PHASE SELECTION DURING CRYSTALLIZATION OF UNDERCOOLED LIQUID EUTECTIC LEAD-TIN ALLOYS

During rapid solidification substantial amounts of undercooling are in general required for formation of metastable phases. Crystallization at varying levels of undercooling and melting of metastable phases were studied during slow cooling and heating of emulsified Pb-Sn alloys. Besides the experimental demonstration of the reversibility of metastable phase equilibria, two different principal solidification paths have been identified and compared with the established metastable phase diagram and predictions from classical nucleation theory. The results suggest that the most probable solidification path is described by the "step rule" resulting in the formation of metastable phases at low undercooling, whereas the stable eutectic phase mixture crystallizes without metastable phase formation at high undercooling.