SOLIDIFICATION REACTIONS IN UNDERCOOLED ALLOYS

It is now recognized that rapid solidification conditions can be achie ved with slow cooling rates provided that the liquid is undercooled su bstantially prior to nucleation. In fact, many of the novel metastable microstructures produced by rapid solidification require the consider ation of an undercooled liquid for analysis. In general, rapid solidif ication techniques involve either constrained growth in which the soli d phase formation is limited by the rate of heat extraction or delayed nucleation of the solid followed by unconstrained growth. With delaye d nucleation methods such as the droplet emulsion technique direct mea surement 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 that is usuall y set by heterogeneous nucleation. Processing variables can be used to control the undercooling and produce a transition in solidification r eactions. In this case the use of metastable phase diagrams is importa nt for the analysis of product structures and pathways during solidifi cation and solid state treatments. A key to the understanding of struc tural evolution is the consideration of competitive nucleation and gro wth kinetics and thermal history, which can also provide a model for c ontrol of solidification reactions as demonstrated in selective alloys .