Alloy Formation and Stability

Abstract

The stability of a binary alloy depends on the excess free energy ΔG given by $$ \Delta G = \Delta H - T\Delta S + p\Delta V\,, $$ where ΔH, ΔS and AV are the heat, entropy and volume of formation. Under equilibrium conditions (p = 0) the volume of formation makes no direct contribution, but we shall see that ΔV has an important — though often neglected — indirect influence through the volume dependence of the heat and also of the entropy of formation. It is immediately clear that the prediction of the stability of an A x B1−x alloy formed by the two compounds A and B is an extremely complex problem: the alloy can be either a random solid solution (with complete substitutional disorder and with a crystal structure corresponding either to that of one of the components or an entirely new structure), or a stoichiometric intermetallic compound (which would then be completely ordered), but of course any intermediate situation is possible as well (partial substitutional disorder in an intermetallic compound, long or short range order in a solid solution, etc.). Moreover, the differences in the atomic sizes and in the interatomic potentials will induce static lattice distortions (and thus a certain degree of topological disorder) for any alloy which shows substitutional disorder.