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The effect of Pd content on microstructure and shape-memory properties of Ti–Ni–Pd–Cu alloys
Materials Science & Engineering A 602 (2014) 19–24
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of Pd content on microstructure and shape-memory properties of Ti–Ni–Pd–Cu alloys Masamine Imahashi a, M. Imran Khan a, Hee Young Kim a,n, Shuichi Miyazaki a,b,nn a b
Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 24 December 2013 Accepted 12 February 2014 Available online 20 February 2014
The effect of Pd content on the microstructure and high-temperature shape memory properties of Ti–Ni– Pd–Cu alloys was investigated. The increase in Pd content led to the increase of transformation temperatures. The volume fraction of two types of precipitates (i.e. TiPdCu and Ti2Pd) also increased with increasing Pd content. Although the formation of these precipitates resulted in an increased resistance against the plastic deformation, the amount of recovery strain decreased during thermal cycling tests at various stress levels. By adjusting the heat treatment time, a high transformation temperature and large amount of recovery strain could be achieved while keeping the enough resistance against the plastic deformation. & 2014 Elsevier B.V. All rights reserved.
Keywords: Shape memory alloys Martensitic transformations Precipitation Thermomechanical processing
1. Introduction Ti–Ni shape memory alloys are well known as excellent superelastic and shape memory materials and have been increasingly used in various fields of industry such as home appliances, medical devices and electronic devices [1,2]. Recently high temperature shape memory actuators for power generation, electronic devices, space exploration and automotive applications have attracted considerable research interest [3–22]. However, binary Ti–Ni alloys are limited to use below 373 K because of their low martensitic transformation temperatures [2,23]. Ti–Ni–Pd alloys have received significant research attention because they offer excellent combination of high transformation temperature, small hysteresis, adequate workability and large recovery strain [24–28]. In these alloys transformation temperatures can be tailored in the range from 373 K to 773 K by changing Pd content from 20 to 50 mol% [29]. However, at high temperatures these materials face a serious threat from thermally driven mechanisms, i. e. dimensional instability due to creep deformation, recovery and recrystallization processes and transformation induced plasticity [3,30–34]. For this reason, recent researches in Ti–Ni–Pd high temperature shape memory alloys have been focusing on the improvement of their functional and dimensional stability at high temperatures. Many techniques have been applied to improve the
n
Corresponding author. Tel./fax: þ 81 29 853 6942. Corresponding author at: Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan. Tel./fax: þ 81 29 853 5283. E-mail addresses: [email protected] (H.Y. Kim), [email protected] (S. Miyazaki). nn
http://dx.doi.org/10.1016/j.msea.2014.02.038 0921-5093 & 2014 Elsevier B.V. All rights reserved.
high temperature performance of Ti–Ni–Pd alloys including thermomechanical treatment, aging, training, annealing after sever plastic deformation and quaternary alloying additions [35–40]. In our previous research it was found that by adding Cu to a Ti–Ni–Pd ternary alloy, very high densities of nano-scale precipitates of TiPdCu and Ti2Pd were preferentially formed at heterogeneous nucleation sites provided by deformation-induced defects as a result of spinodal type decomposition [41]. These two types of precipitates increased the resistance against the creep and transformation induced plasticity. In addition, these precipitates seemed to delay the recovery and recrystallization type of softening processes [42]. Precipitation of these two types of precipitates largely depends on Cu content and these precipitates were found to be stable at temperatures 4773 K. However, the high densities of these precipitates decreased the martensitic transformation temperatures mainly because of the increase in Ni content of the matrix. It is still unknown that whether these precipitates are also effective at even higher temperatures in terms of their resistance against the plastic deformation and creep. In the present study, the effect of Pd content on the microstructure and shape memory properties of Ti–Ni–Pd–Cu alloys was investigated. In addition, the annealing time was optimized for the purpose of improving the shape memory properties.
2. Experimental procedure Ti50Ni45 xPdxCu5 (x ¼25, 30, 35) alloys were fabricated by the Ar-arc melting method. Hereafter each alloy is called by its Pd content such as 25Pd, 30Pd or 35Pd. The ingots were sealed in a
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M. Imahashi et al. / Materials Science & Engineering A 602 (2014) 19–24
quartz tube under vacuum and homogenized at 1223 K for 7.2 ks. The homogenized alloys were cold-rolled up to 40%. Specimens for X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and shape memory testing were cut by an electrodischarge machine. These specimens were heat treated at various temperatures between 673 and 1173 K for 3.6 ks. All the heat treatments were done in Ar-filled quartz tubes followed by water cooling without crushing the quartz tubes. Transformation temperatures were determined by DSC with a heating and cooling rate of 10 K min 1. Phase constitutions at 298 K were determined by XRD analysis using Cu Kα radiation. Microstructural investigations were carried out by SEM and TEM. Shape memory properties of some selected specimens were investigated by thermal cycling tests (with a heating and cooling rate of 10 K min 1) under various constant tensile stress levels. All the samples were first heated up to 673 K, and then a predefined stress was applied at the above mentioned temperature. The constrained samples were then cooled to 273 K and then heated again to 673 K. After that, the next level of stress was applied and the similar procedure was repeated until the end of the cycling experiment. The tests were conducted in nitrogen gas atmosphere in order to prevent oxidation and improve thermal conductivity.
773 K the peaks became stronger slightly but the transformation temperatures remained similar. Heat treatment at 873 K caused an increase of the transformation temperatures; the martensitic transformation start temperature (Ms) increased to 450 K. It is also seen that the transformation peaks became sharp after heat treatment at 873 K. The specimens heat treated at 973 K and above showed very sharp transformation peaks and transformation temperatures similar to the specimen heat treated at 873 K. Similar trends on the change of DSC curves as the increase of heat treatment temperature were observed in the 30Pd and 35Pd alloys as shown in Fig. 1(b) and (c), respectively. For the 30Pd alloy, the transformation peaks were too small and broad to exactly determine the transformation temperatures in the specimen heat treated at 673 K. The 35Pd alloy heat treated at 673 K showed no transformation peaks. After annealing at 773 K, the 30Pd and 35Pd alloys exhibited broad and small transformation peaks. Similar to the 25Pd alloy, the transformation peaks became sharp and transformation temperature increased with increasing heat treatment temperature. The martensitic transformation start temperature (Ms) of each alloy is plotted in Fig. 1(d) as a function of heat treatment temperature. It is seen that the increase of Pd content led to the increase in transformation temperatures at each heat treatment condition. It is also noted that Ms saturated at 873 K for the 25Pd and 30Pd alloys while it saturated at 973 K for the 35Pd alloy.
3. Result and discussion
3.2. XRD analysis and SEM observation
3.1. Effect of Pd content and annealing temperature on martensitic transformation temperature
Fig. 2 shows room temperature XRD profiles of the 25Pd, 30Pd, and 35Pd alloys heat treated at 673, 773, 873, 973 and 1173 K. For the 25Pd alloy, relatively broad peaks of the B19 phase were observed and no other peaks were detected in the specimen heat treated at 673 K. Similar broad peaks of the B19 phase were observed in the 30Pd and 35Pd alloys. These broad peaks indicated
Fig. 1(a) shows DSC curves of the 25Pd alloy heat treated at 673, 773, 873, 973 and 1173 K, respectively. The 25Pd alloy heat treated at 673 K showed broad transformation peaks. After heat treatment at
Fig. 1. DSC curves of the (a) 25Pd, (b) 30Pd and (c) 35Pd alloys heat treated at various temperatures between 673 and 1173 K and (d) Ms of all the alloys as a function of annealing temperature.
M. Imahashi et al. / Materials Science & Engineering A 602 (2014) 19–24
21
Fig. 2. Room temperature XRD profiles of the (a) 25Pd, (b) 30Pd and (c) 35Pd alloys heat treated at various temperatures between 673 and 1173 K.
Fig. 3. Backscattered SEM images of the 25Pd, 30Pd, and 35Pd alloys heat treated at 773 K (a–c) and 873 K (d–f).
the presence of significant lattice distortion introduced during cold deformation. After heat treatment at 773 K, the clear peaks of Ti2Pd and TiPdCu type precipitates were observed along with the peaks of the B19 phase in all the alloys; it is noticed that the relative peak intensity of the precipitates to the B19 phase increased with increasing Pd content. After heat treatment at 873 K, the 25Pd and 30Pd alloys showed only peaks of B19 phase which are sharper than those of peaks observed in the specimens heat treated at 673 K. The 35Pd alloy showed the peaks of Ti2Pd and TiPdCu type precipitates after heat treatment at 873 K although these peaks were weaker than those of specimen heat treated at 773 K. After annealing at 973 K, no phase other than B19 phase was detected in all the alloys. Backscattered SEM images of the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K are shown in Fig. 3(a)–(c). As revealed from the XRD results, the presence of two types of precipitates was confirmed in all the alloys heat treated at 773 K, one with brighter contrast and the other one with darker contrast which correspond to TiPdCu and Ti2Pd-type precipitates, respectively. In all alloys these precipitates have small size of about 100 nm and high densities which are expected to work as a resistance to plastic deformation. The differences in morphologies caused by different Pd content are difficult to measure in this heat treatment condition. Backscattered SEM images of the 25Pd, 30Pd and 35Pd alloys heat treated at 873 K are shown in
Fig. 3(d)–(f). Although the XRD results did not show the presence of precipitates in the 25Pd and 30Pd alloys heat treated at 873 K, the SEM images show the existence of precipitates. A small amount of TiPdCu-type precipitates was observed in the 25Pd alloy (Fig. 3(d)). The volume fraction of TiPdCu-type precipitates increased with increasing Pd content; the 30Pd alloy (Fig. 3(e)) shows relatively larger size and higher density of TiPdCu-type precipitates when compared with the 25Pd alloy (Fig. 3(d)). The 30Pd alloy also shows a tiny amount of Ti2Pd-type precipitates. Fig. 3(f) clearly shows the existence of TiPdCu and Ti2Pd-types of precipitates in the 35Pd alloy. Compared with Fig. 3(c), it is clearly seen that the size of both types of precipitates considerably increased and their densities decreased with increasing heat treatment temperature. From XRD and SEM results, it can be concluded that the increase in Pd content promoted the formation of Ti2Pd and TiPdCu type precipitates and expanded the annealing temperature range where the precipitates are formed. 3.3. Constant stress thermal cycling at various stress levels Constant stress thermal cycling tests were carried out to investigate shape memory properties of the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K, and the results are shown in Fig. 4. The solid and dashed lines correspond to the strain– temperature curves upon cooling and heating, respectively.
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Fig. 4. Strain–temperature curves of the (a) 25Pd, (b) 30Pd and (c) 35Pd alloys heat treated at 773 K for 3.6 ks and (d) measurement scheme of shape memory characteristics on strain–temperature curves.
Fig. 5. Shape memory properties of the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K for 3.6 ks: (a) Ms, (b) recovery strain and plastic strain and (c) work output.
Symbols Ms, Mf, As and Af in Fig. 4(d) are abbreviations of the temperatures for martensitic transformation start, its finish, reverse martensitic transformation start and its finish, respectively. Symbols εr and εp denote the recovery and plastic strains, respectively. Fig. 5(a) shows the change of Ms of the alloys with
applied stress (s). Ms of the alloys increased with increasing applied stress in accordance with the Clausius–Clapeyron relationship. Fig. 5(a) also reveals that Ms increased with increasing Pd content at each applied stress. It is noted that stable shape memory effect with Ms higher than 450 K was successfully
M. Imahashi et al. / Materials Science & Engineering A 602 (2014) 19–24
achieved in the 30Pd and 35Pd alloys. It is also seen that Ms increased to 576 K under 600 MPa in the 35Pd alloy. Fig. 5(b) shows the dependence of εr and εp on applied stress in the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K. As shown in Fig. 5(b), εr increased as the applied stress increased and saturated at 500–550 MPa while εp started to increase at 500 MPa. It is noted that all the alloys showed almost similar resistance against the plastic deformation under each stress levels, although the 30Pd and 35Pd alloys had faced more severe transformation conditions than the 25Pd alloy because of their higher transformation temperatures. This improved resistance against the plastic deformation actually permitted to achieve the constant rise of work output even at high stress levels in all the alloys as shown in Fig. 5(c), where work output is defined as s εr. However, the values of recovery strain decreased with increasing Pd content at each stress level which in turn led to the decrease in the values of work output. This can be explained as follows. As shown in Fig. 2 and Fig. 3, it is clear that the increase in Pd content encouraged the formation of TiPdCu and Ti2Pd type precipitates. Although the formation of these precipitates improved the resistance against the plastic deformation, it also decreased the volume fraction of matrix phase and obstructed the phase transformation. This in turn caused a decrease in the overall recovery strain. 3.4. Effect of annealing time on shape memory properties and microstructure In order to improve the recovery strain, the effect of heat treatment time was investigated using the 30Pd alloy, whose Ms temperature was higher than 473 K under stress free condition and which showed relatively higher recovery strain as compared to that of the 35Pd alloy. Fig. 6(a) and (b) show the strain– temperature curves of the 30Pd alloy heat treated at 773 K for 1.8 ks and 0.6 ks, respectively. Fig. 7(a) shows the evolution of
23
recovery and plastic strain and Fig. 7(b) shows the evolution of work output with respect to the applied stress. The results of the sample heat treated at 773 K for 3.6 ks are also shown for comparison purposes. It is seen that, by shortening the heat treatment time, the transformation strain was remarkably improved. The specimen annealed for 3.6 ks exhibited 1.7% recovery strain when stressed at 500 MPa. On the other hand the specimens heat treated for 1.8 ks and 0.6 ks exhibited 2.4% and 3.0% recovery strain when stressed at 500 MPa, respectively. Although the plastic strain values also increased with decreasing heat treatment time, these differences were not as large as the increase in the recovery strain. As a result this led to improve the overall work output as shown Fig. 7(b). In addition, Ms increased slightly with decreasing heat treatment time. Backscattered SEM images of the 30Pd alloy heat treated at 773 K for 1.8 ks and 0.6 ks are shown in Fig. 8(a) and (b), respectively. Fig. 8(c) shows a bright field TEM image of the 30Pd alloy heat treated at 773 K for 0.6 ks. When compared with Fig. 3(b), it can be clearly seen that the density and size of both types of precipitates decreased by shortening the heat treatment time. Although precipitates in the specimen heat treated for 0.6 ks are difficult to be identified in the SEM image (Fig. 8(b)) because of their small size, very fine nano-scale precipitates were observed in the TEM image (Fig. 8(c)) as indicated by white arrows. From the above results, it is concluded that by changing the density and size of precipitates, the combination of high transformation temperatures and large amount of recovery strain would be achievable while keeping the enough resistance against the plastic deformation.
4. Conclusions In this work the effect of Pd content on microstructure and high temperature shape memory properties of Ti50Ni45 xPdxCu5 (x ¼25,
Fig. 6. Strain–temperature curves of the 30Pd alloy heat treated at 773 K for (a) 1.8 ks and (b) 0.6 ks.
Fig. 7. Shape memory properties of the 30Pd alloy heat treated at 773 K for 0.6, 1.8 and 3.6 ks: (a) recovery strain and plastic strain and (b) work output.
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Fig. 8. Backscattered SEM images of the 30Pd alloy heat treated at 77 3 K for (a) 1.8 ks and (b) 0.6 ks, and (c) a TEM image of the 30Pd alloy heat treated at 773 K for 0.6 ks.
30, 35) alloys was investigated. The main findings of this study can be summarized as follows: 1. The increase in Pd content led to increase the transformation temperatures. The increase in the Pd content also expanded the annealing temperature range where the TiPdCu and Ti2Pd type precipitates are formed. 2. The precipitation of TiPdCu and Ti2Pd type precipitates was found to increase with increasing Pd content, which led to enhance the dimensional stability and reduce the recovery strain. 3. Annealing time was found to be an important parameter to control the density and size of precipitates in Ti–Ni–Pd–Cu alloys. By shortening the annealing time, the density and size of precipitates decreased and the values of recovery strain increased which led to achieve a better combination of high transformation temperatures and good shape memory properties.
Acknowledgment This work was partially supported by JSPS KAKENHI Grant nos. 23360300 and 25289247 and by MEXT KAKENHI Grant nos. 23102503 and 25102704. References [1] S. Miyazaki, K. Otsuka, ISIJ Int. 29 (1989) 353–377. [2] K. Yamauchi, I. Ohkata, K. Tsuchiya, S. Miyazaki, Shape Memory and Superelastic Alloys: Technologies and Applications, First ed., Woodhead Publishing, Cambridge, 2011. [3] J. Ma, I. Karaman, R.D. Noebe, Int. Mater. Rev. 55 (2010) 257–314. [4] R. Arockiakumar, M. Takahashi, S. Takahashi, Y. Yamabe-Mitarai, Mater. Sci. Eng. A 585 (2013) 86–93. [5] A. Wadood, M. Takahashi, S. Takahashi, H. Hosoda, Y. Yamabe-Mitarai, Mater. Sci. Eng. A 564 (2013) 34–41. [6] Y. Yamabe-Mitarai, T. Hara, S. Miura, H. Hosoda, Intermetallics 18 (2010) 2275–2280. [7] K.C. Atli, I. Karaman, R.D. Noebe, D. Gaydosh, Mater. Sci. Eng. A 560 (2013) 653–666. [8] E. Acar, H.E. Karaca, H. Tobe, R.D. Noebe, Y.I. Chumlyakov, J. Alloys Compd. 578 (2013) 297–302. [9] G.S. Bigelow, S.A. Padula , A. Garg, D. Gaydosh, R.D. Noebe, Metall. Mater. Trans. A 41 (2010) 3065–3079.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of Pd content on microstructure and shape-memory properties of Ti–Ni–Pd–Cu alloys Masamine Imahashi a, M. Imran Khan a, Hee Young Kim a,n, Shuichi Miyazaki a,b,nn a b
Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
art ic l e i nf o
a b s t r a c t
Article history: Received 24 December 2013 Accepted 12 February 2014 Available online 20 February 2014
The effect of Pd content on the microstructure and high-temperature shape memory properties of Ti–Ni– Pd–Cu alloys was investigated. The increase in Pd content led to the increase of transformation temperatures. The volume fraction of two types of precipitates (i.e. TiPdCu and Ti2Pd) also increased with increasing Pd content. Although the formation of these precipitates resulted in an increased resistance against the plastic deformation, the amount of recovery strain decreased during thermal cycling tests at various stress levels. By adjusting the heat treatment time, a high transformation temperature and large amount of recovery strain could be achieved while keeping the enough resistance against the plastic deformation. & 2014 Elsevier B.V. All rights reserved.
Keywords: Shape memory alloys Martensitic transformations Precipitation Thermomechanical processing
1. Introduction Ti–Ni shape memory alloys are well known as excellent superelastic and shape memory materials and have been increasingly used in various fields of industry such as home appliances, medical devices and electronic devices [1,2]. Recently high temperature shape memory actuators for power generation, electronic devices, space exploration and automotive applications have attracted considerable research interest [3–22]. However, binary Ti–Ni alloys are limited to use below 373 K because of their low martensitic transformation temperatures [2,23]. Ti–Ni–Pd alloys have received significant research attention because they offer excellent combination of high transformation temperature, small hysteresis, adequate workability and large recovery strain [24–28]. In these alloys transformation temperatures can be tailored in the range from 373 K to 773 K by changing Pd content from 20 to 50 mol% [29]. However, at high temperatures these materials face a serious threat from thermally driven mechanisms, i. e. dimensional instability due to creep deformation, recovery and recrystallization processes and transformation induced plasticity [3,30–34]. For this reason, recent researches in Ti–Ni–Pd high temperature shape memory alloys have been focusing on the improvement of their functional and dimensional stability at high temperatures. Many techniques have been applied to improve the
n
Corresponding author. Tel./fax: þ 81 29 853 6942. Corresponding author at: Division of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan. Tel./fax: þ 81 29 853 5283. E-mail addresses: [email protected] (H.Y. Kim), [email protected] (S. Miyazaki). nn
http://dx.doi.org/10.1016/j.msea.2014.02.038 0921-5093 & 2014 Elsevier B.V. All rights reserved.
high temperature performance of Ti–Ni–Pd alloys including thermomechanical treatment, aging, training, annealing after sever plastic deformation and quaternary alloying additions [35–40]. In our previous research it was found that by adding Cu to a Ti–Ni–Pd ternary alloy, very high densities of nano-scale precipitates of TiPdCu and Ti2Pd were preferentially formed at heterogeneous nucleation sites provided by deformation-induced defects as a result of spinodal type decomposition [41]. These two types of precipitates increased the resistance against the creep and transformation induced plasticity. In addition, these precipitates seemed to delay the recovery and recrystallization type of softening processes [42]. Precipitation of these two types of precipitates largely depends on Cu content and these precipitates were found to be stable at temperatures 4773 K. However, the high densities of these precipitates decreased the martensitic transformation temperatures mainly because of the increase in Ni content of the matrix. It is still unknown that whether these precipitates are also effective at even higher temperatures in terms of their resistance against the plastic deformation and creep. In the present study, the effect of Pd content on the microstructure and shape memory properties of Ti–Ni–Pd–Cu alloys was investigated. In addition, the annealing time was optimized for the purpose of improving the shape memory properties.
2. Experimental procedure Ti50Ni45 xPdxCu5 (x ¼25, 30, 35) alloys were fabricated by the Ar-arc melting method. Hereafter each alloy is called by its Pd content such as 25Pd, 30Pd or 35Pd. The ingots were sealed in a
20
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quartz tube under vacuum and homogenized at 1223 K for 7.2 ks. The homogenized alloys were cold-rolled up to 40%. Specimens for X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and shape memory testing were cut by an electrodischarge machine. These specimens were heat treated at various temperatures between 673 and 1173 K for 3.6 ks. All the heat treatments were done in Ar-filled quartz tubes followed by water cooling without crushing the quartz tubes. Transformation temperatures were determined by DSC with a heating and cooling rate of 10 K min 1. Phase constitutions at 298 K were determined by XRD analysis using Cu Kα radiation. Microstructural investigations were carried out by SEM and TEM. Shape memory properties of some selected specimens were investigated by thermal cycling tests (with a heating and cooling rate of 10 K min 1) under various constant tensile stress levels. All the samples were first heated up to 673 K, and then a predefined stress was applied at the above mentioned temperature. The constrained samples were then cooled to 273 K and then heated again to 673 K. After that, the next level of stress was applied and the similar procedure was repeated until the end of the cycling experiment. The tests were conducted in nitrogen gas atmosphere in order to prevent oxidation and improve thermal conductivity.
773 K the peaks became stronger slightly but the transformation temperatures remained similar. Heat treatment at 873 K caused an increase of the transformation temperatures; the martensitic transformation start temperature (Ms) increased to 450 K. It is also seen that the transformation peaks became sharp after heat treatment at 873 K. The specimens heat treated at 973 K and above showed very sharp transformation peaks and transformation temperatures similar to the specimen heat treated at 873 K. Similar trends on the change of DSC curves as the increase of heat treatment temperature were observed in the 30Pd and 35Pd alloys as shown in Fig. 1(b) and (c), respectively. For the 30Pd alloy, the transformation peaks were too small and broad to exactly determine the transformation temperatures in the specimen heat treated at 673 K. The 35Pd alloy heat treated at 673 K showed no transformation peaks. After annealing at 773 K, the 30Pd and 35Pd alloys exhibited broad and small transformation peaks. Similar to the 25Pd alloy, the transformation peaks became sharp and transformation temperature increased with increasing heat treatment temperature. The martensitic transformation start temperature (Ms) of each alloy is plotted in Fig. 1(d) as a function of heat treatment temperature. It is seen that the increase of Pd content led to the increase in transformation temperatures at each heat treatment condition. It is also noted that Ms saturated at 873 K for the 25Pd and 30Pd alloys while it saturated at 973 K for the 35Pd alloy.
3. Result and discussion
3.2. XRD analysis and SEM observation
3.1. Effect of Pd content and annealing temperature on martensitic transformation temperature
Fig. 2 shows room temperature XRD profiles of the 25Pd, 30Pd, and 35Pd alloys heat treated at 673, 773, 873, 973 and 1173 K. For the 25Pd alloy, relatively broad peaks of the B19 phase were observed and no other peaks were detected in the specimen heat treated at 673 K. Similar broad peaks of the B19 phase were observed in the 30Pd and 35Pd alloys. These broad peaks indicated
Fig. 1(a) shows DSC curves of the 25Pd alloy heat treated at 673, 773, 873, 973 and 1173 K, respectively. The 25Pd alloy heat treated at 673 K showed broad transformation peaks. After heat treatment at
Fig. 1. DSC curves of the (a) 25Pd, (b) 30Pd and (c) 35Pd alloys heat treated at various temperatures between 673 and 1173 K and (d) Ms of all the alloys as a function of annealing temperature.
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Fig. 2. Room temperature XRD profiles of the (a) 25Pd, (b) 30Pd and (c) 35Pd alloys heat treated at various temperatures between 673 and 1173 K.
Fig. 3. Backscattered SEM images of the 25Pd, 30Pd, and 35Pd alloys heat treated at 773 K (a–c) and 873 K (d–f).
the presence of significant lattice distortion introduced during cold deformation. After heat treatment at 773 K, the clear peaks of Ti2Pd and TiPdCu type precipitates were observed along with the peaks of the B19 phase in all the alloys; it is noticed that the relative peak intensity of the precipitates to the B19 phase increased with increasing Pd content. After heat treatment at 873 K, the 25Pd and 30Pd alloys showed only peaks of B19 phase which are sharper than those of peaks observed in the specimens heat treated at 673 K. The 35Pd alloy showed the peaks of Ti2Pd and TiPdCu type precipitates after heat treatment at 873 K although these peaks were weaker than those of specimen heat treated at 773 K. After annealing at 973 K, no phase other than B19 phase was detected in all the alloys. Backscattered SEM images of the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K are shown in Fig. 3(a)–(c). As revealed from the XRD results, the presence of two types of precipitates was confirmed in all the alloys heat treated at 773 K, one with brighter contrast and the other one with darker contrast which correspond to TiPdCu and Ti2Pd-type precipitates, respectively. In all alloys these precipitates have small size of about 100 nm and high densities which are expected to work as a resistance to plastic deformation. The differences in morphologies caused by different Pd content are difficult to measure in this heat treatment condition. Backscattered SEM images of the 25Pd, 30Pd and 35Pd alloys heat treated at 873 K are shown in
Fig. 3(d)–(f). Although the XRD results did not show the presence of precipitates in the 25Pd and 30Pd alloys heat treated at 873 K, the SEM images show the existence of precipitates. A small amount of TiPdCu-type precipitates was observed in the 25Pd alloy (Fig. 3(d)). The volume fraction of TiPdCu-type precipitates increased with increasing Pd content; the 30Pd alloy (Fig. 3(e)) shows relatively larger size and higher density of TiPdCu-type precipitates when compared with the 25Pd alloy (Fig. 3(d)). The 30Pd alloy also shows a tiny amount of Ti2Pd-type precipitates. Fig. 3(f) clearly shows the existence of TiPdCu and Ti2Pd-types of precipitates in the 35Pd alloy. Compared with Fig. 3(c), it is clearly seen that the size of both types of precipitates considerably increased and their densities decreased with increasing heat treatment temperature. From XRD and SEM results, it can be concluded that the increase in Pd content promoted the formation of Ti2Pd and TiPdCu type precipitates and expanded the annealing temperature range where the precipitates are formed. 3.3. Constant stress thermal cycling at various stress levels Constant stress thermal cycling tests were carried out to investigate shape memory properties of the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K, and the results are shown in Fig. 4. The solid and dashed lines correspond to the strain– temperature curves upon cooling and heating, respectively.
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Fig. 4. Strain–temperature curves of the (a) 25Pd, (b) 30Pd and (c) 35Pd alloys heat treated at 773 K for 3.6 ks and (d) measurement scheme of shape memory characteristics on strain–temperature curves.
Fig. 5. Shape memory properties of the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K for 3.6 ks: (a) Ms, (b) recovery strain and plastic strain and (c) work output.
Symbols Ms, Mf, As and Af in Fig. 4(d) are abbreviations of the temperatures for martensitic transformation start, its finish, reverse martensitic transformation start and its finish, respectively. Symbols εr and εp denote the recovery and plastic strains, respectively. Fig. 5(a) shows the change of Ms of the alloys with
applied stress (s). Ms of the alloys increased with increasing applied stress in accordance with the Clausius–Clapeyron relationship. Fig. 5(a) also reveals that Ms increased with increasing Pd content at each applied stress. It is noted that stable shape memory effect with Ms higher than 450 K was successfully
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achieved in the 30Pd and 35Pd alloys. It is also seen that Ms increased to 576 K under 600 MPa in the 35Pd alloy. Fig. 5(b) shows the dependence of εr and εp on applied stress in the 25Pd, 30Pd and 35Pd alloys heat treated at 773 K. As shown in Fig. 5(b), εr increased as the applied stress increased and saturated at 500–550 MPa while εp started to increase at 500 MPa. It is noted that all the alloys showed almost similar resistance against the plastic deformation under each stress levels, although the 30Pd and 35Pd alloys had faced more severe transformation conditions than the 25Pd alloy because of their higher transformation temperatures. This improved resistance against the plastic deformation actually permitted to achieve the constant rise of work output even at high stress levels in all the alloys as shown in Fig. 5(c), where work output is defined as s εr. However, the values of recovery strain decreased with increasing Pd content at each stress level which in turn led to the decrease in the values of work output. This can be explained as follows. As shown in Fig. 2 and Fig. 3, it is clear that the increase in Pd content encouraged the formation of TiPdCu and Ti2Pd type precipitates. Although the formation of these precipitates improved the resistance against the plastic deformation, it also decreased the volume fraction of matrix phase and obstructed the phase transformation. This in turn caused a decrease in the overall recovery strain. 3.4. Effect of annealing time on shape memory properties and microstructure In order to improve the recovery strain, the effect of heat treatment time was investigated using the 30Pd alloy, whose Ms temperature was higher than 473 K under stress free condition and which showed relatively higher recovery strain as compared to that of the 35Pd alloy. Fig. 6(a) and (b) show the strain– temperature curves of the 30Pd alloy heat treated at 773 K for 1.8 ks and 0.6 ks, respectively. Fig. 7(a) shows the evolution of
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recovery and plastic strain and Fig. 7(b) shows the evolution of work output with respect to the applied stress. The results of the sample heat treated at 773 K for 3.6 ks are also shown for comparison purposes. It is seen that, by shortening the heat treatment time, the transformation strain was remarkably improved. The specimen annealed for 3.6 ks exhibited 1.7% recovery strain when stressed at 500 MPa. On the other hand the specimens heat treated for 1.8 ks and 0.6 ks exhibited 2.4% and 3.0% recovery strain when stressed at 500 MPa, respectively. Although the plastic strain values also increased with decreasing heat treatment time, these differences were not as large as the increase in the recovery strain. As a result this led to improve the overall work output as shown Fig. 7(b). In addition, Ms increased slightly with decreasing heat treatment time. Backscattered SEM images of the 30Pd alloy heat treated at 773 K for 1.8 ks and 0.6 ks are shown in Fig. 8(a) and (b), respectively. Fig. 8(c) shows a bright field TEM image of the 30Pd alloy heat treated at 773 K for 0.6 ks. When compared with Fig. 3(b), it can be clearly seen that the density and size of both types of precipitates decreased by shortening the heat treatment time. Although precipitates in the specimen heat treated for 0.6 ks are difficult to be identified in the SEM image (Fig. 8(b)) because of their small size, very fine nano-scale precipitates were observed in the TEM image (Fig. 8(c)) as indicated by white arrows. From the above results, it is concluded that by changing the density and size of precipitates, the combination of high transformation temperatures and large amount of recovery strain would be achievable while keeping the enough resistance against the plastic deformation.
4. Conclusions In this work the effect of Pd content on microstructure and high temperature shape memory properties of Ti50Ni45 xPdxCu5 (x ¼25,
Fig. 6. Strain–temperature curves of the 30Pd alloy heat treated at 773 K for (a) 1.8 ks and (b) 0.6 ks.
Fig. 7. Shape memory properties of the 30Pd alloy heat treated at 773 K for 0.6, 1.8 and 3.6 ks: (a) recovery strain and plastic strain and (b) work output.
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Fig. 8. Backscattered SEM images of the 30Pd alloy heat treated at 77 3 K for (a) 1.8 ks and (b) 0.6 ks, and (c) a TEM image of the 30Pd alloy heat treated at 773 K for 0.6 ks.
30, 35) alloys was investigated. The main findings of this study can be summarized as follows: 1. The increase in Pd content led to increase the transformation temperatures. The increase in the Pd content also expanded the annealing temperature range where the TiPdCu and Ti2Pd type precipitates are formed. 2. The precipitation of TiPdCu and Ti2Pd type precipitates was found to increase with increasing Pd content, which led to enhance the dimensional stability and reduce the recovery strain. 3. Annealing time was found to be an important parameter to control the density and size of precipitates in Ti–Ni–Pd–Cu alloys. By shortening the annealing time, the density and size of precipitates decreased and the values of recovery strain increased which led to achieve a better combination of high transformation temperatures and good shape memory properties.
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