Solid-liquid phase equilibria at 727 °C in the ternary system Fe-Mg-Si

Abstract

The solid-liquid phase equilibria in the Fe-Mg-Si ternary system were experimentally investigated at 727 °C by two complementary approaches: reaction to equilibrium of Fe-Mg-Si powder mixtures and growth of reaction zones at the interface of diffusion couples. X-ray powder diffraction, optical metallography (OM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) were used to characterize the phases formed in these experiments. It has been shown that except silicon and βFeSi₂, all the compounds and solid solutions stable at 727 °C in the Mg-Si and Fe-Si binary subsystems (Mg₂Si, εFeSi, α₁Fe₃Si, α₂Fe₃Si, and αFe) are in equilibrium at that temperature with a magnesium-rich Mg-Si liquid phase. The liquid simultaneously in equilibrium with FeSi and Mg₂Si contains 3.1±0.2 at.% Si; that involved in the three-phased equilibrium with εFeSi (50 at.% Si) and α₁Fe₃Si (27 at.%Si) contains 1.0±0.05 at.%Si. For lower silicon contents in the liquid, the conjugate solid phases are successively α₁Fe₃Si (DO₃ structure, homogeneity range 27 to 14 at.%Si), α₂Fe₃Si (B2 structure, homogeneity range 14 to 11.5 at.%Si), and αFe (A2 structure, homogeneity range 11.5 to 0 at.% Si). It is however to note that for a silicon content in the liquid as low as about 0.05 at.%, the conjugate solid phase is still quasistoichiometric α₁Fe₃Si with 25 at.% Si.