S. Tokui, Y. Takeda, M. Tomozawa, T. Koyano, H. Y. Kim, S. Miyazaki, University of Tsukuba, Tsukuba, Japan
High temperature shape memory alloys (HTSMAs) have attracted attention because of their potential applications for high temperature devices. Ti-Ni-Zr alloys are considered as promising candidates for practical HTSMAs due to the relatively low price of raw materials compared with other HTSMAs such as Ti-Ni-Hf, Ti-Ni-Pt, Ti-Ni-Pd and Ti-Ni-Au. One of the major drawbacks of Ti-Ni-Zr alloys is poor workability. In this study, the effect of Zr content on cold workability and transformation temperatures in Ti-49.5Ni-(0-20)Zr alloys was investigated. In addition, the effect of Ta addition on cold workability and shape memory properties of Ti-49.5Ni-15Zr was investigated in order to improve cold workability of Ti-Ni-Zr alloys. The transformation temperatures increase with increasing Zr content, but workability becomes worse. The total reduction in thickness was about 20% prior to fracture during cold rolling in the Ti-49.5Ni-15Zr alloy. The cold workability improved with increasing Ta content. The (Ti-49.5Ni-15Zr)-20Ta specimen could be cold rolled up to 60% reduction in thickness without fracture. The martensitic transformation start temperature Ms of the Ti-49.5Ni-15Zr alloy was measured to be 460K. The Ms was not changed by addition of Ta up to 20at.%. The shape memory properties were investigated by thermal cycling tests under various constant stresses. Shape memory effect was confirmed in all specimens although Maximum recovery strain decreases with increasing Ta content. Maximum recovery strain of 2.9% was obtained in the (Ti-49.5Ni-15Zr)-20Ta alloys heat treated at 1223K for 7.2ks. The relationships among cold workability, shape memory properties and microstructure will be discussed in terms of Ta addition.
Summary: Ti-Ni-Zr alloys are considered as promising candidates for practical HTSMAs due to the relatively low price of raw materials compared with other HTSMAs such as Ti-Ni-Hf, Ti-Ni-Pt, Ti-Ni-Pd and Ti-Ni-Au. One of the major drawbacks of Ti-Ni-Zr alloys is poor workability. In this study, the effect of Zr content on cold workability and transformation temperatures in Ti-49.5Ni-(0-20)Zr alloys was investigated. In addition, the effect of Ta addition on cold workability and shape memory properties of Ti-49.5Ni-15Zr was investigated in order to improve cold workability of Ti-Ni-Zr alloys.
