Thermodynamic investigation of the Al$_2$O$_3$-CaO-Fe$_2$O$_3$-FeO system: Heat capacity measurement, modeling, and application

The Al$_2$O$_3$-CaO-Fe$_2$O$_3$-FeO system plays a critical role in high-temperature industrial processes, and a thorough understanding of its phase equilibria is essential. Due to extremely complex phase equilibria, no complete thermodynamic modeling for this system has been reported in the literature. A comprehensive thermodynamic assessment of the Al$_2$O$_3$-CaO-Fe$_2$O$_3$-FeO system was conducted using the CALPHAD method, aiming to provide a reliable thermodynamic description for high-temperature industrial applications. As a key ternary compound in the Al$_2$O$_3$-CaO-Fe$_2$O$_3$ subsystem, the CaFe$_3$AlO$_7$ (C$_2$F$_3$A) phase was synthesized by conventional solid-state reaction. Its phase composition and microstructure were analyzed by XRD, EPMA, and SEM. Heat capacity measurements were performed using DSC and incorporated into the thermodynamic optimization. The compound energy formalism (CEF) was applied to model the Gibbs energies of solid solution phases, while the ionic two-sublattice liquid model was employed to describe the Gibbs energy of the liquid phase. A new set of self-consistent thermodynamic parameters was obtained based on critically evaluated phase equilibria data. ... mehrThe calculated phase diagrams, including isothermal sections and liquidus projections under various oxygen partial pressures, demonstrate a satisfactory agreement with experimental data from the literature. Furthermore, the developed thermodynamic description was applied to evaluate the stability of inclusions under representative conditions of calcium-treated Al-killed steel production, demonstrating its important role in guiding inclusion control strategies.