Solidification of highly supercooled liquid metals and alloys

The amount of liquid supercooling is an important factor in determining microstructural development by controlling phase selection during nucleation and morphological evolution during rapid crystal growth. While supercooling is an inherent feature of many solidification techniques, the deepest supercoolings and most controlled studies have been possible in droplet samples. Liquid dispersal into fine droplets yields an effective nucleant isolation which allows for supercoolings approaching 0.3–0.4 Tm before the onset of solidification. At high supercooling, the nucleation of an equilibrium phase may be superseded by metastable product structures to produce a transition in solidification kinetics. In this case, the use of metastable phase diagrams is important for the interpretation and predication of product structures and pathways for metastable phase formation during solidification and solid state treatments. At the same time, a consideration of competitive nucleation and growth and thermal history is essential in the analysis of structure formation. These features highlight a number of reaction paths and provide a basis for an effective alloy design and microstructure control strategy.