Equilibrium Defects and Concentrations in Nickel Aluminide

Perturbed angular correlation of gamma rays was applied to determine properties of equilibrium defects in B2 NiAl near the stoichiometric composition. Point defects were detected through quadrupole interactions they induce at In probe atoms on the Al sublattice. Well-resolved signals were observed for probe atoms having zero, one or two Ni-vacancies (VNi) in the first neighbor shell. The fractions of probes in different sites are analyzed using a thermodynamic model to determine defect properties as follows. The equilibrium high-temperature defect is determined to be the triple defect combination (two VNi and one Ni-antisite atom) through the variation of the vacancy concentration with composition and not, for example, the Schottky vacancy pair. The binding enthalpy of VNi with a probe atom was determined to be in the range 0.18-0.24 eV. Site fractions were measured for three samples having 50.03, 50.14 and 50.91 at.% Ni at temperatures up to 1300 C. Vacancy concentrations were deduced from the site fractions and binding enthalpy. The equilibrium constant for formation of the triple defect was determined as a function of temperature from the vacancy concentrations and sample compositions. The formation enthalpy was found to be in the range 1.65-1.83 eV, depending on the binding enthalpy. The formation entropy was found to be -3.2(4) kB. The large, negative value of the formation entropy probably cannot be explained in terms of a binding entropy, and we speculate that triple defects harden the B2 lattice, perhaps by disrupting the well-known 1/3 <111> ‘soft mode’ lattice instability in B2 and bcc materials.