SOLIDIFICATION OF HIGHLY SUPERCOOLED LIQUID-METALS AND ALLOYS

The amount of liquid supercooling is an important factor in determinin g microstructural development by controlling phase selection during nu cleation and morphological evolution during rapid crystal growth. Whil e supercooling is an inherent feature of many solidification technique s, the deepest supercoolings and most controlled studies have been pos sible in droplet samples. Liquid dispersal into fine droplets yields a n effective nucleant isolation which allows for supercoolings approach ing 0.3-0.4T(m) before the onset of solidification. At high supercooli ng, the nucleation of an equilibrium phase may be superseded by metast able product structures to produce a transition in solidification kine tics. In this case, the use of metastable phase diagrams is important for the interpretation and predication of product structures and pathw ays for metastable phase formation during solidification and solid sta te treatments. At the same time, a consideration of competitive nuclea tion and growth and thermal history is essential in the analysis of st ructure formation. These features highlight a number of reaction paths and provide a basis for an effective alloy design and microstructure control strategy.