A first-principles plane-wave pseudopotential method based upon density functional theory was used to investigate the concentrations of point defects and their interaction in TiNi. The calculations showed that, in the thermal equilibrium state, TiNi developed 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 were considerable amounts of vacancies in Ti-rich compounds. The calculated interaction energies showed that the nearest-neighboring Ni antisite and Ti antisite were attractive to each other, whereas the second-nearest-neighboring Ni antisites repel each other and Ti antisites mutually attract, which indicated that a Ti-rich domain could appear in stoichiometric TiNi, and also explained 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 were explained; based upon charge density calculations. It was shown that the attractive interaction between Ti antisites was due to the accumulation of electron density between the Ti antisites, whereas the repulsion between Ni antisites was due to the depletion of electron density between them.

Point Defects and their Interaction in TiNi from First-Principles Calculations. J.M.Lu, Q.M.Hu, L.Wang, Y.J.Li, D.S.Xu, R.Yang: Physical Review B, 2007, 75[9], 094108 (7pp)