Mean field theory of point defects in β-NiAl

We have used a mean field theory to study the equilibrium concentrations of vacancies and antisite defects in each sublattice of β-NiAl in the composition range between 45 and 55 at% Al and in the temperature range 800–2000 K. The interatomic potentials, which are inputs to the theory, are extracted from recently reported (continuous) pair potentials of Finnis–Sinclair type. The predominant defect in the stoichiometric alloy is a combination of an Ni-antisite defect and two vacancies on the Ni sublattice. The Al-rich alloys of composition (50+Δ) at% contain 2Δ% vacancies; since the alloys are almost perfectly ordered, these vacancies predominantly occupy the Ni sublattice. Similarly, the Ni-rich alloys of composition (50−Δ) at% contain Δ% Ni antisites. Both the vacancies on the Ni sublattice (in Al-rich alloys) and Ni-antisites (in Ni-rich alloys) show negligible temperature dependence, and hence owe their origin to the off-stoichiometry. In all the alloys, the Al-antisites have the lowest concentration (of the order 10−6 even at 2000 K). We have critically compared our theoretical results with available experimental ones.