Thermodynamic analysis of Fe-Al-O system using Redlich-Kister (R-K) polynomial and Wagner’s Taylor series expansion

Abstract

Solution behaviour can be described by determining the activity coefficients of its components. The decrease or increase of an activity depends on attractive or repulsive interactions between atoms/molecules in the solution. In the present work, activity coefficients (Al, O, Fe) and excess molar free energy of a ternary system (Fe-Al-O) have been calculated by employing quadratic and cubic formalisms using the compositional data obtained through Al-O equilibrium experiments in liquid iron. These activity coefficients have been compared with the experimentally (EMF) measured values reported in the literature by several researchers. The activity coefficients estimated using cubic formalism showed a good agreement with experimentally measured values. Drawing analogy with the steelmaking process, the activity coefficients were re-calculated for Fe-Al binary system, considering that the concentration of oxygen becomes insignificant in high Al steels, using two different approaches (a) R-K polynomial of the 3rd order and (b) Wagner’s Taylor series expansion truncated at the 3rd order along with Gibbs-Duhem equation. The activity coefficients and excess molar free energy were found to be nearly same across all the aluminium concentration range using both these approaches. Finally, it can be concluded that Fe-Al-O ternary may well be treated as a Fe-Al binary system, beyond a deoxidation range (above 0.07 wt% Al), as the activity coefficients of Fe and Al were nearly the same, in both binary and ternary considerations.