Effect of an overheating temperature on cyclic isothermal stress-induced transformations in single crystal Cu–13.3Al–4.0Ni (wt.%) shape memory alloys

Motivated by eventual applications as high temperature actuators, cyclic isothermal stress-induced transformations of Cu–13.3Al–4.0Ni (wt.%) single crystal wires with stress-free transformation temperatures: Mf = 80 °C, Ms = 100.5 °C, As = 104.5 °C and Af = 117 °C were studied under the two adverse conditions of high overall strain (9%) and an overheating temperature (175 °C for 30 min). Wires were subjected to isothermal stress cycling at 25 °C using an Instron testing machine with environmental chamber until fairly repetitive stress–strain response was obtained. These tests were repeated on the same specimen at progressively higher temperatures of 40, 60, 80 and 100 °C. It is seen that the initial cyclic response is primarily shape memory whereas the subsequent cyclic response is primarily pseudoelastic, attended by some residual inelastic deformation. Tests on a different virgin specimen at 100, 120, 140 and 160 °C showed a similar trend. This points to the possibility of a downward shift in the stress-free transformation temperatures. Possible reasons are generation of dislocations as well as the domination of α-martensites over β-martensites due to the overheating temperature and stress cycling. The effect of stress cycles on overall strain on full loading, after unloading and after heating in between stress cycles has been discussed.