DSC Investigation on Entropy and Enthalpy Changes in Ni-Rich NiTi Shape Memory Alloy at Various Cooling/Heating Rates

The aim of this work is to experimentally investigate the effect of the cooling/heating rate on both entropy and enthalpy changes during the phase transformation of the Ni-rich NiTi Shape Memory Alloy (SMA). The thermal analysis was performed at zero stress using the Differential Scanning Calorimetry (DSC) technique. From the obtained calorimetric results, it was found that the transformation temperatures are considerably sensitive to the cooling/heating rate variation, mainly the finish temperature of the forward martensitic transformation (M_f). Accordingly, it was shown that enthalpy and entropy changes, during the phase transformation process, are strongly affected by the change of this parameter. Based on Gibbs fundamental equation, the evolution of the Gibbs free energy change was also discussed. The findings of this study can be useful to predict the evolution of the critical stress–temperature diagram. Indeed, the enthalpy change describes the evolution of the diagram slopes as function of the cooling/heating rate which allows to limit each occurred transformation domain and defines the material mechanical behavior whether superelasticity or shape memory.