Point defects and their interaction in TiNi from first-principles calculations

Abstract

A first-principles plane-wave pseudopotential method based on density functional theory is used to investigate the concentrations of point defects and their interaction in TiNi. The calculations show that, in the thermal equilibrium state, TiNi develops antisite-type point defect configurations. In accordance with the sequence of the stability of the B2, B19, and B19(') phases, the concentrations of the point defects in stoichiometric TiNi at the identical temperature decrease from B2 to B19 to B19('). At high temperature, there are considerable amounts of vacancies in Ti-rich compounds. The calculated interaction energies show that the nearest-neighboring Ni antisite and Ti antisite are attractive to each other, whereas the second-nearest-neighboring Ni antisites repel each other and Ti antisites mutually attract, which indicates that a Ti-rich domain may appear in stoichiometric TiNi, and also explains the high solubility of excess Ni and low solubility of excess Ti in TiNi. The electronic structure mechanisms behind the interactions between the point defects are discussed based on charge density calculations. It was shown that the attractive interaction between Ti antisites is due to the accumulation of electron density between the Ti antisites, whereas the repulsion between Ni antisites is due to the depletion of electron density between them.