Materials Science Forum Vols. 561-565 (2007) pp 1541-1544 Online available since 2007/Oct/02 at www.scientific.net © (2007) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.561-565.1541
Martensitic Transformation of TiAu Shape Memory Alloys Hideki Hosoda1,a, Ryosuke Tachi2, Tomonari Inamura1,b Kenji Wakashima1,c and Shuichi Miyazaki3,d 1 2
Precision and Intelligence Laboratory, Tokyo Institute of Technology, JAPAN
Graduate student, Tokyo Institute of Technology, JAPAN (Now with Toyota Motor Co. Ltd.) 3
Institute of Materials Science, University of Tsukuba, JAPAN
a c
[email protected], b
[email protected]
[email protected], d
[email protected]
Keywords: martensitic transformation, TiAu, shape memory alloy, high temperature, lattice parameter, transformation strain.
Abstract. Martensitic transformation temperatures were measured and transformation strains were evaluated in a promising high temperature shape memory alloy TiAu with a compositional range from 46 to 53mol%Au. It was found by differential scanning calorimetry that martensitic transformation start temperature (Ms) is kept almost constant value of 880K in the Au-rich side of the stoichiometric composition. On the other hand, Ms decreases monotonically with decreasing Au content in the Au-poor side. X-ray diffraction analysis revealed that apparent phase of all the alloys at room temperature is B19 martensite phase. Under an assumption that the atomic volume is constant during martensitic transformation, the lattice parameters of B2 parent phase and maximum transformation strain were calculated. It was found that the maximum transformation strain depends on chemical composition and that it reaches 10.75% for Ti-53mol%Au alloy. The value is comparable to that of Ti-Ni.
Introduction Shape memory alloys (SMAs) are power actuator materials. The large output power is caused by the thermoelastic martensitic transformation. Another benefit is that SMAs in nature exhibit their own temperature sensors and that they are automatically actuated at their phase transformation temperatures. Although various SMAs have been reported [1], only Ti-Ni is mostly used for practical applications. However, some limitations are known for Ti-Ni. One is that the highest actuation temperature of binary Ti-Ni is limited to be 393K [2]. For higher temperature applications, high temperature SMA (HTSMA) having higher actuation temperature is required. Several trials were done to increase martensitic transformation temperature of Ti-Ni by addition of ternary additions [3, 4]. The following ternary elements can rise Ms of Ti-Ni: Pt, Pd, Au, Rh, Hf and Zr [4]. However, the ternary Ti-Ni alloys are brittle and shape recovery strain also becomes small. TiAu is a suitable candidate for HTSMA. TiAu exhibits a thermoelastic martensitic transformation from B2 phase (ordered bcc) to B19 martensite phase (orthorhombic) at around 900K [4, 5]. The crystal structures of B2 and B19 are shown in Figure 1. The martensitic transformation temperature and apparent phase of TiAu have been already reported [5]. However, precise data for the B19 and B2 crystal structures and transformation strain are not available at present. Transformation strain is the major part of shape recovery strain. In this study, such precise data of TiAu were collected for the development of TiAu HTSMA.
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B19
B2 bB19
cB19
aB2
aB19 Figure 1 Crystal structures of B2 (4 unit cells) and B19 (aB19
