Kinetic processes in undercooled melts

In the examination of metastable undercooled liquids, a thermodynamic analysis can often provide insight into the range of structural and compositional options that are possible either as precursor reactions in a liquid or as solidification products. The thermodynamic analysis also provides an important basis for the examination of nucleation energetics. The numerous examples of metastable phase formation during solidification of undercooled melts demonstrates that the actual product selection is dominated by kinetics. In fact, the observation of undercooling is linked to a kinetic control. Under most conditions the onset of nucleation and the level of undercooling are controlled by heterogeneous catalysis. Other forms of kinetic control can be identified during rapid quenching and involve constrained growth of solid phases. In both cases, a kinetic competition exists that is essential in the understanding of phase selection which may be modified by the thermal and processing history. With heterogeneous nucleation, the analysis requires a clear identification and description of the multiplicity of sites and the hierarchy of potency. In undercooled liquids, the heterogeneous nucleation behavior and metastable phase diagrams provide an effective basis for analysis of kinetic transitions. Throughout the analysis of kinetic processes, well-defined experiments and physically realistic models are essential in revealing the controlling mechanisms.