Phase selection in containerless solidification of FeSi2 during free fall in drop tube

Abstract

The melts of the Fe-66.7%Si alloy ejected into a drop tube was solidified during its free fall. The spherical samples collected at the bottom of the drop tube were classified into several groups according to their diameters from 300 μm to 1000 μm. The microstructures of the samples were examined and analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential thermal analysis (DTA). In addition to the grain refinement of the constituent phases, it was observed that the primary phase changed from the equilibrium FeSi (e) phase to the metastable Fe2Si5 (α) phase and then returned to the e phase again with decrease in the sample diameter. This result indicates that the microstructure of the sample solidified from the melt during free fall is controlled by the phase competition between α and e depending on the degree of the undercooling. If it is assumed that the phase having the highest growth rate is selected as the primary phase, the dendrite growth model proposed by Boettinger, Coriell and Trivedi predicts the changes of the primary phase from e to α and then to e with increase in the undercooling. This means that the metastable eutectic point as a function of undercooling is expressed by the curve just like a character, C, in the Fe-Si binary phase diagram.