Role of Nucleation in Rapid Solidification

Abstract

During rapid solidification processing (RSP) the amount of liquid undercooling is an important factor in determining microstruetural development by controlling phase selection during nucleation and morphological evolution during crystal growth. When a liquid is dispersed into fine droplets, an effective isolation of the most potent nucleants allows for a deep undercooling in the range of 0.3-0.4 T_m before the onset of nucleation. Liquid undercooling behavior can be influenced by powder size refinement, size distribution characteristics and coating treatments. At high undercooling the nucleation of an equilibrium phase may be superseded by metastable product structures. In this case the use of metastable phase diagrams is important for the interpretation and prediction of solidification products such as supersaturated solutions, metastable intermediate phases and glasses. With known heterogeneous sites in droplets it has been possible to control nucleation and to identify specific pathways for metastable phase formation and microstruetural development. The operative reaction is determined by a competitive nucleation kinetics which can be modified at different undercooling levels by the effect of cooling rate. These advances have allowed for a clearer assessment of the interplay between undercooling, cooling rate and particle size statistics in controlling nucleation during RSP structure formation.