On attempts to make the interaction parameter formalism thermodynamically consistent

Although the interaction parameter formalism serves the needs of steelmakers quite well, it is thermodynamically inconsistent, because corresponding expressions for the activity coefficients do not satisfy the Gibbs–Duhem equation. Pelton and Bale amended the formalism and made it thermodynamically consistent. That new modified interaction parameter formalism preserved the expediency of original Wagner's formulae, because it required nothing in addition to already compiled abundant data on the interaction parameters. It seemed that metallurgists had been given a numerically straightforward and thermodynamically sound approach to handle solutes' activities in steels at finite concentrations. In this work, it is demonstrated that although the Pelton–Bale modification undoubtedly works, there exists an infinite number of corrections bringing a thermodynamic consistency into Wagner's formalism. Such an ambiguity suggests that too much confidence should not be put in numerical values of activities calculated in non-dilute liquid iron solutions with the modified interaction parameter formalism.