Y. Takeda, M. Tomozawa, T. Koyano, H. Y. Kim, S. Miyazaki, University of Tsukuba, Tsukuba, Japan
Shape memory alloys with high transformation temperatures are required for being used in devices that operate at elevated temperatures. Applications of binary Ti-Ni alloys are limited to temperatures lower than 373K, because the martensitic transformation start temperature is lower than 373K. Substitution of Zr or Hf for Ti or substitution of a precious element such as Au, Pd or Pt for Ni is an effective way to increase transformation temperatures of Ti-Ni alloys. The Ti-Ni-Zr system is regarded as most practical due to the relative low price of raw materials. The martensitic transformation start temperature above 373K can be obtained in Ti-Ni-Zr alloys with a Zr content above 10at.%. However, the workability of Ti-Ni-Zr alloys becomes worse with increasing Zr content. Therefore, the workability of Ti-Ni-Zr alloys is needed to be improved for practical applications. In this study, the effect of Nb addition on cold workability and transformation temperatures of the Ti-49.5Ni-15Zr high-temperature shape memory alloy was investigated. The cold workability was investigated through cold rolling. The gap between two rolls was decreased step by step. The total thickness reduction was measured when the specimen was broken. The total thickness reduction prior to fracture was about 20% for the Ti-49.5Ni-15Zr alloy. The total thickness reduction was increased with increasing Nb content. The (Ti-49.5Ni-15Zr)-Nb alloys could be cold rolled up to 80% reduction in thickness without fracture when Nb content is 10 at.% or more. The martensitic transformation start temperature decreased with increasing Nb content. However it was still higher than 373K for alloys with Nb content less than 15 at.%.
Summary: The high transformation temperature is essentially required to be used in devices that operate at elevated temperature. The Ti-Ni-Zr system is regarded as most practical due to the relative low price of raw materials. However, the workability of Ti-Ni-Zr alloys becomes worse with increasing Zr content. Therefore, the workability of Ti-Ni-Zr alloys is needed to be improved for practical application. In this study, the effect of Nb addition on cold workability and transformation temperature of Ti-49.5Ni-15Zr high-temperature shape memory alloy was investigated.
