Phase Transformation of Ti-Ni Containing Platinum-Group Metals
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Phase Transformation of Ti-Ni Containing Platinum-Group Metals Hideki Hosoda, Masahiro Tsuji*, Motoki Mimura*, Yohei Takahashi*, Kenji Wakashima and Yoko Yamabe-Mitarai1 Precision and Intelligence Laboratory (P&I Lab) Tokyo Institute of Technology (Tokyo Tech), 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan *Graduate students, Tokyo Institute of Technology 1 High-Temperature Materials Group, National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan ABSTRACT Since the maximum shape recovery temperature of the binary Ti-Ni alloys is limited to be around 400K, the increase in martensitic transformation temperature (Ms) of Ti-Ni should be done by alloying for the demand of high temperature applications. Although most of additional elements are known to decrease Ms of Ti-Ni, substitutional elements having large atomic size are expected to increase Ms. In this study, phase constitution, phase transformation temperature, lattice parameter of B2 phase and Vickers hardness were investigated for Ti-Ni alloys containing several platinum-group metals (PGM). The alloy systems investigated were the pseudobinary systems of TiNi-TiRh, TiNi-TiIr and TiNi-TiPt where the PGM atoms substitute for the Ni-sites of TiNi. The phase transformation and phase constitution were assessed by differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). It was found by XRD that TiNi can contain a large amount of the PGMs as Ti(Ni, Rh), Ti(Ni, Ir) and Ti(Ni, Pt). Lattice parameters monotonously increase with increasing amount of PGMs. With increasing Pt content, Ms slightly decreases when less than 10mol%Pt while continuously increases as the rate of 26K/mol%Pt when more than 10mol%Pt. On the other hand, Ms decreases and then disappears with increasing Rh or Ir content. Hardness ranges from HV180 to HV570 and the maximum values in the pseudobinary systems lie around 20-30mol%PGM, suggesting solid solution hardening caused by the substitution of PGMs. INTRODUCTION Shape memory alloys (SMAs) exhibiting large shape-recovery strain and force in addition to superelasticity (SE) are important functional materials and the practical applications of SMAs become wide day by day. Although several kinds of SMAs have been reported in the literature such as Cu-based and Fe-based SMAs [1], Ti-Ni can be said to be the only one practical SMA at the present time. However, the applications of Ti-Ni are still limited partially due to the maximum shape recovery temperature of the Ti-Ni alloys: the highest Ms is around 400K in the binary system [2]. Large efforts have been paid in order to change the martensitic transformation temperature (Ms) of Ti-Ni by addition to substitutional elements. It is known that most ternary elements such as Fe, Co and Cr reduce Ms of Ti-Ni [3, 4] but that some substitutional elements such as Zr, Hf and Pd raise Ms [5]. It has been also pointed out that Ms of Ti-Ni exhibits a tendency to increase by adding large atomic-size elements
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