RARE METALS Vol. 28, No. 1, Feb 2009, p. 63 DOI: 10.1007/s12598-009-0012-8
Effect of heat treatment on the transformation behavior and temperature memory effect in TiNiCu wires YU Huajun and XIANG Xia Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China Received 22 December 2007; received in revised form 18 March 2008; accepted 17 April 2008
Abstract The effect of heat treatment on the phase transformation behavior of TiNiCu shape memory alloy wires and the temperature memory effect in this alloy were investigated by the resistance method. These results showed that with increasing annealing temperature and annealing time, the phase transformation temperatures of TiNiCu wires were shifted to higher temperatures in the heating and cooling process. It was also found that incomplete thermal cycles, upon heating the TiNiCu wires, which were arrested at a temperature between the start and finish temperatures of the reverse martensite transformation, could induce a kinetic stop in the next complete thermal cycle. The kinetic stop temperature was closely related to the previous arrested temperature. This phenomenon was defined as the temperature memory effect. The result of this study was consistent with the previous report on the phenomenon obtained using the differential scanning calorimetry method, indicating that temperature memory effect was a common phenomenon in shape memory alloys. Keywords: TiNiCu shape memory alloy; heat treatment; phase transformation temperature; temperature memory effect
1. Introduction During the past decades, shape memory alloys (SMAs) have received considerable research and wide use for novel engineering and mechanical applications owing to their excellent functional properties, shape memory effect, and superelasticity behavior [1-2]. Among SMAs, TiNi based alloys are widely investigated because of their superior memory and structural properties. The unique properties of these alloys are directly related to solid-state martensitic transformation. Recently numerous investigations on the transformation characteristics of TiNi alloys have been carried out using various methods. The transformation characteristics of TiNi-based alloys show great dependence on a variety of alloy compositions [3-4] and heat treatment conditions [5-10]. There have been several reports on the change in the transformation characteristics of TiNi-based SMAs after neutron, proton, and electron irradiation [11-15]. Compared with TiNi SMAs, the ternary TiNiCu shape memory alloys are better candidates for many applications, such as electrical connectors, sensors, and actuators. Unlike the near equatomic TiNi alloys, the martensitic transformation in the TiNiCu alloys is from B2 to B19 without Ti3Ni4 precipitation and R-phase transformation [1]. Thus, the effect of heat treatment on the transformation behavior of TiNiCu may be Corresponding author: XIANG Xia
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different from that of the TiNi alloy. However, there are few reports on the effect of heat treatment on the transformation behavior in TiNiCu SMAs. Some researches have also shown that shape memory alloys have the ability to memorize their thermal history [16-17]. It has been found that after incomplete transformation cycling during the first thermal cycling in an NiTi binary alloy, the subsequent transformation cycling behaves as if it ‘remembers’ the arrest in the first cycle. Zheng et al. [18] have defined the new phenomenon as temperature memory effect (TME) in shape memory alloys. Subsequently, TME has been systemically investigated by Wang et al. [19-20], He et al. [21], and Li et al. [22] in TiNi-based SMAs using differential scanning calorimetry (DSC). TME is a reversible process and can also be erased through an appropriate thermal treatment [19]. The resistance method is another important method to investigate the transformation characteristics of shape memory alloys. There is no report on TME of SMAs measured using the resistance method. The present study is aimed at investigating the role of heat treatment in modifying the behavior of phase transformation temperatures and the TME phenomenon of SMA TiNiCu wires. Systematic investigations of the evolution of transformation characteristics as a function of annealing temperature and time and the TME in TiNiCu wires are perwww.springerlink.com
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formed by electrical resistance measurement using the four-probe technique.
2. Experimental The materials used were wires of Ti-45at.%Ni-5at.%Cu with a diameter of 0.5 mm, which were provided by the Northwest Institute for Nonferrous Metal Research, China. For resistance measurements of different annealing systems, the alloy wires were cut into pieces with the same length of 60 mm. The samples were annealed at 400-550°C for 1 h and at 500°C for 5 h, respectively. After each annealing, the surface of the sample was cleaned by fine-grained emery paper to ensure the removal of the oxide surface for proper electrical contact. Electrical resistance measurements were performed at temperatures from 10 to 90°C with a heating and cooling rate of 2.5°C/min, using the four-probe technique.
3. Results and discussion 3.1. Effect of heat treatment on the phase transformation behavior The temperature dependences of electrical resistance of
the TiNiCu wire samples annealed at 400-550°C for 1 h are shown in Figs. 1(a)-1(d). The start and the end of the transformation temperatures have been determined by noting the intersection of the tangents drawn to the parts of the curves on either side of the site where a sudden change in resistance values takes place, as shown in Fig. 1(a). The points Ms and Mf are the start and the finish temperatures of the martensitic phase transformation, and points As and Af are those of the inverse phase transformation, respectively. The transformation temperatures deduced from the resistance profile of the samples annealed at different temperatures are listed in Table 1. The resistance in the austenitic phase (A) for the TiNiCu wire is lower when compared to that in the martensite (M). From Fig. 1 and Table 1, it can be seen that for all the annealed samples, the phase transformation temperatures (Ms, Mf, As, and Af) increase monotonically with an increase in annealing temperature. In the cooling process, it can be seen from Fig. 1 that there is only one-step phase transformation occurring in the temperature range between 90 and 10°C for the annealed samples. This implies that there is no R-phase occurring in the transition from the high temperature phase to the low temperature phase for TiNiCu wires annealed at 400-550°C. The general behavior of the TiNiCu
Fig. 1. Profiles of electrical resistance as a function of temperature for the TiNiCu wire samples annealed at 400°C (a), 450°C (b), 500°C (c), and 550°C (d) for 1 h.
Yu H.J. et al., Effect of heat treatment on the transformation behavior and temperature memory effect in TiNiCu wires
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wire is different from that of TiNi [5]. Fig. 2 shows the electrical resistance profile corresponding to the sample heat treated at 500°C for 5 h. The transformation temperatures, for the sample annealed at 500°C for 1 h and 5 h, are shown in Table 1. It is worth noticing that the phase transformation temperatures slightly increase with an increase in heat treating time in case of the TiNiCu wire. Table 1. Transformation temperatures of TiNiCu wire samples annealed at different temperatures Annealing temperature / °C 400
Annealing As / °C Af / °C Ms / °C Mf / °C time / h 38.0
48.2
28.4
22.4
450
1 1
51.5
57.9
40.0
34.7
500
1
57.3
63.7
46.1
42.0
550
1
59.7
65.0
46.9
42.5
500
5
60.2
67.4
49.8
43.5
Fig. 3. Profile of the TME of the TiNiCu wire annealed at 500°C with a single incomplete cycle on heating at 59°C (a) and 61°C (b).
Fig. 2. Profile of electrical resistance as a function of temperature in the TiNiCu wire sample annealed at the temperature of 500°C for 5 h.
3.2. Temperature memory effect phenomena in TiNiCu wires The variation of electrical resistance as a function of temperature for the TiNiCu wire annealed at 500°C for 1 h upon heating and cooling is shown in Fig. 1(c). Figs. 3(a) and 3(b) show TMEs of the TiNiCu wire after performing one single incomplete cycle upon heating (ICH) at the stop temperatures Ts = 59 and 61°C, respectively. A plateau, corresponding to the arrested temperature, can clearly be observed for each profile of electrical resistance as a function of temperature. Fig. 4(a) shows the results of the TiNiCu wire performed with two successive ICHs at temperatures Ts = 60 and 58°C with decreasing order, and two plateaus can be clearly observed in this profile. Fig. 4(b) shows the results of the TiNiCu wire after performing two successive
ICHs at Ts = 58 and 60°C (with increasing order). Only one plateau can be clearly found in the profile and the maximum temperature of Ts = 68°C is memorized. The TME can be easily wiped out by the following complete thermal cycle as revealed in Fig. 4(c). TME is a common phenomenon in the shape memory alloy, which is induced by partial reverse transformation [19]. In this study, TME performed by the electrical resistance measurement using the four-probe technique on the TiNiCu wire reveals clearly that TME is one of the significant characteristics. If a number N of ICHs with different arrested temperatures is performed with decreasing order, N temperatures can be memorized. The capability to memorize temperature can be eliminated by an appropriate complete transformation heat cycle. During the partial reverse transformation, the martensite to austenite transformation is stopped at a certain temperature between As and Af, and only part of the martensite transforms into the parent phase, with the rest of the martensite remaining. Here the remaining martensite is called M1. With further decreasing the temperature below Mf, the parent phase transforms back to
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martensite, and the newly formed martensite is called M2. During the following heating process, M2 and M1 transform into the parent phase sequentially, causing a plateau between these transformations.
mation behavior of the shape memory alloy TiNiCu wire. With an increase in annealing temperature between 400 and 550°C, the phase temperatures (Ms, Mf, As, and Af) monotonically increase, and there is only one-step transition upon heating and cooling at the temperature range between 10 and 90°C. (2) Annealing time can also influence the transition temperatures of the TiNiCu wire. The temperatures slightly increase with increasing time. (3) The TME phenomenon in the TiNiCu wire is induced by performing either a single incomplete cycle, or a sequence of repetitious incomplete cycles with different arrested temperatures. The results indicate that TME is a special characteristic in the shape memory alloy, which is induced by a partial reverse transformation.
References
Fig. 4. Profiles of the TME of the TiNiCu wire annealed at 500°C with two incomplete cycles at Ts = 60 and 58°C with decreasing order (a), Ts = 58 and 60°C with increasing order (b), and subsequent global transformation (c).
4. Conclusions (1) Annealing temperature strongly affects the transfor-
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