Triple-defect complexes in theB2intermetallic compound NiAl

The present study combines a theoretical model on the basis of the Bethe-Bragg-Williams approximation with literature data on the pair-interaction energies in ${\beta }^{\prime }$-NiAl with $B2$ (CsCl) structure. In this intermetallic compound, the most interesting point defect complexes are the triple defects since they have been supposed to be mainly responsible for the concentration dependence of the Ni self-diffusion coefficient in the temperature range between 1000 and $1300\mathrm{K}$, which is nearly constant for Al-rich and stoichiometric alloys and shows a pronounced increase with increasing Ni content on the Ni-rich side of the composition range. The theoretical composition dependence of the concentration of triple-defect complexes, derived in the present work, and the experimental Ni diffusivities from the recent literature show excellent correlation, thus pointing to an important contribution of the triple-defect diffusion mechanism in $B2$ NiAl.