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

Shape memory alloys(SMAs) are designed to be used as structural elements at elevated temperatures. The material used in this study was Ni-Ti-Nb SMA with a chemical composition of 51 wt% Ni, 38 wt% Ti, and 11 wt% Nb. The shape-recovery finishing temperature was around 350 K. When the material was heated from room temperature(293 K)to high temperature(561 K), the tensile strength and 0.2%-proof stress slightly decreased, while the elongation and reduction of area increased. Young's modulus increased with the temperature, up to 673 K. The fatigue strength at 561 K in air and in water was slightly higher than at 293 K in air. The fatigue limit in water was close to that in air at 561 K. The fatigue limits at 561K were about 290, 240, and 150 MPa for stress ratios of -1, 0, and 0.5 respectively. Fatigue cracks were found to initiate in the Ti-rich precipitates in the sub-surface, indicating that the fatigue strength is affected by the mean stress. The Ni-Ti-Nb SMA was found to have sufficient mechanical and fatigue strengths to be used as a structural element at elevated temperatures in water.