Point defects in NiAl: The effect of lattice vibrations

Abstract

We investigate the effect of atomic vibrations on point defect free energies and equilibrium concentrations in the B2 NiAl compound using the quasiharmonic approximation in combination with a recently developed embedded-atom potential. The entropy term appears to be the dominant contribution to the Gibbs free energies of point defects. Vibrational entropies of the main defect complexes: triple-Ni defect, exchange defect, divacancy, and even of the interbranch-Al defect turn out to be positive in the whole range of temperatures studied here (0--1700 K). This leads to an increase in the concentrations of all four types of point defects in Ni-rich and stoichiometric NiAl. On the Al-rich side, the effect of lattice vibrations is to shift the minimum on the vacancy concentration versus temperature curve towards lower temperatures. The effect of zero-point vibrations is shown to be too small to affect the type of constitutional defects in NiAl. The constitutional defects remain nickel antisite atoms on the Ni-rich side and nickel vacancies on the Al-rich side.