Phase Stability and Mechanical Properties of Ti(Ni, Ru) Alloys
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Phase Stability and Mechanical Properties of Ti(Ni, Ru) Alloys Masahiro Tsuji*, Hideki Hosoda**, Kenji Wakashima and Yoko Yamabe-Mitarai1 Precision & Intelligence Laboratory (P&I Lab.), Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. 1 High-Temperature Materials Group, National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan. * Graduate student, Tokyo Institute of Technology. ** Corresponding author: Phone&FAX 81-45-924-5057, Email [email protected] ABSTRACT Effects of ruthenium (Ru) substitution on constituent phases, phase transformation temperatures and mechanical properties were investigated for Ti-Ni shape memory alloys. Ti50Ni50-XRuX alloys with Ru contents (X) from 0mol% (binary TiNi) to 50mol% (binary TiRu) were systematically prepared by Ar arc-melting followed by hot-forging at temperatures from 1173K to 1673K depending on chemical composition. Phase stability was assessed by DSC (differential scanning calorimetry), XRD (X-ray diffractometry) and TEM (transmission electron microscopy). Mechanical properties were investigated using hardness and tensile tests at room temperature. With increasing Ru content, it was found that the lattice parameter of B2 phase increases, the martensitic transformation temperature slightly decreases, and the melting temperature increases monotonously. Besides, R-phase appears for Ti-Ni alloys containing 3mol% and 20mol%Ru but no diffusionless phase transformation is seen in Ti-Ni alloy containing 5mol%Ru. Vickers hardness shows the maximum at an intermediate composition (HV1030 at 30mol%Ru); this suggests that large solid solution hardening is caused by Ru substitution for the Ni-sites in TiNi. INTRODUCTION Ti-Ni shape memory alloys (SMAs) are practically important functional materials because they exhibit excellent shape memory effect (SME) and superelasticity (SE) in addition to superior mechanical properties [1-3]. A limitation of the Ti-Ni SMAs for practical applications is that the martensitic transformation temperature (Ms) is limited below 400K. Thus the Ti-Ni binary alloys have a difficulty for high temperature applications such as in steam boilers. Thus high-temperature shape memory alloys (HTSMAs) exhibiting higher actuation temperatures and comparable mechanical properties are required. Several investigations have been done for HTSMAs. It has been reported that the martensitic transformation temperature of Ti-Ni alloys is dramatically raised by Pd addition when Pd is added more than 20mol% [4]. It has also been reported that Ms in Ti-Ni alloys remarkably increases by addition of some platinum-group metals (PGMs) such as Pt and Pd to Ti-Ni alloys [5]. These HTSMAs, however, still exhibit some drawbacks such as brittleness, so that new HTSMAs should be developed. In this study, Ru addition to the binary Ti-Ni was of interest because Ru is a PGM and is less expensive compared to other PGMs. Besides, at least no systematic work has been done for the Ru addition to Ti-Ni to the best
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