Analysis of thermodynamic properties of Ca – Si – Fe melt

The model of ideal associated solutions was used for the analysis of thermodynamic properties of the Ca – Si – Fe melt. Chemical equilibrium, as per the law of mass conservation between associates and monomers in the assumed model version, was performed without consideration of mole fractions of these particles in solution but with consideration of the absolute number of their moles. It allows taking account the changes in the associated solution mole composition depending on the concentration of its components. The understudied binary sub-system Ca – Si was analyzed most comprehensively. Using the latest data of temperature dependency of heat capacity for five types of intermetallics of this sub-system, types of stable associates in it were defined, i.e. Са₂Si, СаSi in the solution range with low contents of silicon in solution and СаSi, СаSi₂ in the solution range with high contents of silicon in solution. Thermodynamic properties of the corresponding intermetallics in the databases Terra, Astra and HSC notably differ from the computed properties of the associates. The reason of disagreement of experimental and reference data consists apparently in the inaccurate reference information based on the previous underestimated studies of intermetallics’ heat capacities. Analysis of mixing energy of Ca – Si alloy components has shown that concentration and temperature dependencies of excessive free energy closely follow the so-called pseudosubregular model of binary solutions. Only two types of stable associates were defined for the other sub-system Fe – Si, i.e. Fe₃Si and FeSi. On the whole, energies of formation of these associates and respective intermetallics agree well. The third sub-system Ca – Fe was not considered because of the very limited mutual solubility of its components. Thus, only three associates, i.e. CaSi, CaSi₂ , FeSi, are valid out of five possible in the triple system Ca – Si – Fe in the range with high concentrations of silicon. A calculation under this condition of thermodynamic properties of calcium silicon melts for CK10 – CK30 grades has shown that activity of silicon in them at temperature 1873 K constituted 0.6 – 0.7, whereas activities of other components do not exceed 0.01.

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