Solidification reactions in undercooled alloys

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It is now recognized that rapid solidification conditions can be achieved with slow cooling rates provided that the liquid is undercooled substantially prior to nucleation. In fact, many of the novel metastable microstructures produced by rapid solidification require the consideration of an undercooled liquid for analysis. In general, rapid solidification techniques involve either constrained growth in which the solid phase formation is limited by the rate of heat extraction or delayed nucleation of the solid followed by unconstrained growth. With delayed nucleation methods such as the droplet emulsion technique direct measurement of undercooling is available for analysis of metastable phase formation. In fine droplet samples an effective nucleation isolation allows for undercoolings of about 0.3 T m with a limit taht is usually set by heterogeneous nucleation. Processing variables can be used to control the undercooling and produce a transition in solidification reactions. In this case the use of metastable phase diagrams is important for the analysis of product structures and pathways during solidification and solid state treatments. A key to the understanding of structural evolution is the consideration of competitive nucleation and growth kinetics and thermal history, which can also provide a model for control of solidification reactions as demonstrated in selective alloys.