Thermodynamics of the Interactions in Fe–Mg–Al–La–O Melts

Today, rare-earth elements are mainly introduced in alloys in the form of misch metal, which is a natural mixture of rare-earth elements (with atomic numbers 57–71). This mixture contains about 50 wt % cerium. The primary components of the mixture are generally cerium, lanthanum, and niobium. The specific composition depends on the particular field from which they were drawn. The variable composition of this mixture may significantly impair its effectiveness in alloying. The ratio of the individual elements in the mixture cannot be determined experimentally for each grade of steel, because of the prohibitive expense of such separation. However, thermodynamic modeling permits determination of the optimal concentration of each rare-earth element and the composition of the misch metal. In the present work, thermodynamic modeling is applied to the interaction of magnesium, aluminum, and lanthanum with oxygen in liquid iron. The thermodynamic model of the reduction of steel by mixtures of such active metals is considered. On the basis of literature data regarding the phase diagrams of the MgO–Al₂O₃, MgO–La₂O₃, and La₂O₃–Al₂O₃ systems, the coordinates of points of invariant equilibrium in the MgO–La₂O₃–Al₂O₃ system are established. The MgO–La₂O₃–Al₂O₃ phase diagram is plotted. That permits the identification of all possible equilibria in the reduction of steel by magnesium, aluminum, and lanthanum and also the determination of corresponding descriptive equations. The activity of the components in liquid oxide melts is determined by the theory of subregular ionic solutions, taking account of the influence of the oxide melt’s composition on the coordination number of the cations. The activity of the components in metallic melts that are conjugate with the oxide systems is determined by means of the Wagner theory on the basis of first-order reaction parameters. The equilibrium constants of the reduction of steel by the given elements are established by indirect thermodynamic calculations. On the basis of the results, the solubility surface of the components in Fe–Mg–Al–La–O melts is constructed. That permits the identification of regions where the composition of the liquid metal corresponds to a particular oxide phase.