Molecular statics calculations were used to study the  = 3 (111) twin boundary in stoichiometric, Ni-rich and Al-rich samples. The study involved a consistent treatment of excess energies. These depended upon the chemical potentials which, in turn, depended upon the point defect formation energies and upon the stoichiometry. It was found that the grain boundary which best agreed with high-resolution transmission electron microscopic data was stoichiometric and symmetrical; with Al atoms occupying the mirror plane. The expansion of the grain boundary was equal to 0.05nm. Particular care was taken to incorporate chemical potentials into the calculations in a consistent manner. This introduced a marked dependence of the boundary energy not only upon its structure and composition, but also upon the assumed bulk stoichiometry. Thus, in Ni-rich material, the Ni antisite defect predominated and it could be thought of as favoring a Ni-rich boundary by segregation to any Al-rich rivals. In this case, the antisite formation energy entered the expression for the excess boundary energy via the chemical potentials. General expressions were derived for all of the other possibilities for the chemical potential in an ordered AmBn alloy close to stoichiometry.

M.Hagen, M.W.Finnis: Materials Science Forum, 1996, 207-209, 245-8