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 appl ied to describe the intermediate B2 compound in order to cope with the orde r-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 ene rgy function. The B2 phase is the parent phase of the martensitic transform ation in the TiNi shape memory alloys (SMAs), and its thermodynamic propert ies are then reassessed with emphasis on its composition range that is crit ical for SMAs. The low temperature B19' phase is also evaluated on the basi s of the selected experimental data from the martensitic transformation. Pr operties related to the transformation are studied in comparison with exper imental data. The magnetic contribution is examined for the martensitic tra nsformation. All calculations are in satisfactory agreement with experiment al phenomena. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.