Nonequilibrium phenomena in rapid solidification:theoretical treatment for process modeling

Traditionally, a solidification process has been treated as a pure thermal problem with an implicit assumption that phase change takes place under equilibrium conditions and the solid-liquid interface follows the equilibrium phase diagram. Rapid solidification in many advanced material process, such as atomization, melt substrate quenching e.g., splat cooling, melt spinning, and thermal spray deposition, surface melting with high-energy beams, however, takes advantage of nonequilibrium phenomena which lead to materials with special properties. It is therefore essential that any treatment of rapid solidification problems includes the nonequilibrium kinetics. This article reviews various kinds of nonequilibrium phenomena associated with rapid solidification processes, including melt undercooling, metastable phase formation, crystalline growth kinetics, solute trapping, etc. The historical development of how to incorporate these phenomena in a thermal model is described. The effects of various nonequilibrium phenomena on rapid solidification characteristics are discussed.