Origin of microstructure in Bi–Pb and Bi–Sn binary eutectics

Abstract

To study in detail the nature of the advancing solid/liquid interface, thin specimens of the binary eutectic alloys Bi-Pb2 Bi and Bi-Sn were solidified at various low speeds (5.7–115 nm S⁻¹) and quenched to form a representative interface for subsequent examination. Both eutectic alloys were found to be of the faceted-non-faceted type, with Bi the faceting phase. The solid/liquid interface was observed to be nearly planar and isothermal at the lowest growth rate used, but became increasingly irregular and non-isothermal with increasing growth rate. The maximum observed temperature difference between the most and least advanced parts of the interface was estimated to be ∼ 1 K. Massive Bi particles were formed at the lowest growth rate. Although still recognisable in outline, these Bi regions became subdivided at higher growth rates. An extracted particle of subdivided Bi was shown to be a single crystal. In the case of the Bi-Sn eutectic alloy, at the higher growth rate used, two phase projections were sometimes observed to form on the interface, indicating the presence of a layer of liquid undercooled with respect to both phases. An explanation is proposed to account for the formation of this layer.