Rapid solidification of undercooled Fe-Sn-Si monotectic alloys

Rapid solidification of Fe_60.4Sn_29.6Si₁₀, Fe_43.6Sn_21.4Si₃₅ and Fe_20.1Sn_9.9Si₇₀ ternary monotectic alloys was accomplished by drop tube technique. Two or three layer core-shell microstructures were formed during experiment. Phase characteristics was obtained by X-ray diffraction analysis. The microstructure of Fe_60.4Sn_29.6Si₁₀ alloy is composed of α-Fe and β-Sn solid solutions phases together with FeSn and FeSi intermetallic compound phases; the microstructure of Fe_43.6Sn_21.4Si₃₅ alloy consists of β-Sn, FeSn₂, FeSi and Fe₂Si phases; and the microstructure of Fe_20.1Sn_9.9Si₇₀ alloy contains β-Sn solid solution, FeSi₂ intermetallic compound and Si semiconducting phase. Thermal history of alloy droplets was calculated by Newtonian heat transfer model combined with classical nucleation theory. According to the analysis of thermal profile, the undercooling of alloy droplet is the controlling factor for different microstructures during the rapid solidification in drop tube. Calculated results show that undercooling decreases when droplet diameters increase. Tip-splitting structures of FeSn phase are observed in the solidification microstructures of Fe_60.4Sn_29.6Si₁₀ monotectic alloy. Structural analyses indicate that the instability of solid-liquid interface aroused by deep undercooling may cause a morphological transition of FeSn phase from dendrite growth to spontaneous tip-splitting growth. It has also been found that there is close relationship between dendrite breaking and tip-splitting structures, which may be the practical reason for spontaneous grain refinement.