Thermodynamic prediction of the equilibrium chemical composition of critical nuclei: Bcc Cu precipitation in α-Fe

The chemical composition of Cu-rich precipitates from body-centered Fe–Cu is treated in the framework of classical nucleation theory. Although the analysis is based on purely thermodynamic arguments and does not account for kinetic effects, it is demonstrated that, when taking into account the composition dependence of interfacial energy and driving force, the predicted equilibrium composition of nanometer-sized Cu precipitates differs substantially from the equilibrium composition of large precipitates. Evidence is provided to the experimental finding that early Cu precipitates contain substantial amounts of Fe.