Thermodynamics of oxygen solutions in manganese-containing Ni – Co melts

Abstract

Nickel-cobalt alloys are widely used in modern technology. Manganese is one of the alloying components in these alloys. Oxygen is a harmful impurity in Ni – Co alloys; it presents in the metal in dissolved form or in the form of oxide nonmetallic inclusions. The presence of oxygen in these alloys degrades their service properties. The study of oxygen solution thermodynamics in manganese-containing Ni – Co melts is of considerable interest for the practice of such alloys production. Thermodynamic analysis of oxygen solutions in manganese-containing Ni – Co melts has been carried out. The equilibrium constant of interaction of manganese and oxygen dissolved in the nickel-cobalt melts, the activity coefficients at infinite dilution, and the interaction parameters characterizing these solutions were determined for melts of different composition. In the interaction of manganese with oxygen in Ni – Co melts, the oxide phase, in addition to MnO, contains NiO and CoO. The values of the mole fractions of MnO, NiO and CoO in the oxide phase were calculated at 1873 K for different manganese concentrations in Ni – Co melts. In the case of nickel melt, even when manganese content is higher than 0.1 %, the mole fraction of manganese oxide is close to unity. As the cobalt content in the melt increases, the mole fraction of manganese oxide in the oxide phase decreases. In the case of pure cobalt, it is close to unity with manganese contents above 0.7 %. The dependences of the oxygen solubility on contents of cobalt and manganese in the studied melts were calculated. In nickel-cobalt melts, manganese is characterized by a high affinity for oxygen. The deoxidizing ability of manganese decreases with increasing cobalt content in the melt. In pure cobalt it is significantly lower than in pure nickel. The oxygen solubility curves in manganese-containing Ni – Co melts pass through a minimum, the position of which shifts to the higher manganese content with an increase in cobalt content in the melt.