Oxygen Solubility in Boron-Containing Ni–Co Melts

Abstract

Alloys of the nickel–cobalt system are widely used in industry. Boron is one alloying constituents of the alloys. The study of the thermodynamics of oxygen solutions in boron-containing Ni–Co melts is of substantial interest for manufacturing practice. Thermodynamic analysis of the oxygen solutions in the boron-containing Ni–Co melts is performed. The equilibrium constants of the reaction of boron with oxygen dissolved in the Ni–Co melts, the activity coefficients at infinite dilution, and the interaction parameters for the melts differing in the composition are determined at 1873 K. The interaction parameters are calculated; these are \(\varepsilon _{{{\text{B(Ni)}}}}^{{{\text{Co}}}}\) = –0.238 and \(\varepsilon _{{{\text{B(Co)}}}}^{{{\text{Ni}}}}\) = 0.674. When boron interacts with oxygen in the Ni–Co melts, the oxide phase contains the NiO and CoO oxides along with B₂O₃. The molar fractions of B₂O₃, NiO, and CoO in the oxide phase are calculated for different boron concentrations in the Ni–Co melts at 1873 K. In the case of the nickel melt with boron contents of more than 0.01%, the molar fraction of boron oxide is close to unity. As the cobalt content in the melts increases to 20%, the molar fraction of boron oxide in the oxide phase decreases and then is almost unchanged. The dependences of the oxygen solubility in the studied melts on the cobalt and boron contents are calculated. The deoxidizing ability of boron slightly decreases as the cobalt content increases to 20%, and after that increases as the cobalt content in the melt increases. The boron concentrations corresponding to the minima in the oxygen solubility curves and associated minimum oxygen concentrations are determined.