Determination of the atomistic structure of the Sigma 3(111) twin boundary in NiAl

We have applied molecular static calculations to examine the Sigma 3 (111) twin boundary in NiAl. Stoichiometric, Ni-rich and Al-rich grain boundaries were considered. We include a consistent treatment of the excess energies, which depend on the chemical potentials, which depend in turn on the point defect formation energies and the stoichiometry. The grain boundary which best agrees with HRTEM experiments is stoichimetric and symmetric, with Al atoms occupying the mirror plane. The expansion of the grain boundary is 0.05 nm.