Effect of Environment on Static Tensile and Fatigue Properties of Ni-Ti-Nb Shape Memory Alloy.

Abstract

A shape memory alloy (SMA) is intended to be used as structural elements at elevated temperatures. The static tensile and fatigue properties of Ni-Ti-Nb SMA were examined at room temperature and 561 K. The chemical composition of Ni-Ti-Nb SMA is 51 wt.% Ni, 38 wt.% Ti and 11 wt.% Nb. The shape recovery temperature is around 350 K. The 0.2% proof stress and tensile strength are slightly decreasing, but the elongation and the reduction of area are increasing with increasing temperature below 561 K. Young's modulus increases with increasing temperature. The fatigue strength at high temperature is slightly higher than that at room temperature in air. The fatigue limit in water almost agrees with that in air at 561 K. Fatigue limits at 561 K are about 290, 240 and 150 MPa for stress ratios of R=-1, 0 and 0.5 respectively. Since fatigue crack is found to initiate from the Ti-rich precipitates at the sub-surface, the fatigue strength is considered to be affected by mean stress. The SMA is found to have sufficient mechanical and fatigue strength for structural elements at elevated temperature in water.