Dissociation Behavior of the Congruently Melting FeSi Compound in the Fe-Si System: A Bjerrum–Guggenheim Thermodynamic Analysis

This study presents a thermodynamic analysis of the dissociation and association behavior of the Fe–Si system using the Bjerrum–Guggenheim osmotic coefficient. An equilibrium thermodynamic approach was applied to evaluate the Gibbs free energy, equilibrium constant, and degree of association of the congruently melting compound FeSi over a wide temperature range. The Fe–Si system was analyzed across three characteristic crystallization regions: Fe-rich, FeSi, and Si-rich. It was established that the Fe-rich region exhibits behavior approaching ideality with a nearly linear dependence of the osmotic coefficient, whereas the Si-rich region is characterized by strong deviations from ideality due to intensive association processes. The FeSi crystallization region represents a transitional regime in which association and dissociation processes occur simultaneously. The formation and partial dissociation of [FexSiy] clusters significantly affect the thermodynamic behavior of the melt. It was shown that accounting for FeSi dissociation leads to a linearization of the osmotic coefficient dependence and improves the accuracy of thermodynamic description. The proposed analytical approximations demonstrate high correlation coefficients (R2 ≈ 0.99), confirming the reliability of the developed approach. The results provide a consistent thermodynamic framework for describing phase transformations and structural evolution in Fe–Si melts and can be applied to the optimization of metallurgical processes involving silicon-containing alloys.