NUCLEATION REACTIONS IN UNDERCOOLED LIQUIDS

The amount of liquid undercooling is an important factor in determinin g solidification microstructure development by exposing nucleation rea ctions that yield an initial phase selection and by controlling morpho logical evolution during rapid crystal growth. At high undercooling th e nucleation of an equilibrium phase may be superseded by metastable p roduct structures to produce a transition in solidification kinetics t hat is often controlled by heterogeneous nucleation. In this case the use of metastable phase diagrams is important for the interpretation a nd prediction of nucleation reaction structures and pathways for phase formation. While the study of homogeneous nucleation requires a sampl e free from all external nucleants, the study of heterogeneous nucleat ion 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 even ts controlling the heterogeneous nucleation reactions and a relative r anking of potency provides some insight into the dynamics of catalysis mechanisms operating at high undercooling. These features can be high lighted by examples of several types of reaction paths to provide a pe rspective on nucleation reactions and microstructure control.