A statistical thermodynamic model for ordering in non-stoichiometric B2 phases was developed which was based upon the mean-field approximation, with constant pair-wise interaction energies between nearest-neighbor atoms. This approximation was limited to the description of the distribution of atoms and vacancies on the various sub-lattices. Any short-range ordering tendency of the point defects was neglected. Expressions for the point-defect concentrations as a function of composition and temperature were derived from the grand canonical ensemble. In B2-phases, the vacancy concentration depended upon the composition and temperature and it was therefore necessary to consider an open system; where transfer of atoms to, or from, the outside was allowed. In B2-phases, 4 defect equilibria had to be considered. It was shown that, if a series of defect equilibria was involved, it was necessary to apply the principle of microscopic reversibility in order to generate the necessary number of independent equilibrium conditions. Expressions were derived for the activities and partial molar enthalpies of the alloy components as a function of composition and temperature. A comparison with experimental data on ß'-FeAl demonstrated the applicability of this approach.

Application of the Grand Canonical Ensemble to the Study of Equilibrium Point Defect Concentrations in Binary Intermetallic Phases with the B2 Structure. R.Krachler, H.Ipser: Intermetallics, 1999, 7[2], 141-51