The thermodynamics of non-equilibrium interfaces during phase transformations in concentrated multicomponent alloys

A unified thermodynamic description of moving non-equilibrium interfaces is developed for both solid/solid and solid/liquid transformations. The theory is applicable to concentrated multicomponent alloys where diffusion is possible in both phases or in just the parent phase, and where energy is dissipated due to solute drag. To be consistent with energy dissipation, we find that solute drag affects both the velocity of the interface and distribution coefficients for the compositions of the two phases at the interface. In the limit of binary alloy solidification, the theory predicts significant changes in the interfacial compositions from that given by the phase diagram at velocities commonly found during additive manufacturing. Since the distribution coefficient is affected by solute drag, the solute-trapping behavior observed in molecular dynamics simulations implies that the interfacial diffusivity lies between the diffusivity of the bulk solid and liquid. A comparison to past work on non-equilibrium interfaces during solid/solid and solid/liquid phase transformations is also given