Thermodynamics of oxygen solutions in Fe-Ni, Fe-Co, and Co-Ni melts

Abstract

By thermodynamic analysis of oxygen solutions in Fe-Ni-O, Fe-Co-O, and Co-Ni-O melts, the composition of the oxide phase is established for the first time. In addition, the equilibrium oxygen concentrations in these melts are determined over the whole range of alloy compositions. In Fe-Ni-O and Fe-Co-O melts, the oxide phase mainly contains FeO over a relatively broad of alloy compositions. Sharp increase in NiO content is only observed when the molar fraction of nickel exceeds one; sharp increase in CoO content is only observed when the molar fraction of cobalt exceeds 0.8. In the Co-Ni-O system, the oxide phase contains both CoO and NiO over the whole range of alloy compositions. In the Fe-Ni system, adding nickel to the melt reduces the solubility of oxygen as a result of weakening of the oxygen bonds in the melt by nickel and consequent increase in oxygen’s activity. With further increase in nickel content in the melt, the oxygen content rises at first slowly and then very sharply. In the Fe-Co system, analogously, adding cobalt to the melt reduces the solubility of oxygen as a result of weakening of the oxygen bonds in the melt and consequent increase in oxygen’s activity. With further increase in cobalt content, the oxygen content rises at first slowly and then relatively rapidly. In the Co-Ni system, adding nickel to cobalt increases the solubility of oxygen over the whole range of alloy compositions, on account of the significantly greater solubility of oxygen in nickel than in cobalt.