Formation of microstructure during solidification of Bi–Pb–Sn ternary eutectic

Abstract

To investigate the nature of the Bi–Pb–Sn ternary eutectic, specimens were solidified unidirectionally at very low speeds and quenched to form a representative solid/liquid interface for subsequent study. Specimens made using the generally accepted composition, as reported by Ho et al., did not form all three solid phases from the start of freezing. Specimens produced using the composition reported by Sakurai, i.e., 54 wt-%Bi, 28 wt-%Pb, and 18 wt-%Sn, did give all three phases from the beginning of freezing, indicating that it is the correct eutectic composition. It was found that this eutectic is of the faceted (Bi) non-faceted (X phase) non-faceted (Sn) type. Under the freezing conditions used, a double binary microstructure was formed, with one component consisting of Sn fibres in the X phase and the other of a Bi–Sn complex regular microstructure. While the occurrence of a double binary microstructure was predicted by C. S. Smith for a lamellar ternary eutectic, the current observation shows that it can also occur in a system with one fibrous phase.