Ni–Co alloy B2 R tansformation start temp. = 280 K Test temp. = 295 K. 200 0. 0. 1. 2 3 Strain, ( ). 4. 5. Fig. 1 Stress versus strain curves of Ti–Ni–Co shape memory alloys aged at 623 K and 723 K for 1.8 ks.4, 5). 2. Experimental Procedure The material investigated was prepared by high frequency vacuum induction melting, and its chemical composition was Ti–49.7 at%Ni–1.38 at%Co. The ingot was cold-rolled to the shape of plate with 5.0 mm width and 0.5 mm thickness. The.

(2) Relation between Tensile Deformation Behavior and Microstructure in a Ti–Ni–Co Shape Memory Alloy. 3. Results and Discussion 3.1 Transformation behavior Figure 2 shows a few examples of the DSC curves obtained for the Ti–Ni–Co aged alloys. Only one peak is clearly observed in both the cooling and heating processes of the 623T alloys, while two peaks are done in both the processes of the 723-T alloys for over 14.4 ks, which are due to the B2 ↔ R ↔ B19 two-stage transformations. For the 723–1.8 alloy, however, one peak due to the B2 → R transformation and two peaks due to the B19 → R → B2 transformations are observed in the cooling and heating processes, respectively. The R → B19 transformation was not detected in the cooling process of the 723–1.8 alloy, probably because of a gradual progress of the transformation. The one DSC peak in the 623-T aged alloys is believed to be due to the B2 ↔ R transformations, because the R-phase martensites are observed in the alloys by TEM (described in Section 3.3). Transformation behavior of just-solution-treated (not-aged) alloys was also examined, but no peak was observed in the alloys. The reason for the no-peak seems to be that the transformation temperatures of these alloys are lower than the refrigerant (liquid nitrogen) and/or the transformation proceeds. Cooling. 2 mW. Heat Flow. Heating. 100. 200. 2 mW. Heating. 100. 623–1.8 alloy. 623–14.4 alloy. 623–230.4 alloy. Cooling. Heat Flow. plate-shaped specimens were solution-treated at 1223 K for 1.8 ks in argon atmosphere followed by water quenching, and then they were aged at 623 K and 723 K for 1.8, 14.4, 28.8, 115.2 and 230.4 ks in argon atmosphere followed by water quenching. Ti–51 at%Ni binary alloy was also prepared and heat-treated under the same conditions, in order to compare a few characteristics with the heat-treated Ti–Ni–Co alloys. These heat-treated alloys are labeled 623-T alloy and 723-T aged alloy, T meaning the aging period. Martensitic transformation behavior of the aged alloys was examined by a differential scanning calorimetry (ShinkuRiko, MTS9000). TEM observations were carried out by using a JEOL-2000FXII and a Hitachi-HF2000 at 200 kV. Chemical compositions of the matrix and precipitates were quantitatively determined by a Kevex-SIGMA EDX system attached with a Hitachi-HF2000. Specimens for the TEM observations were mechanically polished, and then electropolished by using a twin-jet type electro-polishing apparatus. All the TEM observations were carried out at room temperature (≈300 K). X-ray diffraction profiles were obtained by using a Rigaku-RINT2400 X-ray apparatus with Cu–Kα radiation. Cu–Kβ radiation was removed by a graphite monochromater attached with the scintillation counter. Specimens for tensile tests were cut into ribbons with the shape of L (active gauge length) = 7.0 mm, w (width) = 3.5 mm and t (thickness) = 0.5 mm from the above plates by using an electric discharge machine. The ribbon-shaped specimens were then lightly polished with alumina powder in order to remove a thin oxide surface layer. Tensile properties of 623-T and 723-T aged alloys were examined at temperatures of Ms (B2 → R transformation start temperature) + 15 K by using a computer controlled material testing machine (JT Toshi, SVF-200/25-CPU), the initial tensile speed being 3.0 × 10−5 s−1 . The temperatures during the tests was kept constant by an environmental controlled chamber.. 835. 300. 400. 723–1.8 alloy. 723–14.4 alloy. 723–230.4 alloy. 200 300 Temperature , t / K. 400. Fig. 2 DSC curves of 623-T and 723-T Ti–Ni–Co alloys.. over a wide temperature range little by little. Figure 3 shows aging period dependences of Ms (B2 → R transformation start temperature) and Af (R → B2 transformation finish temperature) for Ti–Ni–Co and Ti–51 at%Ni aged alloys. The Ms and Af of the ternary Ti–Ni–Co aged alloys increase with increasing aging time in a similar manner to the binary Ti–Ni alloys. However, hysteresis of the B2 ↔ R transformations for the former alloys are very small compared to those for the latter alloys, and not dependent on aging time. Moreover, those of the 723-T aged alloys are lower than those of the 623-T alloys when compared at the same aging period. 3.2 Tensile properties Figures 4 and 5 show the stress-strain curves measured for Ti–Ni–Co and Ti–51 at%Ni aged alloys, respectively. Apparent plastic deformation is observed following to the linear elastic deformation on all the stress-strain curves, which is due to the martensitic transformation. Critical stress for inducing B19 martensites, σM , was determined to be the intersection of the line extended from the linear elastic region with that from the linear plastic region. The Ti–Ni–Co alloys are known to be strengthened by the addition of Co atoms, because σM and fracture strength of the 623-T Ti–Ni–Co al-.

(3) 836. Y. Kishi, Z. Yajima and K. Shimizu. 340. Ms’ and Af’ (K). 320 300 280 Ti–Ni–Co alloy Ms' Af'. 260 240 220 0 10. Ti–51Ni alloy Ms' Af'. 623–T alloy. 623–T alloy. 723–T alloy. 723–T alloy. 1. 2. 3. 10 10 Aging Period , T / ks. 10. 0. 1. 10. 2. 3. 10 10 Aging Period , T / ks. 10. Fig. 3 Relation between transformation temperatures and aging period of Ti–Ni–Co and Ti–51 at%Ni alloys. Ms and Af denote B2 → R transformation start temperature and R → B2 transformation finish temperature, respectively.. Stress ,. / MPa. 1000 800 600 400 200 0. 623-1.8 alloy 0. 2. 4. 6. 623-28.8 alloy 8. 10 12 0. 2. 4. 6. 623-230.4 alloy 8. 10 12 0. 2. 4. 6. 8. 10 12. Stress ,. / MPa. 1000 800 600 400 200 0. 723-1.8 alloy 0. 2. 4. 6. 723-28.8 alloy 8. 10 12 0. 2. 4 6 8 10 12 0 Strain , ( ). 723-230.4 alloy 2. 4. 6. 8. 10 12. Fig. 4 Stress-strain curves for 623-T and 723-T Ti–Ni–Co alloys. Test temperature is (Ms + 15) K.. loys (Fig. 4) are higher than those of the 623-T Ti–51 at%Ni alloys (Fig. 5). The σM tends to decrease with increasing aging temperature and period, while elongation at the tensile fracture point shows the converse tendency to σM . The stressstrain curves for the Ti–Ni–Co alloys aged at 623 K and 723 K for 1.8 ks are of a work-hardening type, but those for the alloys aged at those temperatures over 28.8 ks are nearly of a constant-stress type, as for Ti–Ni binary alloys. 3.3 TEM and X-ray observations Figure 6 shows typical TEM images and selected-area electron diffraction (SAED) patterns taken from 623-T ((a), (b). and (c)) and 723-T ((d), (e) and (f)) alloys, (a) and (d) being for the aged ones for 1.8 ks, and (b) and (e) for the aged ones for 230.4 ks. Fine lenticular precipitates are observed parallel to the traces of {111}B2 planes on those specimen surfaces. The size of the precipitates increases with increasing aging temperature and period, as known from the comparison between (a) and (b) and between (d) and (e). Dark contrasts surrounding the fine precipitates, which are caused by some strain field and/or R-phase, are observed in both the aged alloy, especially for very fine precipitates. The SAED pattern taken from the encircled area in Fig. 6(b), which was taken from a 623–230.4 aged alloy and in-.

(4) Relation between Tensile Deformation Behavior and Microstructure in a Ti–Ni–Co Shape Memory Alloy. / MPa. 500. Stress ,. 750. 250 623-1.8 alloy 0. 0. 1. 2. 3. 4. 5. / MPa. 500. Stress ,. 750. 250 623-28.8 alloy 0. 0. 1. 2 3 Strain , ( ). 4. 5. Fig. 5 Stress-strain curves for 623–1.8 and −28.8 Ti–51 at%Ni alloys. Test temperature is (Af + 15) K.. cludes small precipitates, is shown in Fig. 6(c), and two kinds of zone patterns are observed with different intensities. Similar SAED patterns have been observed in nickel-rich Ti–Ni binary aged alloy,6) Ti–Ni–Fe,7, 8) and Ti–Ni–Al9) ternary alloys and so on. One pattern with strong reflections, which are indexed with subscription B2, is from the B2 type matrix lattice. The other weak reflections align along the 110 B2 reciprocal lattice vector, and are located at the positions dividing the length of the vectors into one-third. Then, the week reflections are concluded to be ones from the R-phase, referring to the previous studies. The SAED pattern taken from the encircled area in Fig. 6(e), which was taken from a 723–230.4 aged alloy and includes a comparatively large precipitate, is shown in Fig. 6(f), and two kinds of zone patterns are also observed with different intensities. The weak reflections align along the 321 B2 reciprocal lattice vector, and are located at the positions dividing the length of the vectors into one-seventh. Similar SAED patterns have been observed for a binary Ti–Ni aged alloy,10) and the weak reflections are concluded to be ones from the rhombohedral Ti3 Ni4 precipitate, following to the previous study. In order to confirm the above conclusions obtained by electron diffraction study, X-ray diffraction study was carried out at room temperature. Figure 7 shows X-ray diffraction profiles for solution treated, 623–230.4 aged and 723– 230.4 aged Ti–Ni–Co alloys. Profiles consisting of reflections from the B2 and R phases are obtained from the latter two aged alloys, although un-indexed reflections, which are probably caused by Ti2 Ni and Ti4 Ni2 Ox precipitates, are. 837. also seen as marked with ▼. Since the X-ray profiles were taken at room temperature, reflections from the B19 martensite formed at sub-zero temperature should not be observed in Fig. 7. Hence, one peak observed on the DSC curves for the 623-T aged alloys may be attributed to the B2 ↔ R transformations. X-ray diffraction peaks from the Ti3 Ni4 precipitates are also obtained from 623–14.4, −115.2, −230.4 and 723– 14.4, −115.2, −230.4 aged alloys. Therefore, the lenticular precipitates observed in the Ti–Ni–Co alloys aged at 623 K and 723 K may be concluded to be the Ti3 Ni4 , as observed in Ti–Ni binary alloys.10) Figure 8 shows relations between the aging period and the inter-precipitate distance for ternary Ti–Ni–Co and binary Ti– Ni aged alloys. The inter-precipitate distance depends on the aging period and temperature, as well as the precipitate size. The distance of the 723-T Ti–Ni–Co alloys rapidly increases with increasing aging period, while that of the 623-T Ti–Ni– Co alloys slightly increases. The distance of the 623–230.4 aged Ti–Ni-Co alloy is far smaller than that of the 723–230.4 aged Ti–Ni–Co alloy. According to the transformation behavior (shown in Fig. 2), the R ↔ B19 transformations are observed in the alloys aged at 723 K for over 14.4 ks, and therefore the R-phase may transform to the B19 martensite when the inter-precipitate distance would grow over that of 723–14.4 aged alloy. For this reason, the R-phase of 623-T alloys may not transform to the B19 martensite during the DSC measurements. Figure 9 shows relations between the aging period and the Ni and Co atoms composition in the matrix of the 723-T aged Ti–Ni–Co alloys. The composition decreases with increasing aging period, and they seemed to be consumed in order to form the precipitates. Since Co atoms are known to behave in the same way as Ni atoms,11) the lenticular precipitates seem to possess the composition of Ti3 (Ni, Co)4 . 3.4 Influence of microstructure on stress-strain behavior According to the previous and present studies, critical stress for inducing B19 martensites, σM , of the 723–1.8 Ti– Ni–Co alloy is nearly equal to that of the 623–1.8 Ti–Ni– Co alloy. However, elongation at tensile fracture point of the former alloy is about 2 times larger than that of the latter alloy. Such a difference in stress-strain behavior between the 723–1.8 and 623–1.8 aged alloys is thought to be due to some difference in distribution of precipitates in the two alloys. According to TEM observations, the number of precipitates per unit area in the 723–1.8 aged alloy is smaller than that in the 623–1.8 aged alloy, that is, distance among precipitates is larger in the former alloy than the latter alloy. Therefore, it seems that the growth of stress-induced martensites in the 623–1.8 aged alloy is more difficult than that in the 723–1.8 aged alloy. As a result, there has been observed the above mentioned difference in stress-strain behavior between the two alloys. Stress-strain behavior of the alloys aged at 623 K and 723 K over 28.8 ks is similar to that of a constant-stress type in Ti– Ni binary alloys. This may be because the composition of Co atoms in the matrix decreases as aging proceeds. According to the TEM and EDX analyses, in fact, the Co atoms composition was observed to decrease with increasing aging period. The solution-strengthening by Co atoms in the Ti–Ni–Co al-.

(5) 838. Y. Kishi, Z. Yajima and K. Shimizu. (d). (a). 111B2 100B2 000. (b). 011B2. (e) C. F. (f). (c). 000. 211B2. 011B2 110B2 101B2 000. 312B2 323B2. Fig. 6 TEM images and SAED patterns in Ti–Ni–Co aged alloys: (a) Bright field image (BFI) of 623–1.8 alloy, (b) BFI of 623–230.4 alloy, (c) SAED pattern taken from the encircled area C in (b), (d) BFI of 723–1.8 alloy, (e) BFI of 723–230.4 alloy, (f) SAED pattern taken from the encircled area F in (e).. loys may be also known from the stress-strain curves shown in Figs. 4 and 5. Hence, the chemical composition of the matrix in Ti–Ni–Co aged alloys is considered to approach that in Ti–Ni aged alloys as aging proceeds, that is, Ti:(Co + Ni) composition ratio of the former alloys approaches Ti:Ni one of the latter alloys. As a result, the stress-strain behavior of the Ti–Ni–Co alloys aged over 28.8 ks may become nearly a constant-stress type, as that of Ti–Ni binary alloys. 4. Conclusions Transformation characteristics, TEM microstructures and tensile properties of aged Ti–49.7 at%Ni–1.38 at%Co alloys have been investigated. Only one DSC peak was clearly observed in both the cooling and heating processes of the 623-T aged alloys. The one DSC peak was concluded to be due to the B2 ↔ R transformations from the TEM observations. Two DSC peaks were observed for the 723–14.4, 115.2 and. 230.4 aged alloys, which were due to the B2 ↔ R ↔ B19 two-stage transformations. However, for the 723–1.8 aged alloy, one peak due to the B2 → R transformation was observed in the cooling process, and two peaks due to the B19 → R → B2 transformations were in the heating process. Transformation characteristics of ternary Ti–Ni–Co aged alloys were thus almost identical to that of binary Ti– Ni aged alloys. Fine lenticular precipitates were observed in TEM images of both the 623 K and 723 K aged alloys. Distance among the precipitates increased with increasing aging temperature and period, as well as the size of the precipitates. From EDX analyses, Ni and Co atoms composition in the matrix was determined to decrease with increasing aging period, and so the Ni and Co atoms are supposed to be consumed in order to form the precipitate, whose composition becomes Ti3 (Ni, Co)4 . The stress-strain behavior of the Ti–Ni–Co alloys aged over 28.8 ks was nearly of a constant-stress type as that in Ti–Ni binary alloys. The constant-stress is supposed.

(6) 110B2. 400. Solution treated alloy 111B2. 300 200. 0 400 300 200. 111B2. 623-230.4 alloy. 723-230.4 alloy 111B2. 100. 110B2. 200. 110B2. 300. 102R 131Ti3Ni4. 0 400. 102R 131Ti3Ni4. Intensity (arb. unit). 100. 100 0 35. 40. 45 Diffraction angle , 2. 50. 55. Fig. 7 X-ray diffraction profiles of solution treated, 623–230.4 and 723–230.4 Ti–Ni–Co alloys. Mark ▼ denotes un-indexed reflections.. Composition of Co and Ni Atoms in the Matrix (at ). Relation between Tensile Deformation Behavior and Microstructure in a Ti–Ni–Co Shape Memory Alloy. 52 50 48. : Ni + Co : Ni. 46 2.2 2.0 1.8 1.6. : Co. 1.4 -1. 10. Distance between the Precipitates , dp / nm. 400 300. Ti-Ni-Co alloy : 623- T alloy : 723- T alloy. 1. 2. 3. 10. completing the manuscript. This research was partially supported by Grant-in-Aid for Encouragement of Young Scientists (A), 11750577, and Scientific Research (C), 12650663 and 12650703, of the Ministry of Education, Science, Sports and Culture, Japan. The support is greatly appreciated.. 100. 300. 0. 10 10 10 Aging Period , T / ks. Fig. 9 Compositions of Co and Ni atoms in the 723-T aged Ti–Ni–Co alloys as a function of aging period.. 200. 0 400. 839. Ti-51Ni alloy : 623- T alloy : 723- T alloy. REFERENCES. 200 100 0 -1 10. 0. 1. 2. 10 10 10 Aging Period , T / ks. 3. 10. Fig. 8 Relation between inter-precipitate distance and aging period.. to appear due to that the chemical composition of the matrix approaches that of Ti–Ni aged alloy as aging proceeds. Elongation at tensile fracture point of the 623–1.8 aged alloy was about 2 times larger than that of the 723–1.8 aged alloy. Such a difference was also observed previously, and from the present TEM observations, the difference in elongation may be attributed to the fact that stress-induced martensites in the 623–1.8 alloy are hard to grow compared to those in the 723– 1.8 aged alloy. Acknowledgements The authors are indebted to Mr. T. Hayashi, a graduate student, for his help in carrying out the present study and in. 1) Internet home page of Daido Steel Co., Ltd. (www.daido.co.jp). 2) T. Saburi: Shape Memory Materials, ed. by K. Otsuka and C. M. Wayman (Cambridge University Press, 1998) pp. 49–96. 3) Y. Kishi, Z. Yajima and K. Shimizu: Proc. of China-Japan Bilateral Symposium on Shape Memory Alloys (C-J SMA ’97): Hangzhou, China, 1997, ed. by C. Youyi and K. Otsuka (International Academic Publishers, Beijing, P. R. China, 1998) pp. 75–80. 4) Y. Kishi, Z. Yajima, K. Shimizu and K. Morii: Mater. Sci. and Eng. A, 273–275 (1999) 654–657. 5) Y. Kishi, Z. Yajima, K. Shimizu and K. Morii: Proc. of the International Symposium and Exhibition on Shape Memory Materials (SMM ’99), Kanazawa, Japan, 1999, ed. by T. Saburi (Trans Tech Publications, Switzerland, 2000) pp. 123–126. 6) S. Miyazaki and K. Otsuka: Metall. Trans. A 17A (1986) 53–63. 7) C. M. Hwang, M. Meichle, M. B. Salamon and C. M. Wayman: Philos. Mag. A 47 (1983) 9–30. 8) C. M. Hwang, M. Meichle, M. B. Salamon and C. M. Wayman: Philos. Mag. A 47 (1983) 31–62. 9) see Ref. 2) p. 57. 10) T. Tadaki, Y. Nakata, K. Shimizu and K. Otsuka: Mater. Trans., JIM 27 (1986) 731–740. 11) Y. Nakata, T. Tadaki and K. Shimizu: Proc. of International. Symposium on Displacive Phase Transformation & Their Application in Materials Engineering, (The Minerals, Metals & Materials Society, 1998) pp. 187–196..

(7)

No results found