On the transition from diffusion-limited to kinetic-limited regimes of alloy solidification

An abrupt transition from diffusion-limited solidification to diffusionless, kinetic-limited solidification with complete solute trapping is explained as a critical phenomenon which arises due to local non-equilibrium diffusion effects in the bulk liquid. The transition occurs when the interface velocity V passes through the critical point V = VD, where V = VD is the bulk liquid diffusive velocity. Analytical expressions are developed for velocity–temperature and velocity–undercooling functions, using local non-equilibrium partition coefficient based on the Jackson et al. kinetic model and the local non-equilibrium diffusion model of Sobolev. The calculated functions demonstrate a sharp break in the velocity–undercooling and velocity–temperature relationships at the critical point V = VD. At this point the local non-equilibrium solidus and liquidus lines coincide with the T0 temperature. The relationship to pertinent experiments and the influence of the local non-equilibrium diffusion effects on grain refinement and disorder trapping phenomena are discussed.