Segregation Behavior of Impurity Iron in Primary Silicon During Directional Solidification of a Hypereutectic Silicon-Titanium Alloy

Iron (Fe) in metallurgical-grade silicon (MG-Si) is the main metal impurity and is difficult to remove. Strengthening the segregation of impurity Fe at the solid-liquid interface is the key to purifying MG-Si; therefore, a refinement method for the directional solidification of hypereutectic silicon-titanium (Si-Ti) alloy was proposed, and the segregation ability was quantified by calculating the equilibrium segregation coefficient of impurity Fe. By analyzing the microstructure, segregation thermodynamics, and equilibrium segregation coefficient of impurity Fe in a Si-rich layer, it was confirmed that the segregation ability of impurity Fe during Si-Ti alloy refining was stronger than that of MG-Si. The results showed that impurity phases containing Fe precipitates in the Si-rich layer included tau(5) and tau(1) phases. After introducing Ti into the Si melt, the equilibrium segregation coefficient of Fe impurities was significantly reduced, and the equilibrium segregation coefficient of Si-10 wt pct Ti-0.396 wt pct Fe was k(0Fe) = 8.68 x 10(-7), which was much lower than that of MG-Si (6.4 x 10(-6)). During the directional solidification and refining of Si-10 wt pct Ti-0.396 wt pct Fe, the removal rate of impurity Fe increased from 90.5 to 96.75 pct of MG-Si to 99.86 pct. This work helps provide understanding of the segregation behavior of impurities at the solid-liquid interface during the directional solidification of Si-Ti alloys and lays a theoretical foundation for the deep removal of impurities. (C) The Minerals, Metals & Materials Society and ASM International 2022