NUCLEATION REACTIONS IN UNDERCOOLED LIQUIDS

The amount of liquid undercooling is an important factor in determining solidification microstructure development by exposing nucleation reactions that yield an initial phase selection and by controlling morphological evolution during rapid crystal growth. At high undercooling the nucleation of an equilibrium phase may be superseded by metastable product structures to produce a transition in solidification kinetics that is often controlled by heterogeneous nucleation. In this case the use of metastable phase diagrams is important for the interpretation and prediction of nucleation reaction structures and pathways for phase formation. While the study of homogeneous nucleation requires a sample free from all external nucleants, the study of heterogeneous nucleation requires a sample which contains only well-known and characterized nucleants. With alloy samples and in melts with dispersed particles, droplet experiments may be designed to expose known nucleant surfaces to the undercooled liquid. Even with known catalysts a multiplicity of nucleant sites is observed indicating a hierarchy of potency which is influenced by kinetic effects. A model analysis of the atomistic events controlling the heterogeneous nucleation reactions and a relative ranking of potency provides some insight into the dynamics of catalysis mechanisms operating at high undercooling. These features can be highlighted by examples of several types of reaction paths to provide a perspective on nucleation reactions and microstructure control.