Effect of thermal cycling on the thermomechanical behaviour of NiTi shape memory alloys

This paper analyses the influence of thermal cycling at zero stress on the thermomechanical response of NiTi shape memory alloy (SMA) wire in the martensitic, R-phase and austenitic transformation ranges. An in depth study compares the mechanical properties of a sample thermally cycled at zero stress and a sample without thermal or mechanical cycling. The critical stress-temperature diagram for each sample is presented taking into account the R-phase transformation zone. The results show that thermal cycling has considerable impact on the transformation behaviour of the alloy and on the mechanical properties of the R-phase: the values of the apparent modulus and the critical transformation stresses are higher. The parameters that describe the SMA macromechanical behaviour, then, undergo a considerable change due to thermal cycling, suggesting that if an SMA has to be used throughout the R-phase range, the material should be thermally cycled to obtain better mechanical properties.