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 10 5  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 10 5  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’.

No abstract

The results of microstructure investigation of eutectic alloys Sn Bi and Sn Pb, obtained at cooling rates of the melt 10-2 K/s, 1 K/s and 105 K/s are represented. Various cooling rates are achieved by graphite mold casting and by ultra fast quenching from the melt. Microstructure and grain structure are investigated by scanning electron microscopy and the electron backscatter diffraction technique. It has found that the melt cooling rate increasing at rapid quenching leads to microcrystalline structure and to a change of the solidification mechanism. The reasons for the transition from lamellar to the microcrystalline structure are discussed.