Diffusionless (chemically partitionless) crystallization and subsequent decomposition of supersaturated solid solutions in Sn–Bi eutectic alloy

Results of a study on microstructural evolution of eutectic Sn-57 wt.% Bi processed with cooling rates of 10−2 , 1 K s−1 and approximately 105  K s−1 are presented. In order to distinguish different mechanisms of microstructure formation, a comparison with microstructures of different hypoeutectic alloys with compositions down to below the maximum solubility of Bi in Sn–Bi is undertaken. It is found that at the cooling rates of 10−2 and 1 K s−1 , coupled eutectic growth occurs, leading to lamellar structures with different length scales. At the rapid quenching rates of approximately 105  K s−1 , structure formation in the eutectic alloy is qualitatively different. Partitionless solidification resulting in a supersaturated solid solution with the initial composition is observed in both eutectic and hypoeutectic alloys. It is shown that the observed microstructure of the rapidly solidified alloys forms by the decomposition of the supersaturated solid solution.This article is part of the theme issue ‘Heterogeneous materials: metastable and non-ergodic internal structures’.