THERMODYNAMIC ANALYSIS OF ALUMINATE FORMATION AT FE/AL2O3 AND CU/AL2O3 INTERFACES

Subsolidus ternary phase relationships in the systems Fe-Al-O and Cu-Al-O are reviewed in order to provide a basis for thermodynamic analysis of aluminate formation at Fe/alpha-Al2O3 and Cu/alpha-Al2O3 interfaces. Expressions for the critical oxygen activity in the Fe-rich and Cu-rich solid solutions necessary to stabilize the equilibrium aluminate phases (FeAl2O4 and CuAlO2, respectively) in the presence of alpha-Al2O3 are derived using data available in the literature. As in previous work in the Ni-alpha-Al2O3 system, aluminate formation by solid state reaction of Fe and Cu with alpha-Al2O3 was calculated to require a threshold concentration of oxygen in the metal, which is of the order of 115 the solubility limit. The results are presented in stability diagrams and compared with previous results on the Ni/alpha-Al2O3 system in terms of the free energies of formation of the aluminates and the relative free energies of solution of oxygen in the metals. The results are also compared with available experimental observations on Fe/alpha-Al2O3 and Cu/alpha-Al2O3 interfaces.