On the thermodynamic driving force for diffusion-induced grain boundary migration, discontinuous precipitation and liquid film migration in binary alloys

Abstract

An expression for the net thermodynamic driving force (free energy change per unit distance of reaction front migration per unit area of reaction front) for the migration of reaction fronts during diffusion-induced grain boundary migration (DIGM), discontinuous precipitation (DP) and liquid film migration (LFM) in binary alloys is determined from a two-dimensional model which considers individual segments of the migrating reaction front as open systems which can receive solute and solvent atoms from or give them up to the surrounding material. In the case of DIGM and DP the reaction front is treated as a separate grain boundary phase of thickness δ. In the case of LFM it is treated as a liquid film of thickness δ. The magnitude of the force on the reaction front is calculated for DIGM in AuCu and for LFM in NiMo using available thermodynamic data. Finally, a criterion for the stability of a reaction front, when a small change in composition across a reaction front occurs and the reaction front composition is slightly changed, is established which shows that a driving force for reaction front migration can develop spontaneously as the result of solute and solvent diffusion along it, as long as the curvature of the free energy curve for the reaction front is positive.