Activities in the FeTiO₃-NiTiO₃ solid solution from alloy-oxide equilibria at 1273 K

Abstract

Nine tie-lines between Fe-Ni alloys and FeTiO₃-NiTiO₃ solid solutions were determined at 1273 K. Samples were equilibrated in evacuated quartz ampoules for periods up to 10 days. Compositions of the alloy and oxide phases at equilibrium were determined by energy-dispersive x-ray spectroscopy. X-ray powder diffraction was used to confirm the results. Attainment of equilibrium was verified by the conventional tie-line rotation technique and by thermodynamic analysis of the results. The tie-lines are skewed toward the FeTiO₃ corner. From the tie-line data and activities in the Fe-Ni alloy phase available in the literature, activities of FeTiO₃ and NiTiO₃ in the ilmenite solid solution were derived using the modified Gibbs-Duhem technique of Jacob and Jeffes [K.T. Jacob and J.H.E. Jeffes, An Improved Method for Calculating Activities from Distribution Equilibria, High Temp. High Press., 1972, 4, p 177-182]. The components of the oxide solid solution exhibit moderate positive deviations from Raoult’s law. Within experimental error, excess Gibbs energy of mixing for the FeTiO₃-NiTiO₃ solid solution at 1273 K is a symmetric function of composition and can be represented as: ΔG^E=8590(±200)X_FeTiO3X_NiTiO3J/mol Full spectrum of tie-lines and oxygen potentials for the three-phase equilibrium involving Fe-Ni alloys, FeTiO₃-NiTiO₃ solid solutions, and TiO₂ at 1273 K were computed using results obtained in this study and data available in the literature.