Ordering, phase separation, and phase transformations in Fe-$M$ alloys

Concepts of ordering, phase separation, and phase transformations in metallic solid solutions are discussed in the context of the latest experimental data on the microstructure of Fe-$M$ alloys. It is shown that the type of chemical interaction between alloy component atoms depends on the degree of the localization of 3d-valence electrons on atoms rather than on the ratio of energies of ordering in different coordination shells. The morphology of the products of phase transformations in alloys with a tendency to ordering or phase separation is considered, and the known mechanisms of phase decomposition are analyzed. The decomposition of alloys having a tendency to ordering and to phase separation is described in thermodynamic terms. It is shown that solid-solution fields in phase diagrams are in fact regions where the alloys have a tendency to phase separation and where, at certain correlations between free-energy components, separation microstructures are formed. It is noted that phase transformations in alloys can occur at different levels of the structure of matter: the ‘primary’ occur at the level of changes (ordering–phase separation) in the electronic structure, and the ‘secondary’ are realized at the level of changes (order–disorder) in the microstructure.