Homogeneous granular microstructures developed by phase separation and rapid solidification of liquid Fe-Sn immiscible alloy

Abstract

Liquid Fe41.5Sn58.5 alloy was containerlessly solidified under the free-fall microgravity condition inside drop tube. Conspicuous liquid phase separation took place in the broad miscibility gap of 325 K and formed the dispersed pattern microstructures of Fe-rich phase distributed in the matrix of Sn-rich phase. Although the equilibrium phase constitution includes only FeSn and FeSn2 compounds in the phase diagram, the high cooling rate and large undercooling lead to the formation of four new metastable phases αFe, Fe3Sn, (Sn)1 and (Sn)2 respectively. The granular patterns of Fe3Sn phase dominates the droplet solidification process. Phase field simulation reveals that the granular microstructure experiences three evolution stages during liquid phase separation: the nucleation, aggregation, or coalescence and Ostwald ripening of secondary liquid phase, which agrees well with the experimental results. Meanwhile, two kinds of Marangoni migrations play an important role to develop the homogeneous granular structures. EDS analysis shows that the solute contents of various phases increase with the decrease of alloy droplet diameter, while the primary αFe phase is found to contain as much as 18.07 at.% Sn at droplet diameter 73 μm owing to the significant solute trapping during rapid solidification.