SOLIDIFICATION STRUCTURE FORMATION IN HIGHLY UNDERCOOLED BINARY ALLOYS

During rapid solidification processing of metals and alloys, structure formation often occurs under non-equilibrium conditions involving metastable phase generation from an initially highly undercooled liquid state. The formation of metastate phases and structure morphologies in slowly cooled (10-20(DEGREES)C/min) droplet samples has also been observed in "splat-quenched" (10('6)-10('8)(DEGREES)C/sec) samples, but the undercooling levels have not been determined accurately in rapid cooling studies. In addition, manipulations of solidification structures in droplet samples were achieved by controlling factors such as sample surface catalysis, droplet size, cooling rate and alloy composition.; Unlike rapid quenching studies, the ability to observe the melting behavior of metastable phases in droplet samples, allows for the accurate determination of the undercooling levels and heats of fusion. An observation common to rapid quenching and undercooling studies, is the bypassing of the peritectic reaction by, either the formation of the peritectic phase, e.g. the (epsilon)-phase in Pb-Bi alloys, or by the direct formation of the primary phase as a supersaturated product, e.g. the (beta)Sn(,ss)-phase in Sn-Cd alloys. Also, the eutectic reaction in the Pb-Bi system is bypassed by the formation of the metastable X-phase, which was established to have solidified well within its hypercooling regime.; The characterization of the undercooling behavior of an alloy sample is important in understanding the operation of phase selection at nucleation and the nature of the post nucleation processes that determine the solidification structure. In Pb-Bi alloys, phase selection kinetics between several phases was found to be a function of undercooling and composition. In Pb-Sn and Pb-Sn alloys, while homogeneous structures such as the a(,ss)-phase formed at the highest undercooling levels, segregated (non-hypercooled) morphologies developed at the lowest undercooings. Although phase selection is important in deciding the nature of the solidification products, a combination of varying degrees of growth kinetics, recalescence and interface stability effects can play a crucial role in the morphological evolution of the phase distributions. In addition, the observed microstructures are usually modifications of solidified structures as a result of coarsening and decomposition precipitation reactions.