Metal-spinel-corundum three phase equilibria in the system Ni-Cr-Al-O at 1373 K

Abstract

The tie-lines representing the inter-crystalline ion exchange equilibria between the NiCr2O4-NiAl2O4 spinel solid solution and Cr2O3-Al2O3 corundum solid solution are determined by electron microprobe and EDAX point count analysis of the oxide phases equilibrated with metallic Ni at 1373 K. The component activities in the spinel solid solution are derived from the tie-lines and thermodynamic data for Cr2O3-Al2O 3 solid solution available in the literature. The Gibbs energy of mixing of the spinel solid solution calculated from the experimental data is discussed in relation to the values derived from the cation distribution models which assume random mixing of cations on both tetrahedral and octahedral sites. Positive deviation from the models is observed indicating significant positive enthalpy contribution arising form the size mismatch between Al+3 and Ni+2 ions on the tetrahedral site and Al+3, Ni +2 and Cr+3 on the octahedral site. Variation of the oxygen potential for three-phase equilibrium involving metallic nickel, spinel solid solution and corundum solid solution is computed as a function of composition of the solid solutions at 1373 K. The oxygen potential exhibits a minimum at aluminum cationic fraction ηAl/ (ηAl + ηCr) = 0.524 in the oxide solid solutions.