Microstructural effect on the thermomechanical behavior of aged Ni-rich NiTi SMA

In this study, we investigate the microstructural effect on the thermomechanical behavior of an aged 50.6 at% Ni NiTi shape memory alloy. The thermal and structural characterizations at zero-stress were performed by means of Differential Scanning Calorimetry (DSC) and x-ray Diffraction (XRD) analyzes, supported by Scanning Electron Microscopy (SEM) observations. It was reported that increasing the aging temperature induces a significant change in microstructural features and thermal properties such as transformation process, transformation temperatures and transformation rate. The precipitation phase kinetic as function of aging temperature was also discussed. In order to understand the influence of the observed microstructural changes on the NiTi mechanical behavior, the stress-induced martensitic transformation was studied through an isothermal compression tests. The hardness and the superelastic stress-strain loops were strongly affected by the microstructural behavior. The correlation between thermal and mechanical behaviors was carried out by the construction of the critical stress-temperature diagram. Two different transformation processes were selected to study the effect of the Ti3Ni4 precipitation phase on the global NiTi thermomechanical behavior. The obtained results can be very useful to predict the occurrence of the shape memory and superelastic properties for an aged Ni-rich NiTi alloy.