Factors for Controlling Martensitic Transformation Temperature of TiNi Shape Memory Alloy by Addition of Ternary Element
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Factors for Controlling Martensitic Transformation Temperature of TiNi Shape Memory Alloy by Addition of Ternary Elements Hideki Hosoda, Kenji Wakashima, Shuichi Miyazaki1 and Kanryu Inoue2 Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan. 1 Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan. 2 Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195-2120, USA. ABSTRACT Correlations between the changes in martensitic transformation start temperature (Ms) by addition of ternary elements X and several factors of the ternary additions were investigated for TiNi shape memory alloy. The change of Ms by addition of 1mol%X is referred to as ∆Ms (in K/mol%), and ∆Ms was systematically evaluated by differential scanning calorimetry experimentally using (Ti, X)50Ni50 solution-treated at 1273K for 3.6ks where the Ni content was kept constant to be 50mol%. The ternary additions X investigated are the transition metal (TM) elements selected from 4th period group (Zr, Hf) to 10th period group (Pd, Pt). The factors investigated are (1) the number of total outer d- and s-electrons (Nele), and electron hole number (NV), (2) electronegativity (EN), (3) atomic volume (VX) and (4) Mendeleev number (NM). It was found that the values of ∆Ms are different even in a same period group; ∆Ms of 6th period group are -133K/mol%Cr (3d-TM), -152K/mol%Mo (4d-TM) and -64K/mol%W (5d-TM) for example. The results found in the correlations between ∆Ms and those factors are summarized as follows. (1) ∆Ms depends on Nele and NV. However, the data are scattered because same Nele and NV are often given in a same period group. Then, other factors than Nele and NV are required for clear understanding of ∆Ms. (2) ∆Ms seems to become lowered slightly with increasing EN. (3) ∆Ms weakly depends on atomic volume VX. Ternary addition with large VX increases ∆Ms slightly, and with small VX decreases ∆Ms largely. Since the stress field must be formed by substitution due to size mismatch, the type of stress field, tension/compression, may be an important role to determine the sign of ∆Ms. (4) ∆Ms shows a good correlation with NM as -9.4Kmol%-1/∆NM where ∆NM is the difference in NM. This suggests that a ternary alloying element with smaller (larger) NM stabilizes the B19’ martensite (B2 parent) phase. Effect of site occupancy on Ms is also discussed only for Cr. INTRODUCTION 100
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TiNi alloy is a representative actuator material due to its unique shape memory effect and superelasticity. In order to develop better actuator material based on shape memory alloy (SMA), the precise control of martensitic transformation temperature (Ms) is required. It was already reported that Ms of TiNi is influenced by applied and/or internal stress based on the Clausius-Clapeyron relationship [1]. Besides, Ms depends on chemical compositions; several works have been done for the effects of stoichiometry [2, 3
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