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Thermo-mechanical cyclic transformation behavior of Ti–Ni shape memory alloy wire
Materials Science and Engineering A 509 (2009) 8–13
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Thermo-mechanical cyclic transformation behavior of Ti–Ni shape memory alloy wire Y.F. Li ∗ , X.J. Mi, J. Tan, B.D. Gao State Key Laboratory for Fabrication & Processing of Nonferrous Metals, General Institute of Non-ferrous Metals, No. 2 Xinjiekou Wai Str., Beijing 100088, PR China
a r t i c l e
i n f o
Article history: Received 16 July 2008 Received in revised form 3 December 2008 Accepted 18 February 2009 Keywords: Ti–Ni shape memory alloy Thermo-mechanical cycling Recovery strain Fatigue life Transformation temperatures
a b s t r a c t The present paper deals with recovery strain and fatigue life of Ti49.8 Ni50.2 shape memory alloy (SMA) wire on thermo-mechanical cycling (TMC). Recovery strain of SMA wire with several heat treatments shows different changes when being subjected to TMC at a series of constant loads. These changes are significant in the initial 200 cycles and recovery strain tends to reach a steady state in the further cycling. The effect of heat treatment on the fatigue life is analyzed, the SMA wire with shorter heat treatment exhibits lower fatigue life while higher recovery strain. Transformation temperatures of SMA wire on TMC is investigated by electrical resistance measurements. It was found that both annealing time and TMC has obvious influence on transformation behavior. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ti–Ni-based shape memory alloys (SMAs) have been researched for nearly half of a century due to their unique thermo-mechanical properties associated with thermo-elastic martensitic transformations. There are several typical martensitic transformations in Ti–Ni-based SMAs. The martensitic transformation in solutiontreated near equi-atomic Ti–Ni SMAs takes place from the B2 to the B19 structure [1], while B2 → R → B19 two stage transformation always happens in Ti–Ni–Fe SMAs and aged Ni-rich Ti–Ni SMAs [2,3]. Ti–Ni–Cu SMAs with different compositions experience three kinds of martensitic transformations [4–6], including B2 → B19 , B2–B19–B19 and B2 → B19 . Actuator is a representative application of SMAs in the field of industry. When being applied as line actuators, Ti–Ni-based SMA wires are subjected to repeated thermal cycling under a constant load through the transformation range, which is generally called as thermo-mechanical cycling (TMC). Since various applications of SMAs are sensitive to processing procedures, it is of vital importance to investigate effects of composition, cold work, heat treatment and loading conditions on stabilization of shape memory effect upon TMC. Rong et al. [7] have found that annealing temperature has obvious effect on transformation temperatures and TMC fatigue life of Ti50 Ni40 Cu10 . Zadno et al. [8] have researched stability of recovery strain in cycling of Ti50 Ni40 Cu10 actuator wire, and Saikrishna
∗ Corresponding author. Tel.: +86 10 8224 1161; fax: +86 10 8224 0096. E-mail address: [email protected] (Y.F. Li). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.02.041
et al. [9] recommended that TMC at a particular stress level for 25 cycles stabilize the Ti50 Ni44.5 Cu5.5 wires for lower stress levels but reduce the subsequent recovery strain compared with no TMC. Miller and Lagoudas [10] have analyzed the influence of cold work and heat treatment on the transformation strain and plastic strain of Ti–Ni under constant applied stress. Moreover, it has been found that transformation temperatures of Ti–Ni-based SMAs experience huge changes in the initial number of cycles upon thermal cycling [11]. Most of the above research concentrated on the effect of cold work and heat treatment on the recovery strain, plastic strain and fatigue life of Ti–Ni–Cu SMAs upon TMC. To the author’s knowledge, there is no systematic study of the influence of heat treatment and TMC on recovery strain, fatigue life and transformation behavior of Ti–Ni SMAs. This information is the key to the application of SMAs on actuators, which need stable and larger recovery strain and long fatigue life. The focus of the present paper includes the following two aspects: the effects of heat treatment and TMC on recovery strain and fatigue life of Ti–Ni wires were investigated. On the other hand, transformation temperatures of annealing specimens experiencing different number of cycles were measured to research the influence of annealing time and TMC on transformation behavior.
2. Experimental procedure The alloy of nominal composition Ti49.8 Ni50.2 was prepared using a high frequency induction furnace. The as-melted ingot was hot forged, and then hot rolled to wire of diameter 8.5 mm. The wire was process by multi-step hot drawing and cold drawing in turn, the
Y.F. Li et al. / Materials Science and Engineering A 509 (2009) 8–13
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Fig. 1. Schematic of self-assembled setup used for thermo-mechanical cycling test.
final specimen was in the form of wire of diameter 0.105 mm and included 50% cold work. Specimens prepared for test were annealed at identical temperature 773 K for different times (5, 10, 15, and 30 min) in an evacuated silica tube followed by air cooling. The self-assembled setup used for testing recovery strain and fatigue life upon TMC is shown in Fig. 1. The length of specimen for each test was about 100 mm. The specimen was loaded with a pre-determined static load through a connecting insulator rod between the specimen and the load. Three proper constant loads were chosen in the present study based on the results of previous experiments, including 250, 280 and 310 MPa. An ample direct current was used for heating the specimen over than 373 K in 2 s, which made sure that the reverse martensitic transformation could be completed. The specimen was air-cooled down to room temperature in 4 s when the current was cut down, so a cycle is 6 s. The recovery strain of the specimen in each cycle was monitored through a displacement sensor of resolution 0.04 mm. In order to avoid random error that the inhomogeneity of specimens may bring, the recovery strain of the specimen in identical annealing state being subjected to certain constant load was measured three times. Fatigue life of specimens was counted through TMC tests. The transformation temperatures of original annealing specimens and specimens being subjected to different numbers of cycles were determined using four-probe electrical resistance measurements. The heating/cooling rates during the measurements were approx. 3 K/min. 3. Results 3.1. Recovery strain response of specimens annealed for different times upon TMC Fig. 2a shows the change of recovery strain upon TMC to failure against a constant load of 250 MPa. Recovery strain decreased sharply with increase in annealing time. Specimens annealed for 5 min can reach a stable recovery strain (defined as the recovery strain of specimen after 500 cycles) of 5.2%. Stable recovery strain decreased to 3.56% when annealing time was extended to 30 min. It should be noted that recovery strain experienced significant changes during the initial 200 cycles (Fig. 2b), which is obviously different from that were reported in other open literatures [8,9]. Specimens annealed for 5, 10 and 15 min respectively all reached maximum recovery strains (6, 6.24 and 6.2%, respectively) in the first cycle and minimum recovery strains (0.92, 3 and 4.88%,
Fig. 2. Recovery strain versus number of cycles upon TMC at 250 MPa of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min during (a) the whole fatigue life and (b) the initial 200 cycles.
respectively) in the second cycle. Subsequently, recovery strains increased to second peak values in a few number of cycles, which are slightly smaller than the first ones. The longer annealing time was, the sharper the increase was. However, specimens annealed for 30 min, showing the same trend recorded in the above mentioned literatures, obtained maximum (5.4%) in the first cycle and suffered a considerable reduction in a few cycles. Figs. 3 and 4 depict recovery strain of specimens as a function of number of cycles upon TMC at 280 and 310 MPa, respectively. It is clear that stable recovery strain upon TMC at 280 and 310 MPa also decreased with the increase of annealing time like the situation of 250 MPa. Fig. 2b demonstrates that only the recovery strain of specimens annealed for 5 min still experienced a maximum and a minimum in the first two cycles and increased to another maximum during a few number of cycles, and then decreased slowly, while the three others obtained maxima and decreased directly. When the constant load was elevated to 310 MPa (Fig. 3b), recovery strains of all the specimens decreased from maxima monotonically. 3.2. Fatigue life of specimens annealed for different times upon TMC TMC fatigue life of specimens annealed at 773 K for different times is illustrated in Fig. 5. Fatigue life in this bar chart was calculated as a mean value of three times repeated TMC tests of each
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Fig. 3. Recovery strain versus number of cycles upon TMC at 280 MPa of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min during (a) the whole fatigue life and (b) the initial 200 cycles.
specimen. For identical annealing time (10, 15 and 30 min), fatigue life increased slightly with the elevation of constant load. However, specimen annealed for 5 min showed almost the same fatigue life at every constant load levels. For identical constant load (280 and 310 MPa), fatigue life increased with the increment of annealing time. Being subjected to TMC at 250 MPa, specimens annealed for 5, 10 and 15 min did not show behavior much difference in fatigue life, while the fatigue life of specimens annealed for 30 min was much higher than the three others.
Fig. 4. Recovery strain versus number of cycles upon TMC at 310 MPa of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min during (a) the whole fatigue life and (b) the initial 200 cycles.
Specimen annealed for 10 min upon TMC at 250 MPa was chosen as a representative to investigate the effect of TMC on transformation temperatures. As evident from Fig. 8, R transformation became inconspicuous with increasing N. As was not notated since it was difficult to find an inflexion on the electrical resistance versus tem-
3.3. Changes of transformation temperatures upon TMC Fig. 6 shows the electrical resistance versus temperature curves for specimens annealed at 773 K for different times. It was confirmed that annealing specimens all experienced B2 → R → B19 two stage martensitic transformations and B19 → B2 reverse martensitic transformation. The notation for determining the transformation temperatures on the electrical resistance versus temperature curves followed the example established by Wu et al. [12]. The transformation temperatures were indicated in Fig. 7. It is clear that Ms and Mf increased with the increase of annealing time. At the same time, the value of (Mf − Ms ) kept a constant of about 6 K. Af obtained a maximum and then decreased slightly. However, As and TR did not show obvious changes for different annealing times.
Fig. 5. Fatigue life of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min upon TMC at different constant loads.
Y.F. Li et al. / Materials Science and Engineering A 509 (2009) 8–13
Fig. 6. Effect of annealing time on the electrical resistance versus temperature curves of Ti49.8 Ni50.2 SMA specimens annealed at 773 K. The arrows indicate Af , As , TR , Ms and Mf .
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Fig. 9. Effect of TMC at 250 MPa on the transformation temperatures of specimens annealed at 773 K for 10 min.
perature curves during heating. Changes of Af , Ms and Mf during TMC are depicted in Fig. 9. Both Ms and Mf decreased slightly during the first 10 cycles, subsequently increased with increasing N, and then reached saturated values after 100 cycles. Af did not show a monotonous variation, but it still increased by 13 K after 500 cycles. 4. Discussion 4.1. Effect of annealing time on recovery strain upon TMC
Fig. 7. Effect of annealing time on the transformation temperatures of Ti49.8 Ni50.2 SMA specimens annealed at 773 K.
Fig. 8. Effect of TMC at 250 MPa on the electrical resistance versus temperature curves of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 10 min. N = number of cycles. The arrows indicate Af , Ms and Mf .
Chouf et al. [13] introduced that different temperatures-times treatments had the same effect on transformation temperatures, electrical resistivity and thermo-mechanical properties. Undoubtedly, annealing time is considered to be an important variant in heat treatment. This point was confirmed by the data from Figs. 1–3. All the specimens used in this study underwent 50% cold work before heat treatment, which induced much deformation textures. Liu et al. [14] have indicated that the annealing treatment has no significant influence on the textures in Ti–Ni alloys. Moreover, deformation textures in polycrystalline Ti–Ni alloys, which were caused by cold work, influenced shape memory strain to a great extent [15,16]. Although specimens experienced different annealing, deformation textures existing in them kept almost the same state, and maximum recovery strains that they bring during transformations should not have much discrepancy. Therefore, deformation textures could not be utilized to explain how annealing time affected recovery strain. Heat treatment of Ti–Ni alloys is always associated with the recovery and recrystallization processes [7,17]. According to Khelfaoui’s study [17], for annealing temperature above 723 K, recrystallize will occur, rapidly reducing the dislocation density and lattice distortion by cold drawn working. Specimen annealed at 773 K for increasing time in this study underwent increasing percentage of crystallization. Specimens annealed for 5 min just finished recovering and started to crystallize; much dislocations and lattice distortion remained in specimens were believed to be responsible for the high critical stress of them, which means they could resist plastic deformation well. Therefore, small plastic strain and large recovery strain developed upon TMC. On the contrary, specimen annealed for 30 min had considerable crystallization; little dislocations and lattice distortion remained so that specimens slipped on different slip planes easily to lead a larger plastic strain and a smaller recovery strain upon TMC. Above all, the different percentage of crystallization of specimens, caused by different
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Fig. 10. Recovery strain versus number of cycles upon TMC at different constant loads of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for (a) 5 min, (b) 10 min, (c) 15 min and (d) 30 min during the initial 200 cycles.
annealing, is the main reason for the difference of recovery strains between these four kinds of specimens. 4.2. Effect of TMC on recovery strain As Fig. 7 shows, Stable Ms of specimens experiencing 500 TMC cycles at 250 MPa, which was the highest, was still 16 K below the ambient temperature (298 K). Hence, specimens would only undergo B2 → R transformation that could bring about maximum transformation strain of 0.8% [18] if cooled to room temperature without constant loads. In the present study, constant loads lead stress-induced martensitic transformations above room temperature during the TMC, which is the resource of recovery strain during the heating process. Recovery strains of specimens annealed for identical time at different constant loads are shown in Fig. 10 in order to analyze the effect of TMC. Stress-induced martensitic transformations take place when being subjected to constant loads above certain critical stress. Critical stress increases when the deformation temperature (above Ms ) is elevated [19]. Stress-induced martensite variants tend to become preferential orientation. If the constant load is high, preferential orientation of stress-induced martensite variants is uniform, which contributes large recovery strain. As Fig. 10a shows, specimens obtained slightly increasing maximum recovery strain with the increase of constant load in the first cycle. Although 250 MPa is higher than critical stress that could induce martensites, it is still a relatively low stress
so that stress-induced martensitic transformation is believed to need more response time than 4 s (set cooling time) to finish. The same specimens still tested at 250 MPa, extending the cooling time to 15 s, confirmed the above assumption. Recovery strain versus number of cycles curve was the same as that of 310 MPa (Fig. 1). Therefore, in the set cooling stage (4 s), 250 MPa only produced self-accommodating martensites that did not developed much preferential orientation martensites; consequently, a minimum recovery strain occurs in the second cycle. Critical stress that could induce martensite is believed to decrease with the number of cycles. The constant load could induce more preferred martensite variants during the cooling and then bring larger recovery strain during the heating. After a few cycles, stress-induced martensitic transformation could be completed in the set cooling time, and the recovery strain experienced another maximum. On the contrary, TMC at 310 MPa needs less time than 4 s to finish stress-induced martensitic transformation, so the second cycle could reach a recovery strain slightly smaller that of the first one. The increasing load caused generation of more dislocations and in turn stabilized more martensites. Moreover, martensite stabilized by dislocations increases with the number of cycles. The increase in the amount of stabilized martensite resulted in the decrease in the recovery strain since less amount of martensite was available for providing transformation strain [9]. After 500 cycles, recovery strain of all specimens underwent a slightly decrease (about 0.2%) until cycled to failure (Figs. 2–4). Under the circum-
Y.F. Li et al. / Materials Science and Engineering A 509 (2009) 8–13
stances, recovery strain was approximately thought to reach a steady state. 4.3. Effect of annealing time on TMC fatigue life A large amount of dislocations and lattice distortion were removed by considerable crystallization of specimens through heat treatment. Critical slip stress decreased with the increasing annealing time. Low critical slip stress made specimens easily occur plastic deformation and introduced new dislocations. Compared with dislocations in original specimens after heat treatment, the density of new dislocations induced by TMC was much lower since the constant load was much smaller than the stress that cold work exerted on specimens. When the density of dislocations obtains a saturated level, dislocations on different slip planes move and intersect with each other, a crack will be formed. The crack propagates through the further piling up of dislocations and finally caused fracture. As Fig. 10 depicts, Specimens annealed for 5 min, undergoing relatively low percentage of crystallization, possess the highest dislocation density before TMC. Thus they could develop a saturated dislocation structure rapidly and in turn behavior the lowest fatigue life. On the contrary, specimens annealed for 30 min obtain the highest fatigue life at every constant load level. 4.4. Effect of annealing time and TMC on transformation temperatures Recrystallization reduced the dislocation density and lattice distortion, thus the transformation temperatures increased slightly with the annealing time. The extension of annealing time does not restrain B2 → R transformation (Fig. 6). The present measurements (Fig. 8) indicated that TMC influenced both B2 → R and R → B19 transformation behaviors. The stress field caused by the plastic deformation during TMC decreased the transformation temperatures slightly in the initial 10 cycles. Compared with thermal cycling without load [11,20], TMC was believed to have a different effect on the transformation temperatures. Dislocations induced by TMC assisted in the nucleation of the preferred variants of stress directed thermo-elastic martensite during cooling [7] as Fig. 9 shows, both Ms and Mf increase after 10 cycles; on the other hand, dislocation structure developed during TMC would be an obstacle in the growth of the martensite variants and extend the temperature range of martensitic transformation. The value of (Ms − Mf ) increased from the original 5 K (N = 0) to 18 K (N = 100) and kept constant in the following cycles. The increase in Ms with the number of cycles, causing B2 → R and R → B19 transformations to overlap, was in accord with results observed in another literature [21]. 5. Conclusion Based on the experimental results and analysis above, the following conclusions can be drawn:
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1. Recovery strain upon TMC was measured at three predetermined constant loads respectively. When the constant load is much higher than the critical stress of certain specimens, the recovery strain decreased sharply during the initial cycles. After 500 cycles, specimens were approximately thought to reach stable recovery strain. Stable recovery strain decreased with the increasing constant load. However, if the constant load is relatively low, specimens could not complete stress-induced martensitic transformation in the set cooling time at the beginning of TMC. Consequently, an abnormal change of recovery strain occurred. 2. There was certain discrepancy between recovery strain of specimens annealed at 773 K for different times (5, 10, 15 and 30 min) upon TMC. Specimens annealed for longer time obtained a smaller stable recovery strain. 3. Fatigue life of specimens tested at identical constant load (280 and 310 MPa) increased gradually with the increase of annealing time. Moreover, specimens annealed for 10, 15 and 30 min respectively obtained larger fatigue life with the increasing constant load. 4. Ms and Mf increased monotonically with the extension of annealing time, and B2 → R transformation was not restrained. During TMC, Ms and Mf decreased slightly in the first 10 cycles and increased to a saturated value after 100 cycles. B2 → R and R → B19 transformations gradually overlapped with increasing cycles. References [1] H. Nakayama, K. Tsuchiya, M. Umemoto, Scripta Mater. 44 (2001) 1781–1785. [2] C.M. Hwang, C.M. Wayman, Scripta Mater. 17 (1983) 1345–1350. [3] J.I. Kim, Y. Liu, S. Miyazaki, Acta Mater. 52 (2004) 487–499. [4] T.H. Nam, T. Saburi, K. Shimizu, Mater. Trans. JIM 31 (1990) 262–269. [5] T.H. Nam, T. Saburi, K. Shimizu, Mater. Trans. JIM 31 (1990) 814–820. [6] J.H. Lee, T.H. Nam, H.J. Ahn, Y.W. Kim, Mater. Sci. Eng. A 438–440 (2006) 691–694. [7] L.J. Rong, D.A. Miller, D.C. Lagoudas, Mater. Sci. Forum 394/395 (2002) 329–332. [8] R. Zadno, J.W. Simpson, M.A. Imran, Stability in cycling of Ni–Ti–Cu actuator wire, in: Proceedings of the 1994 conference on Shape Memory and Superelastic Technologies, California, 1994, pp. 317–321. [9] C.N. Saikrishna, K. Venkata Ramaiah, S.K. Bhaumik, Mater. Sci. Eng. A 428 (2006) 217–224. [10] D.A. Miller, D.C. Lagoudas, Mater. Sci. Eng. A 308 (2001) 161–175. [11] S. Miyazaki, Y. Igo, K. Otusuka, Acta Mater. 34 (10) (1986) 2045–2051. [12] S.K. Wu, H.C. Lin, T.Y. Lin, Mater. Sci. Eng. A 438–440 (2006) 536–539. [13] S. Chouf, M. Morin, S. Belkahla, G. Guenin, Mater. Sci. Eng. A 438–440 (2006) 671–674. [14] Y. Liu, Z.L. Xie, J. Van Humbeeck, L. Delaey, Acta Metall. 47 (1999) 645–660. [15] H. Inoue, N. Miwa, N. Inakazu, Acta Mater. 44 (1996) 4825–4834. [16] K. Kitamura, S. Miyazaki, H. Iwai, M. Kohl, Mater. Sci. Eng. A 273–275 (1999) 758–762. [17] F. Khelfaoui, G. Guenin, Mater. Sci. Eng. A 355 (2003) 292–298. [18] S. Miyazaki, K. Otsuka, Metall. Trans. 17A (1986) 53–63. [19] S. Miyazaki, Y. Ohmi, K. Otsuka, Y. Suzuki, J. Phys. (Suppl. 12) (1982), C4-255-261. [20] P.G. Mccormick, Y. Liu, Acta Mater. 42 (1994) 2407–2413. [21] G.B. Stachowiak, P.G. Mccormick, Acta Mater. 36 (1988) 291–297.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Thermo-mechanical cyclic transformation behavior of Ti–Ni shape memory alloy wire Y.F. Li ∗ , X.J. Mi, J. Tan, B.D. Gao State Key Laboratory for Fabrication & Processing of Nonferrous Metals, General Institute of Non-ferrous Metals, No. 2 Xinjiekou Wai Str., Beijing 100088, PR China
a r t i c l e
i n f o
Article history: Received 16 July 2008 Received in revised form 3 December 2008 Accepted 18 February 2009 Keywords: Ti–Ni shape memory alloy Thermo-mechanical cycling Recovery strain Fatigue life Transformation temperatures
a b s t r a c t The present paper deals with recovery strain and fatigue life of Ti49.8 Ni50.2 shape memory alloy (SMA) wire on thermo-mechanical cycling (TMC). Recovery strain of SMA wire with several heat treatments shows different changes when being subjected to TMC at a series of constant loads. These changes are significant in the initial 200 cycles and recovery strain tends to reach a steady state in the further cycling. The effect of heat treatment on the fatigue life is analyzed, the SMA wire with shorter heat treatment exhibits lower fatigue life while higher recovery strain. Transformation temperatures of SMA wire on TMC is investigated by electrical resistance measurements. It was found that both annealing time and TMC has obvious influence on transformation behavior. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ti–Ni-based shape memory alloys (SMAs) have been researched for nearly half of a century due to their unique thermo-mechanical properties associated with thermo-elastic martensitic transformations. There are several typical martensitic transformations in Ti–Ni-based SMAs. The martensitic transformation in solutiontreated near equi-atomic Ti–Ni SMAs takes place from the B2 to the B19 structure [1], while B2 → R → B19 two stage transformation always happens in Ti–Ni–Fe SMAs and aged Ni-rich Ti–Ni SMAs [2,3]. Ti–Ni–Cu SMAs with different compositions experience three kinds of martensitic transformations [4–6], including B2 → B19 , B2–B19–B19 and B2 → B19 . Actuator is a representative application of SMAs in the field of industry. When being applied as line actuators, Ti–Ni-based SMA wires are subjected to repeated thermal cycling under a constant load through the transformation range, which is generally called as thermo-mechanical cycling (TMC). Since various applications of SMAs are sensitive to processing procedures, it is of vital importance to investigate effects of composition, cold work, heat treatment and loading conditions on stabilization of shape memory effect upon TMC. Rong et al. [7] have found that annealing temperature has obvious effect on transformation temperatures and TMC fatigue life of Ti50 Ni40 Cu10 . Zadno et al. [8] have researched stability of recovery strain in cycling of Ti50 Ni40 Cu10 actuator wire, and Saikrishna
∗ Corresponding author. Tel.: +86 10 8224 1161; fax: +86 10 8224 0096. E-mail address: [email protected] (Y.F. Li). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.02.041
et al. [9] recommended that TMC at a particular stress level for 25 cycles stabilize the Ti50 Ni44.5 Cu5.5 wires for lower stress levels but reduce the subsequent recovery strain compared with no TMC. Miller and Lagoudas [10] have analyzed the influence of cold work and heat treatment on the transformation strain and plastic strain of Ti–Ni under constant applied stress. Moreover, it has been found that transformation temperatures of Ti–Ni-based SMAs experience huge changes in the initial number of cycles upon thermal cycling [11]. Most of the above research concentrated on the effect of cold work and heat treatment on the recovery strain, plastic strain and fatigue life of Ti–Ni–Cu SMAs upon TMC. To the author’s knowledge, there is no systematic study of the influence of heat treatment and TMC on recovery strain, fatigue life and transformation behavior of Ti–Ni SMAs. This information is the key to the application of SMAs on actuators, which need stable and larger recovery strain and long fatigue life. The focus of the present paper includes the following two aspects: the effects of heat treatment and TMC on recovery strain and fatigue life of Ti–Ni wires were investigated. On the other hand, transformation temperatures of annealing specimens experiencing different number of cycles were measured to research the influence of annealing time and TMC on transformation behavior.
2. Experimental procedure The alloy of nominal composition Ti49.8 Ni50.2 was prepared using a high frequency induction furnace. The as-melted ingot was hot forged, and then hot rolled to wire of diameter 8.5 mm. The wire was process by multi-step hot drawing and cold drawing in turn, the
Y.F. Li et al. / Materials Science and Engineering A 509 (2009) 8–13
9
Fig. 1. Schematic of self-assembled setup used for thermo-mechanical cycling test.
final specimen was in the form of wire of diameter 0.105 mm and included 50% cold work. Specimens prepared for test were annealed at identical temperature 773 K for different times (5, 10, 15, and 30 min) in an evacuated silica tube followed by air cooling. The self-assembled setup used for testing recovery strain and fatigue life upon TMC is shown in Fig. 1. The length of specimen for each test was about 100 mm. The specimen was loaded with a pre-determined static load through a connecting insulator rod between the specimen and the load. Three proper constant loads were chosen in the present study based on the results of previous experiments, including 250, 280 and 310 MPa. An ample direct current was used for heating the specimen over than 373 K in 2 s, which made sure that the reverse martensitic transformation could be completed. The specimen was air-cooled down to room temperature in 4 s when the current was cut down, so a cycle is 6 s. The recovery strain of the specimen in each cycle was monitored through a displacement sensor of resolution 0.04 mm. In order to avoid random error that the inhomogeneity of specimens may bring, the recovery strain of the specimen in identical annealing state being subjected to certain constant load was measured three times. Fatigue life of specimens was counted through TMC tests. The transformation temperatures of original annealing specimens and specimens being subjected to different numbers of cycles were determined using four-probe electrical resistance measurements. The heating/cooling rates during the measurements were approx. 3 K/min. 3. Results 3.1. Recovery strain response of specimens annealed for different times upon TMC Fig. 2a shows the change of recovery strain upon TMC to failure against a constant load of 250 MPa. Recovery strain decreased sharply with increase in annealing time. Specimens annealed for 5 min can reach a stable recovery strain (defined as the recovery strain of specimen after 500 cycles) of 5.2%. Stable recovery strain decreased to 3.56% when annealing time was extended to 30 min. It should be noted that recovery strain experienced significant changes during the initial 200 cycles (Fig. 2b), which is obviously different from that were reported in other open literatures [8,9]. Specimens annealed for 5, 10 and 15 min respectively all reached maximum recovery strains (6, 6.24 and 6.2%, respectively) in the first cycle and minimum recovery strains (0.92, 3 and 4.88%,
Fig. 2. Recovery strain versus number of cycles upon TMC at 250 MPa of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min during (a) the whole fatigue life and (b) the initial 200 cycles.
respectively) in the second cycle. Subsequently, recovery strains increased to second peak values in a few number of cycles, which are slightly smaller than the first ones. The longer annealing time was, the sharper the increase was. However, specimens annealed for 30 min, showing the same trend recorded in the above mentioned literatures, obtained maximum (5.4%) in the first cycle and suffered a considerable reduction in a few cycles. Figs. 3 and 4 depict recovery strain of specimens as a function of number of cycles upon TMC at 280 and 310 MPa, respectively. It is clear that stable recovery strain upon TMC at 280 and 310 MPa also decreased with the increase of annealing time like the situation of 250 MPa. Fig. 2b demonstrates that only the recovery strain of specimens annealed for 5 min still experienced a maximum and a minimum in the first two cycles and increased to another maximum during a few number of cycles, and then decreased slowly, while the three others obtained maxima and decreased directly. When the constant load was elevated to 310 MPa (Fig. 3b), recovery strains of all the specimens decreased from maxima monotonically. 3.2. Fatigue life of specimens annealed for different times upon TMC TMC fatigue life of specimens annealed at 773 K for different times is illustrated in Fig. 5. Fatigue life in this bar chart was calculated as a mean value of three times repeated TMC tests of each
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Fig. 3. Recovery strain versus number of cycles upon TMC at 280 MPa of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min during (a) the whole fatigue life and (b) the initial 200 cycles.
specimen. For identical annealing time (10, 15 and 30 min), fatigue life increased slightly with the elevation of constant load. However, specimen annealed for 5 min showed almost the same fatigue life at every constant load levels. For identical constant load (280 and 310 MPa), fatigue life increased with the increment of annealing time. Being subjected to TMC at 250 MPa, specimens annealed for 5, 10 and 15 min did not show behavior much difference in fatigue life, while the fatigue life of specimens annealed for 30 min was much higher than the three others.
Fig. 4. Recovery strain versus number of cycles upon TMC at 310 MPa of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min during (a) the whole fatigue life and (b) the initial 200 cycles.
Specimen annealed for 10 min upon TMC at 250 MPa was chosen as a representative to investigate the effect of TMC on transformation temperatures. As evident from Fig. 8, R transformation became inconspicuous with increasing N. As was not notated since it was difficult to find an inflexion on the electrical resistance versus tem-
3.3. Changes of transformation temperatures upon TMC Fig. 6 shows the electrical resistance versus temperature curves for specimens annealed at 773 K for different times. It was confirmed that annealing specimens all experienced B2 → R → B19 two stage martensitic transformations and B19 → B2 reverse martensitic transformation. The notation for determining the transformation temperatures on the electrical resistance versus temperature curves followed the example established by Wu et al. [12]. The transformation temperatures were indicated in Fig. 7. It is clear that Ms and Mf increased with the increase of annealing time. At the same time, the value of (Mf − Ms ) kept a constant of about 6 K. Af obtained a maximum and then decreased slightly. However, As and TR did not show obvious changes for different annealing times.
Fig. 5. Fatigue life of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 5, 10, 15, and 30 min upon TMC at different constant loads.
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Fig. 6. Effect of annealing time on the electrical resistance versus temperature curves of Ti49.8 Ni50.2 SMA specimens annealed at 773 K. The arrows indicate Af , As , TR , Ms and Mf .
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Fig. 9. Effect of TMC at 250 MPa on the transformation temperatures of specimens annealed at 773 K for 10 min.
perature curves during heating. Changes of Af , Ms and Mf during TMC are depicted in Fig. 9. Both Ms and Mf decreased slightly during the first 10 cycles, subsequently increased with increasing N, and then reached saturated values after 100 cycles. Af did not show a monotonous variation, but it still increased by 13 K after 500 cycles. 4. Discussion 4.1. Effect of annealing time on recovery strain upon TMC
Fig. 7. Effect of annealing time on the transformation temperatures of Ti49.8 Ni50.2 SMA specimens annealed at 773 K.
Fig. 8. Effect of TMC at 250 MPa on the electrical resistance versus temperature curves of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for 10 min. N = number of cycles. The arrows indicate Af , Ms and Mf .
Chouf et al. [13] introduced that different temperatures-times treatments had the same effect on transformation temperatures, electrical resistivity and thermo-mechanical properties. Undoubtedly, annealing time is considered to be an important variant in heat treatment. This point was confirmed by the data from Figs. 1–3. All the specimens used in this study underwent 50% cold work before heat treatment, which induced much deformation textures. Liu et al. [14] have indicated that the annealing treatment has no significant influence on the textures in Ti–Ni alloys. Moreover, deformation textures in polycrystalline Ti–Ni alloys, which were caused by cold work, influenced shape memory strain to a great extent [15,16]. Although specimens experienced different annealing, deformation textures existing in them kept almost the same state, and maximum recovery strains that they bring during transformations should not have much discrepancy. Therefore, deformation textures could not be utilized to explain how annealing time affected recovery strain. Heat treatment of Ti–Ni alloys is always associated with the recovery and recrystallization processes [7,17]. According to Khelfaoui’s study [17], for annealing temperature above 723 K, recrystallize will occur, rapidly reducing the dislocation density and lattice distortion by cold drawn working. Specimen annealed at 773 K for increasing time in this study underwent increasing percentage of crystallization. Specimens annealed for 5 min just finished recovering and started to crystallize; much dislocations and lattice distortion remained in specimens were believed to be responsible for the high critical stress of them, which means they could resist plastic deformation well. Therefore, small plastic strain and large recovery strain developed upon TMC. On the contrary, specimen annealed for 30 min had considerable crystallization; little dislocations and lattice distortion remained so that specimens slipped on different slip planes easily to lead a larger plastic strain and a smaller recovery strain upon TMC. Above all, the different percentage of crystallization of specimens, caused by different
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Fig. 10. Recovery strain versus number of cycles upon TMC at different constant loads of Ti49.8 Ni50.2 SMA specimens annealed at 773 K for (a) 5 min, (b) 10 min, (c) 15 min and (d) 30 min during the initial 200 cycles.
annealing, is the main reason for the difference of recovery strains between these four kinds of specimens. 4.2. Effect of TMC on recovery strain As Fig. 7 shows, Stable Ms of specimens experiencing 500 TMC cycles at 250 MPa, which was the highest, was still 16 K below the ambient temperature (298 K). Hence, specimens would only undergo B2 → R transformation that could bring about maximum transformation strain of 0.8% [18] if cooled to room temperature without constant loads. In the present study, constant loads lead stress-induced martensitic transformations above room temperature during the TMC, which is the resource of recovery strain during the heating process. Recovery strains of specimens annealed for identical time at different constant loads are shown in Fig. 10 in order to analyze the effect of TMC. Stress-induced martensitic transformations take place when being subjected to constant loads above certain critical stress. Critical stress increases when the deformation temperature (above Ms ) is elevated [19]. Stress-induced martensite variants tend to become preferential orientation. If the constant load is high, preferential orientation of stress-induced martensite variants is uniform, which contributes large recovery strain. As Fig. 10a shows, specimens obtained slightly increasing maximum recovery strain with the increase of constant load in the first cycle. Although 250 MPa is higher than critical stress that could induce martensites, it is still a relatively low stress
so that stress-induced martensitic transformation is believed to need more response time than 4 s (set cooling time) to finish. The same specimens still tested at 250 MPa, extending the cooling time to 15 s, confirmed the above assumption. Recovery strain versus number of cycles curve was the same as that of 310 MPa (Fig. 1). Therefore, in the set cooling stage (4 s), 250 MPa only produced self-accommodating martensites that did not developed much preferential orientation martensites; consequently, a minimum recovery strain occurs in the second cycle. Critical stress that could induce martensite is believed to decrease with the number of cycles. The constant load could induce more preferred martensite variants during the cooling and then bring larger recovery strain during the heating. After a few cycles, stress-induced martensitic transformation could be completed in the set cooling time, and the recovery strain experienced another maximum. On the contrary, TMC at 310 MPa needs less time than 4 s to finish stress-induced martensitic transformation, so the second cycle could reach a recovery strain slightly smaller that of the first one. The increasing load caused generation of more dislocations and in turn stabilized more martensites. Moreover, martensite stabilized by dislocations increases with the number of cycles. The increase in the amount of stabilized martensite resulted in the decrease in the recovery strain since less amount of martensite was available for providing transformation strain [9]. After 500 cycles, recovery strain of all specimens underwent a slightly decrease (about 0.2%) until cycled to failure (Figs. 2–4). Under the circum-
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stances, recovery strain was approximately thought to reach a steady state. 4.3. Effect of annealing time on TMC fatigue life A large amount of dislocations and lattice distortion were removed by considerable crystallization of specimens through heat treatment. Critical slip stress decreased with the increasing annealing time. Low critical slip stress made specimens easily occur plastic deformation and introduced new dislocations. Compared with dislocations in original specimens after heat treatment, the density of new dislocations induced by TMC was much lower since the constant load was much smaller than the stress that cold work exerted on specimens. When the density of dislocations obtains a saturated level, dislocations on different slip planes move and intersect with each other, a crack will be formed. The crack propagates through the further piling up of dislocations and finally caused fracture. As Fig. 10 depicts, Specimens annealed for 5 min, undergoing relatively low percentage of crystallization, possess the highest dislocation density before TMC. Thus they could develop a saturated dislocation structure rapidly and in turn behavior the lowest fatigue life. On the contrary, specimens annealed for 30 min obtain the highest fatigue life at every constant load level. 4.4. Effect of annealing time and TMC on transformation temperatures Recrystallization reduced the dislocation density and lattice distortion, thus the transformation temperatures increased slightly with the annealing time. The extension of annealing time does not restrain B2 → R transformation (Fig. 6). The present measurements (Fig. 8) indicated that TMC influenced both B2 → R and R → B19 transformation behaviors. The stress field caused by the plastic deformation during TMC decreased the transformation temperatures slightly in the initial 10 cycles. Compared with thermal cycling without load [11,20], TMC was believed to have a different effect on the transformation temperatures. Dislocations induced by TMC assisted in the nucleation of the preferred variants of stress directed thermo-elastic martensite during cooling [7] as Fig. 9 shows, both Ms and Mf increase after 10 cycles; on the other hand, dislocation structure developed during TMC would be an obstacle in the growth of the martensite variants and extend the temperature range of martensitic transformation. The value of (Ms − Mf ) increased from the original 5 K (N = 0) to 18 K (N = 100) and kept constant in the following cycles. The increase in Ms with the number of cycles, causing B2 → R and R → B19 transformations to overlap, was in accord with results observed in another literature [21]. 5. Conclusion Based on the experimental results and analysis above, the following conclusions can be drawn:
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1. Recovery strain upon TMC was measured at three predetermined constant loads respectively. When the constant load is much higher than the critical stress of certain specimens, the recovery strain decreased sharply during the initial cycles. After 500 cycles, specimens were approximately thought to reach stable recovery strain. Stable recovery strain decreased with the increasing constant load. However, if the constant load is relatively low, specimens could not complete stress-induced martensitic transformation in the set cooling time at the beginning of TMC. Consequently, an abnormal change of recovery strain occurred. 2. There was certain discrepancy between recovery strain of specimens annealed at 773 K for different times (5, 10, 15 and 30 min) upon TMC. Specimens annealed for longer time obtained a smaller stable recovery strain. 3. Fatigue life of specimens tested at identical constant load (280 and 310 MPa) increased gradually with the increase of annealing time. Moreover, specimens annealed for 10, 15 and 30 min respectively obtained larger fatigue life with the increasing constant load. 4. Ms and Mf increased monotonically with the extension of annealing time, and B2 → R transformation was not restrained. During TMC, Ms and Mf decreased slightly in the first 10 cycles and increased to a saturated value after 100 cycles. B2 → R and R → B19 transformations gradually overlapped with increasing cycles. References [1] H. Nakayama, K. Tsuchiya, M. Umemoto, Scripta Mater. 44 (2001) 1781–1785. [2] C.M. Hwang, C.M. Wayman, Scripta Mater. 17 (1983) 1345–1350. [3] J.I. Kim, Y. Liu, S. Miyazaki, Acta Mater. 52 (2004) 487–499. [4] T.H. Nam, T. Saburi, K. Shimizu, Mater. Trans. JIM 31 (1990) 262–269. [5] T.H. Nam, T. Saburi, K. Shimizu, Mater. Trans. JIM 31 (1990) 814–820. [6] J.H. Lee, T.H. Nam, H.J. Ahn, Y.W. Kim, Mater. Sci. Eng. A 438–440 (2006) 691–694. [7] L.J. Rong, D.A. Miller, D.C. Lagoudas, Mater. Sci. Forum 394/395 (2002) 329–332. [8] R. Zadno, J.W. Simpson, M.A. Imran, Stability in cycling of Ni–Ti–Cu actuator wire, in: Proceedings of the 1994 conference on Shape Memory and Superelastic Technologies, California, 1994, pp. 317–321. [9] C.N. Saikrishna, K. Venkata Ramaiah, S.K. Bhaumik, Mater. Sci. Eng. A 428 (2006) 217–224. [10] D.A. Miller, D.C. Lagoudas, Mater. Sci. Eng. A 308 (2001) 161–175. [11] S. Miyazaki, Y. Igo, K. Otusuka, Acta Mater. 34 (10) (1986) 2045–2051. [12] S.K. Wu, H.C. Lin, T.Y. Lin, Mater. Sci. Eng. A 438–440 (2006) 536–539. [13] S. Chouf, M. Morin, S. Belkahla, G. Guenin, Mater. Sci. Eng. A 438–440 (2006) 671–674. [14] Y. Liu, Z.L. Xie, J. Van Humbeeck, L. Delaey, Acta Metall. 47 (1999) 645–660. [15] H. Inoue, N. Miwa, N. Inakazu, Acta Mater. 44 (1996) 4825–4834. [16] K. Kitamura, S. Miyazaki, H. Iwai, M. Kohl, Mater. Sci. Eng. A 273–275 (1999) 758–762. [17] F. Khelfaoui, G. Guenin, Mater. Sci. Eng. A 355 (2003) 292–298. [18] S. Miyazaki, K. Otsuka, Metall. Trans. 17A (1986) 53–63. [19] S. Miyazaki, Y. Ohmi, K. Otsuka, Y. Suzuki, J. Phys. (Suppl. 12) (1982), C4-255-261. [20] P.G. Mccormick, Y. Liu, Acta Mater. 42 (1994) 2407–2413. [21] G.B. Stachowiak, P.G. Mccormick, Acta Mater. 36 (1988) 291–297.