THERMAL VACANCIES IN B2 AND L1(2) ORDERING ALLOYS

Equilibrium configurations in Ll2- and B2-ordering ternary alloys A3B1 -xCx and AB1-xCx (0 less-than-or-equal-to x less-than-or-equal-to 1) c ontaining thermal vacancies are studied within the Bragg-Williams appr oximation. A new approach proposed for the estimation of vacancy conce ntration in a crystal consists of analysing a state of equilibrium wit h respect to vacancy migration between the crystal and its surrounding s considered as an infinite ''vacant'' crystalline lattice. Direct num erical minimisation of the alloy free energy yields a full overview of an interplay between vacancy formation, vacancy ordering and atomic o rdering in alloys characterised by particular interrelations between p air interaction energies of atoms. It is found out that, without affec ting the character of chemical ordering, various relations between ato mic pair interaction energies control the vacancy formation process an d lead to qualitatively different vacancy configurations. The origin o f considerable discrepancies between the present results and some of t he literature data is discussed.