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 co mpositional options that are possible either as precursor reactions in a liquid or as solidification products. The thermodynamic analysis al so provides an important basis for the examination of nucleation energ etics. The numerous examples of metastable phase formation during soli dification of undercooled melts demonstrates that the actual product s election is dominated by kinetics. In fact, the observation of underco oling is linked to a kinetic control. Under most conditions the onset of nucleation and the level of undercooling are controlled by heteroge neous catalysis. Other forms of kinetic control can be identified duri ng rapid quenching and involve constrained growth of solid phases. In both cases, a kinetic competition exists that is essential in the unde rstanding of phase selection which may be modified by the thermal and processing history. With heterogeneous nucleation, the analysis requir es a clear identification and description of the multiplicity of sites and the hierarchy of potency. In undercooled liquids, the heterogeneo us nucleation behavior and metastable phase diagrams provide an effect ive basis for analysis of kinetic transitions. Throughout the analysis of kinetic processes, well-defined experiments and physically realist ic models are essential in revealing the controlling mechanisms. (C) 1 997 Elsevier Science S.A.