Thermodynamic modeling of the Ni-Ti-Cr system and the B2/B19′ martensitic transformation

The Ni-Ti-Cr ternary is a critical system for shape memory alloys. In this work, a thermodynamic reassessment of the Ni-Ti-Cr system was performed using the CALPHAD (CALculation of PHAse Diagrams) method. For the first time the martensitic transformation product phase B19′ was successfully introduced into the description of the Ni-Ti-Cr system. Firstly, the thermodynamic descriptions of the binary Ni-Ti and Ti-Cr systems were updated by focusing on the heat capacities of the Ni3Ti and NiTi2 phases and the homogeneity range of the Laves phases, respectively. The determinations of the end-member parameters of the Laves phases were supported by ab initio calculations. The present model parameters can well reproduce the phase equilibria and thermochemical properties of the Ni-Ti and Ti-Cr systems. Then, the Ni-Ti-Cr system was assessed by combining the updated binaries with available experimental data. The B19′ phase was described using a two-sublattice model, i.e., (Cr,Ni,Va)0.5(Cr,Ni,Ti)0.5, based on the atomic occupancy reported in the literature. The B2/B19′ martensitic transformation starting temperatures (MS) of the Ni-Ti and Ni-Ti-Cr alloys were accurately predicted with the assumption that the energy barrier for the martensitic transformation was 150 J/mol. The reliable prediction of MS is expected to promote the development of shape memory alloys.