The interaction and concentration of vacancies in NiAl

In stoichiometric NiAl, the concentration of thermal vacancies is unusually high because vacancy pairs form a low-energy triple defect consisting of two Ni vacancies and a NiAl antisite defect. The formation energy of the triple defect has been calculated to be 2.28 eV and measured to be 2.18±0.3 eV. On the Al-rich side of stoichiometry, in addition to the thermal vacancies, there are even higher concentrations of constitutional vacancies. All the point defects interact elastically through the image pressure. In Al-rich Ni45Al55, the interaction is estimated to reduce the effective formation energy of a Ni vacancy by ∼0.1 eV.