New modelling of the B2 phase and its associated martensitic transformation in the Ti–Ni system

A symmetric two-sublattice model (Ni, Ti, Va) 0.5(Ni, Ti, Va) 0.5 is applied to describe the intermediate B2 compound in order to cope with the order–disorder transition in the Ti–Ni system. Using this model, the ordered B2 and the disordered Ti-rich b.c.c. are described by a single Gibbs free energy function. The B2 phase is the parent phase of the martensitic transformation in the TiNi shape memory alloys (SMAs), and its thermodynamic properties are then reassessed with emphasis on its composition range that is critical for SMAs. The low temperature B19′ phase is also evaluated on the basis of the selected experimental data from the martensitic transformation. Properties related to the transformation are studied in comparison with experimental data. The magnetic contribution is examined for the martensitic transformation. All calculations are in satisfactory agreement with experimental phenomena.