Nucleation controlled phase selection during solidification

Abstract

An important and long-standing goal in the analysis and modeling of solidification processing is the identification and prediction of microstructural evolution in order to provide a control over solidification structure. The advances towards achieving the goal are significant especially with regard to the role of growth kinetics competition in microstructure selection. At the same time there have been advances in the analysis of the initial phase selection during nucleation that plays a central role in the overall evolution of solidification microstructure. During product phase transitions that develop with changing undercooling level or alloy composition, competitive nucleation can yield mixed phase microstructures of both stable and metastable phases as shown for rapid solidification of eutectic, peritectic and glass forming alloys. At low undercooling the key role of interface catalysis is evident as well as an inhibited growth to allow for the optimization of the nucleation density during grain refining processes. At high undercooling, during crystallization of bulk melts or devitrification of metallic glass, the nucleation onset and the initial phase selection are usually determined by a heterogeneous nucleation on catalytic sites that are developed through either ex situ or in situ processes. The complete analysis of solidification microstructure evolution requires the consideration of the dynamic kinetic competition that is the basis of phase selection transitions.