ORIGIN OF MICROSTRUCTURE IN 350 K EUTECTIC OF BI-IN-SN TERNARY-SYSTEM

Specimens of the 350 K (77 degrees C) ternary eutectic of the Bi-ln-Sn system were solidified unidirectionally at very low speeds and quench ed reform a representative solid/liquid interface for subsequent study . From such interfaces, this eutectic was observed to be of the facete d-faceted-non-face ted type, freezing to form the phases Bi (faceted), BiIn (faceted), and gamma-Sn, a hexagonal Sn phase (non-faceted). A ' double binary' microstructure was formed, consisting of regions of qua siregular BiIn-gamma-Sn structure and regions of complex regular Bi-ga mma-Sn structure. In each of these regions, the phase spacing II and g rowth, are R were found to obey the usual law found for binary eutecti c alloys, namely, lambda(2)R = constant. It was found that the gamma-S n phase decomposes on cooling reform the phases Bi, BiIn, and (tetrago nal) beta-sn. Observation of precipitates in the Biln phase showed som e solubility for Bi and Sn in this phase at the ternary eutectic tempe rature.