Study On Phase Separation And Purification Of Ti-85wt.%Si Alloy By Vacuum Directional Solidification

Vacuum directional solidification combines vacuum refining with directional solidification and the directional crystal growth,polysilicon with relatively high purity can be obtained at the bottom,and crystal growth of Ti-Si alloy can be controlled.At present,there are few theoretical studies on diffusion mass transfer and volatilization removal of impurities during directional solidification in vacuum,and the mechanism of phase separation and directional crystal growth during directional solidification is also not well studied.?1?In this paper,the separation coefficients are modified and the theoretical separation characteristics of Al and Ca are discussed;Relative separation ability of impurities Al,Ca and Mg is discussed in combination with gas-liquid boundary layer;Vacuum directional solidification can remove impurities better than vacuum refining,and the content of impurities in vacuum refining decreases with the increase of holding time and melting temperature,and the volatility of impurities increases with the increase of holding time and melting temperature.?2?Vacuum directional solidification makes the distribution of impurity Al,Ca and Fe from bottom of crucible?silicon enrichment zone?to top of crucible?alloy enrichment zone?increase first and then decrease.The content of impurity Mg reaches the highest at solid-liquid interface,but the distribution of impurity Mg decreases gradually in the longitudinal direction;The vertical distribution of coagulation coefficients of impurities Al,Fe,Mg and Ca increases first and then decreases.The preferential solidification stage of primary silicon is in the range of1414?¹331?,followed by the co-solidification stage of TiSi₂ and Si crystals below 1331?;?3?Quantitative analysis of XRD showed that the pull-down rate of 3?m/s made the content of Si crystal 79.9wt%,higher than 15?m/s?55.5wt%?,while the number of TiSi₂ crystal decreased relatively.?3?The diffusion coefficient of Al in the Si melts is lower than that of the Ti–85wt.%Si melts;Under the same melting temperature and melt moving speed,the mass transfer coefficient of Al in the Ti–85 wt.%Si melts is larger than that of the Si melts,and the phase boundary layer thickness of Al in the TiSi₂ phase is larger than that of the Si phase.Line scanning showed that the content of Al in TiSi₂ phase was higher than that in Si phase,but the highest content at the phase boundary of the two phases.