Transformation intervals and elastic strain energies of B2-B19′ martensitic transformation of NiTi

This study investigates a fundamental thermodynamic question about the temperature intervals of the B2-B19′ martensitic transformation in a Ti-50.2 at%Ni alloy. Temperature intervals of thermoelastic martensitic transformations are considered to be caused by internal elastic energies produced by the formation of the martensite (in the case of the forward transformation). The intervals have been traditionally measured by differential scanning calorimetry analysis. This study demonstrates that such measurements are subjective to heating and cooling rate of the measurements and the commonly values reported in the literature are mostly overestimates of the true values, causing errors in the determination of thermodynamic parameters. In this work, by means of varying thermal scanning rate, the true temperature intervals of the B2-B19′ martensitic transformation in a Ti-50.2 at%Ni alloy are determined to be 5.1 K for the forward and 10.1 K for the reverse processes. These values are significantly smaller than those generally reported in the literature. Based on these measurements, variations of the specific elastic strain energy stored between the start and the finish of the transformations are estimated to be 0.386 J/g and 0.778 J/g for the forward and the reverse processes, respectively.