Undercooling and Nucleation during Solidification.

Undercooling of liquids is a common occurrence in solidification.The level of undercooling inflvences both the microstructural developmentby controlling phaseselection during nucleation andthe morphological evolution during the growth phase of solidification.The development of large liquid undercooling is linked to kinetic control of solidification processes including suppression of heterogeneous nucleation during slow cooling and constrained growth during rapid quenching.The deepest undercoolings have been measured in droplet samp[es and approach 0.3-0.4•T~.At high undercooling, solidification can yield metastable product structures, whoseconstituents are the result of kinetic competition which usually is determined during the nucleation phaseand controlled by heterogeneous nucleation.Recently, theoretical, experimental andsimulation advanceshave led to a better understanding of nucleation catalysis reactions andthe influence of thermal history during processing at high and low undercoolings.Detailed analysis of the kinetic competition process during solidification along with novel experiments have probed the limits of nucleation theory in both size and time scales and have stimulated new developments in theoretical modeling and computer simu[ation studies.