Experimental and computational characterization of alloy–spinel–corundum equilibrium in the system Ni–Co–Al–O at 1873 K

Abstract

The three-phase equilibrium between alloy, spinel solid solution and α-Al₂O₃ in the Ni–Co–Al–O system at 1873K has been fully characterized as a function of alloy composition using both experimental and computational methods. The equilibrium oxygen content of the liquid alloy has been measured by suction sampling and inert gas fusion analysis. The oxygen potential corresponding to the three-phase equilibrium has been determined by emf measurements on a solid state galvanic cell incorporating (Y₂O₃)ThO₂ as the solid electrolyte and Cr+Cr₂O₃ as the reference electrode. The equilibrium composition of the spinel phase formed at the interface between the alloy and alumina crucible was measured by electron probe microanalysis (EPMA). The experimental results are compared with the values computed using a thermodynamic model. The model uses values for standard Gibbs' energies of formation of pure endmember spinels and Gibbs' energies of solution of gaseous oxygen in liquid nickel and cobalt available in the literature. The activity-composition relationship in the spinel solid solution was computed using a cation distribution model. The variation of the activity coefficient of oxygen with alloy composition in the Ni–Co–O system was estimated using both the quasichemical model of Jacob and Alcock, and Wagner's model along with the correlations of Chiang and Chang. The computed results of spinel composition, oxygen potential and oxygen concentration corresponding to the three-phase equilibrium are in excellent agreement with the experimental data.