Ti-Ni-Zr 合金の形状記憶特性と加工性に及ぼす Zr 濃度の影響

Abstract

Shape memory alloys with high transformation temperatures are required for being used in devices that operate at elevated temperatures. Ti-Ni-Zr alloys are regarded as most practical high temperature shape memory alloys due to the relative low price of raw materials compared with other high temperature shape memory alloys such as Ti-Ni-Hf, Ti-Ni-Pt, Ti-Ni-Pd and Ti-Ni-Au. In this study, the effect of Zr content on transformation temperatures, shape memory characteristics and cold workability in Ti-49.5Ni-(0-15)Zr alloys was investigated. The reverse transformation finish temperature increased monotonously with increasing Zr content, while the martensitic transformation temperatures decreased with increasing Zr content, reaching the minimum at 5 at%Zr and then increased by further increasing Zr content. The martensitic transformation temperatures above 373 K were obtained in Ti-49.5Ni-Zr alloys with a Zr content above 10 at%. The shape memory properties were investigated by thermal cycling tests under various constant stresses. The critical stress for slip increased with increasing Zr content. The maximum recovery strain of about 5% was obtained in the Ti-49.5Ni-(0-15)Zr alloys. The Ti-49.5Ni-(0-5)Zr alloys could be cold rolled up to 90% reduction in thickness without fracture. The cold workability became worse with increasing Zr content. The total thickness reduction prior to fracture was about 30% for the Ti-49.5Ni-15Zr alloy