Effect of Heat-treatment Time on Performance Recovery of Ti-50.4at%Ni Shape Memory Alloy

Abstract

In this research, the effect of heat treatment time on the performance recovery of Ti-Ni shape memory alloy (SMA) is investigated. The chemical composition of specimen is Ti-50.4 at%Ni, and cold working ratio is 30 %. At first, the specimen is heat-treated at 673 K for 3.6 ks. And then, the specimen is heat-treated for recovery of function after isothermal 1000 times loading-unloading test under constant strain. The heat-treatment temperature for performance recovery is 573 K, and heat-treatment time is varied from 0.6 to 3.6 ks. The functional deterioration process of specimens heat-treated for performance recovery is investigated by the isothermal loading-unloading tests under constant strain. Recovery strain and critical stress for inducing martensite increase with increasing heat-treatment time, and are saturated when heat-treatment time is more than 1.8 ks. Moreover, the number of cycle to failure increases with increasing heat-treatment time up to 1.8 ks, has a maximum at 1.8 ks, and then followed by a decrease. These tendencies are caused by a variation of the dislocation density owing to the variation of the heat-treatment condition. From these results, the optimal heat-treatment time for performance recovery is 1.8 ks when heat-treatment temperature is 573K.