The effect of thermal cycling and applied stress level on post cycling - Stress free transformation behaviour in NiTi alloys

Abstract
A number of NiTi alloy wires were thermally cycled against a constant applied stress to analyse changes in transformation temperatures and heats of transformation. It was hoped that subsequent differential scanning calorimetry analysis (DSC) at zero stress could be used to establish if permanent changes to internal friction and stored elastic energy occur and what relationship these have to the level of the previously applied stress. It was found that the post cycling M transformation temperatures increased at a rate consistent with the pre-cycling Clausius-Clapeyron gradient. This not only confirms the presence of internal stress fields but also implies that the level of internal stress generated may be measured using the Clausius-Clapeyron gradient. However, beyond a certain stress the M and A temperatures actually began to decrease again. It was also found that after cycling the heats of transformation (as measured by DSC) gradually decreased with increasing stress level. In addition, the heat loss between the forward and reverse réactions also decreased with increasing applied stress level. All these changes are discussed in relation to internal friction and stored elastic energy.