Solid solution domains in equilibrium phase diagrams: Our concepts and reality

Abstract

According to the model of interatomic pair interaction, there are two possible types of interaction between components of binary metallic substitutional alloys: the attraction between the A and B atoms, resulting eventually in the formation of chemical compound A m B n (the mixing energy E mix = u AA + u AB — 2 u AB is negative), and the repulsion between A and B atoms, giving rise to a mixture of crystals formed by pure components (the mixing energy is positive). In thermodynamics, these two types of interatomic interactions are related to the tendency of a system to ordering (the sign of deviations from the Raoult law is negative) and to phase separation (the sign of deviations is positive), respectively. These two types of interatomic interactions correspond to two basic types of equilibrium phase diagrams: for the case E mix 0, the phase diagrams consist of the regions corresponding to the mechanical mixture of crystals formed by components of the alloy. However, there exists a large number of equilibrium phase diagrams involving domains (bounded and unbounded) in which, as expected, solid solutions are formed. The presence of the solid solution domains in the phase diagrams means that these solid solutions are equilibrium phases. If the solid solution is an equilibrium phase at temperatures for which the diffusion of components is sufficient for its decomposition, it is possible to say that such a solid solution is ideal. Such a conclusion is in striking contrast with thermodynamic data which suggest that the overwhelming majority of solid solutions are nonideal and exhibit either positive or negative deviations from the Raoult law. As a consequence, they demonstrate the tendency either to phase separation or to ordering, which should certainly lead to the decomposition of the solid solution when the system approaches equilibrium.