Influence of the post-deformation annealing heat treatment on the low-cycle fatigue of NiTi shape memory alloys

Shape memory alloys (SMA) suffer from the same impairing mechanisms experienced during cycling loading by classic alloys. Moreover, SMA fatigue behavior is greatly influenced by thermomechanical cycling through the zone of thermoelastic phase transformation, which is the basis of shape memory and superelasticity effects. Since the fatigue resistance of any material can be improved by an appropriate thermomechanical treatment, in the present work combined differential scanning calorimetry and microhardness testing were used to determine an optimum annealing temperature for the cold-worked Ni-50.1%Ti alloy. The optimization is based on the assumption that latent heat of transformation is proportional to the mechanical work generated by SMA upon heating, while material hardness is related to the yield stress of the material. It is supposed that an optimum trade-off in these two properties guarantees the best dimensional and functional stability of SMA devices. The level and stability of the mechanical work generated by the material during low-cycle fatigue testing are considered criteria for the material performance and thus of the validity of the proposed optimization procedure.