Solidification and Melting – Asymmetries and Consequences

Solidification and melting are phase transitions from the liquid to solid state or vice versa and are thus often assumed to be similar processes with only opposite direction. However, they can be fundamentally different, i.e. asymmetric, in aspects of both thermodynamics and kinetics. It is known that superheat in the solid is difficult to obtain, unlike supercooling in the liquid. This is often attributed to the fact that nucleation in the liquid can occur (homogeneously or heterogeneously) in the bulk, in the solid it will occur at outer or inner surfaces of the crystal. A further asymmetry is evident as the growing phase is a phase with fast diffusion kinetics in the case of melting, with slow diffusion kinetics in the case of solidification. Two types of experiments (solutal melting and melting/resolidification in a temperature gradient) are presented that allow an evaluation and quantification of the consequences of these asymmetries.