Oxygen Solubility in Ni–Co Melts at Complex Deoxidation with Aluminum and Silicon

Abstract

Ni–Co alloys are broadly used in various industries. One of the harmful impurities in these alloys is oxygen, which is found in metals in dissolved form or as nonmetallic inclusions. One of the main tasks of melting out these alloys is to produce a ready-made metal with a minimal oxygen concentration. In the case of the metal melt’s complex deoxidation, the activities of oxides formed by deoxidation are below one. This allows metal production with a lower oxygen concentration and, therefore, is more deeply deoxidized at the same deoxidizing element content. The dominant role in the joint deoxidation with two deoxidants is played out by the stronger of the two. However, if the oxides of the deoxidizing elements form chemical compounds, the weaker deoxidant in deoxidation will stand out. The joint influence of aluminum and silicon on the oxygen solubility in Ni-Co melts is exposed to thermodynamic analysis. The compounds that can form in deoxidation products include both, mullite (3Al2O3 ⋅ 2SiO2) and kyanite (Al2O3 ⋅ SiO2). Silicon presence in the melt slightly enhances the deoxidizing ability of aluminum for the formation 3Al2O3 ⋅ 2SiO2 and significantly for the formation of Al2O3 ⋅ SiO2. The oxygen solubility curves for the formation of Al2O3 ⋅ SiO2 pass through the minimum point in which the position depends on the melt’s aluminum content and does not depend on the silicon content. As in the case of Ni–Co–Al melts, the aluminum content in the minimum points slightly go down from Ni to Co. Further aluminum additives increase the oxygen concentration. The article determines the regions, where Al2O3, 3Al2O3 ⋅ 2SiO2, Al2O3 ⋅ SiO2, and SiO2 are formed due to the aluminum and silicon content in the melt. The deoxidizing ability of aluminum and silicon in Ni–Co alloys become enhanced with increasing cobalt content in the melt; however, the higher is the cobalt content in the melt, the lesser is the extent to which silicon enhances the deoxidizing ability of aluminum.