Phase selection transitions during undercooled melt solidification

Alloy phase selection is often controlled by nucleation during solidification of undercooled melts. The structure options are defined by thermodynamics, but the selection process is determined by a kinetics competition. During product phase transitions that develop with changing undercooling level or alloy composition, competitive nucleation can yield mixed phase microstructures as shown for rapid solidification of eutectics and glass forming alloys. Alternatively, at increasing rates, an initial solidification reaction can be superseded by a new nucleation controlled phase selection transition due to growth kinetic limitations that can develop during resolidification following surface melting or due to modification of heterogeneous nucleants. The complete analysis of solidification microstructure evolution during phase selection transitions also requires the consideration of solid-state reactions.