Composition Dependences of Entropy Change and Transformation Temperatures in Ni-rich Ti–Ni System

For Ni-rich Ti–Ni alloys, physical properties such as specific heat and electric resistance were systematically investigated. The B2/B19′ martensitic transformation temperatures ranging from 180 to 373 K were determined for Ni contents of 49.98–51.09 %, and a sudden disappearance of martensitic transformation was confirmed for Ni contents greater than 51.23 %, which has also been well reported in the literatures. The entropy change was also evaluated from differential scanning calorimeter measurement, and it was clarified that the entropy change plotted to T₀ temperature shows an S-shaped curve, starting to drastically decrease at about 300 K. Thermodynamic approaches were then carried out attempting to determine the reason for the disappearance of transformation. The entropy change estimated from direct measurements of specific heats for 51.75 Ni (B2) and 50.92 Ni (B19′) was found to be more consistent with the experimental data, rather than the calculated curve based on the Debye model for vibration specific heat. It was proposed that the equilibrium between the parent and martensite phases obeys the Clausius–Clapeyron relationship in the composition–temperature system. Using the constructed composition–temperature diagram, the disappearance of martensitic transformation in the Ti–Ni system can be well understood as being due to the drastic increase of hysteresis at low temperature.