An investigation was made of the effect of atomic vibrations, upon point defect free-energies and equilibrium concentrations in this B2 compound, by using the quasi-harmonic approximation; combined with a new embedded-atom potential. The entropy term appeared to make the predominant contribution to the Gibbs free energies of point defects. The vibrational entropies of the main defect complexes (triple-Ni defect, exchange defect, divacancy, interbranch-Al defect) were found to be positive at 0 to 1700K. This led to an increase in the concentrations of all 4 types of point defect in Ni-rich and stoichiometric NiAl. On the Al-rich side, the effect of lattice vibrations was to shift the minimum, in the vacancy concentration versus temperature curve, towards lower temperatures. The effect of zero-point vibrations was shown to be too small to affect the types of constitutional defect. They continued to be Ni antisite atoms on the Ni-rich side, and Ni vacancies on the Al-rich side.

Point Defects in NiAl - the Effect of Lattice Vibrations. A.Y.Lozovoi, Y.Mishin: Physical Review B, 2003, 68[18], 184113 (10pp)