NUCLEATION-CONTROLLED SOLIDIFICATION KINETICS

Solidification microstructures are usually classified with the respect to the growth processes responsible for the observed morphology. Howe ver, experience has demonstrated that several regimes of processing co nditions can be identified in which nucleation is the determining fact or in the phase selection and the evolution of the solidification micr ostructure. Several of these regimes are analyzed to identify the infl uence of nucleation on the final microstructure. In most cases, nuclea tion occurs by heterogeneous catalysis with competitive kinetics betwe en different sites and product structures. The proper analysis of hete rogeneous nucleation requires a clear identification and description o f active catalytic sites which has been a continuing challenge for bot h experiments and analysis since nucleation signatures can be masked b y subsequent reactions. A strategy for the systematic exploration of h eterogeneous nucleation in alloys is described that yields some inform ation about possible atomistic and dynamic effects at the catalyst/mel t interface that is of theoretical and practical interest. Additionall y, computer simulation studies are described that elucidate some of th e atomistic features of nucleation which become increasingly important when a multiplicity of nucleation sites is observed with a hierarchy of potency.