Effect of Loading Rate on Mechanical Properties and Fracture Toughness for Ni-Ti-Nb Shape Memory Alloy.

Abstract

The static tensile and fracture toughness properties of Ni-Ti-Nb shape memory alloy (SMA) were examined at room temperature and 561 K. Its chemical composition is 51 wt.% Ni, 38 wt.% Ti, and 11 wt.% Nb. The shape recovery temperature is around 350 K. The tensile strength and the reduction of area at 561 K are slightly higher than those at 293 K. And the 0.2% proof stress, tensile strength, and reduction of area are constant with increasing loading rate below 10000 MPa/s. With increasing temperature Kmax slightly decrease below 413K, but slightly increases below 561 K, and KQ slightly decreases below 561 K. With increasing loading rate below 10000 MPa√(m) Kmax is constant, but KQ increases. The fracture toughness of this SMA is lower than that of general structural elements, because, micro-cracks initiate from the Ti-rich precipitates at the bottom of a dimple. K (JIC) was obtained as 51.7 MPa√(m), so the material crack was evaluated conservatively using at K (JIC). In conclusion Ni-Ti-Nb SMA has sufficient mechanical and fracture toughness strength for structural elements at elevated temperatures.