Estimation of self and ternary interaction coefficients in solid Fe-C, Fe-Mn, Fe-Mn-C and Fe-Si-C alloys at 1623 K

Most reported interaction coefficients of solutes, epsilon(j)(i), and activity coefficients of solutes at their zero concentration, gamma(0)(i), in Fe-based systems have been measured in liquid solutions. In comparison, the data with solid iron-base alloys are meager. Utilising the Thermo-Calc generated equilibrium compositions of the coexisting liquid iron and austenite in Fe-C and Fe-Mn binaries, and Fe-C-Mn and Fe-C-Si pseudobinaries, and the first-principle activity relationship of each solute in the two phases at equilibrium, seven important thermodynamic data (epsilon(C)(C), gamma(0)(Mn), epsilon(Mn)(Mn), epsilon(Mn)(C), gamma(0)(Si), epsilon(Si)(Si), and epsilon(Si)(C)) in solid iron at 1623 K have been generated in this work. A quadratic solution model for the activity coefficient (gamma(i)) vs. composition (x(i)) relation is employed. The published values of the gamma(0)(C) in solid iron, and the interaction coefficients and gamma(0)(i)'s in liquid iron are used, after temperature corrections. For this purpose, the temperature dependences of interaction coefficients and activity coefficients have been formulated under the simplifying condition of ideal entropy of mixing. The reliability of the current method of evaluation of data has been verified by comparing the calculated value of gamma(0)(C) in solid iron with the experimental data. The generated data are as follows: epsilon(C)(C)(gamma) = -5.85, gamma(0)(Mn) (gamma) = 1.63, epsilon(Mn)(Mn) (gamma) = -1.05, epsilon(Mn)(C)(gamma) -5.79, gamma(0)(Si)(gamma) = 0.002, epsilon(Si)(Si)(gamma) = 38.50 and epsilon(si)(C) (gamma) = 4.41.