The temperature and composition dependence of the formation of thermal vacancies in the intermetallic compound ${\mathrm{Ni}}_3\mathrm{Al}$ with the ${L1\mathrm{}}_2$ structure was studied by means of positron lifetime spectroscopy. High values of the effective vacancy formation enthalpy $H_{\mathrm{V}}^{\mathrm{F}}$ increasing monotonically from 1.65$\pm$0.08 eV to 2.01$\pm$0.08 eV with increasing Ni content from 74.1 to 76.5 at. % are found. A fit of a model based on the interaction of nearest-neighbor atoms to the experimental data demonstrates predominant formation of thermal vacancies on the Ni sublattice and high concentrations of antisite atoms. The results are discussed with respect to recent tracer-diffusion measurements in ${\mathrm{Ni}}_3\mathrm{Al}$.

• Received 6 November 1996

DOI:https://doi.org/10.1103/PhysRevB.56.3032