SOLIDIFICATION OF UNDERCOOLED SN-SB PERITECTIC ALLOYS .1. MICROSTRUCTURAL EVOLUTION

The droplet emulsion technique, which involves dispersal of a bulk liquid alloy into a collection of fine droplets ( 5 to 30-mu-m), was applied to Sn-Sb alloys to yield high levels of controlled undercooling. The maximum undercooling levels achieved varied from 179-degrees-C for pure Sn to 113-degrees-C for a Sn-16 at. pct Sb alloy. Analysis of hypoperitectic alloy samples (alloys with an Sb content less than that of the liquid at the peritectic temperature) indicates that solute trapping occurs during solidification at the levels of undercooling and cooling rate investigated, yielding nearly homogeneous beta-tin solid solutions with compositions approaching those of the bulk alloys. With increasing undercooling and/or cooling rate, hyperperitectic alloys exhibit a transition from a highly segregated structure consisting of faceted primary intermetallic phase and cellular beta to a structure consisting primarily of a supersaturated tin-rich solid solution. Lattice constant measurements confirm that virtually complete supersaturation of beta-tin was achieved in emulsion samples cooled at 200-degrees-C s-1 for compositions up to approximately 20 at. pct Sb. The development and characteristics of subsequent solid-state precipitation were used to guide the interpretation of the often complex solidifaction reaction sequences in the hyperperitectic alloys. The formation of supersaturated beta-tin solid solutions in the undercooled samples is related to the appropriate metastable phase equilibria and the development of solute trapping.