Evaluation of kinetic equations describing the martensite-austenite phase transformation in NiTi shape memory alloys

Analysis and comparison of existing phenomenological equations for transformation kinetics has been performed for the evolution of the austenite fraction as a function of temperature during the athermal reverse martensite → austenite transformation in NiTi shape memory alloys (SMA) of different compositions. The evaluation has been carried out using experimental data presented in the literature and experimental data obtained in this work, using differential scanning calorimetry, for thermally-cycled equiatomic and Ni-rich NiTi SMAs. Large discrepancies exist between the experimental kinetics and the kinetics, as described by the previously proposed phenomenological equations. A new model has been presented which adequately describes the experimental data, in particular as sensitively expressed by the transformation rate as a function of the degree of transformation.