Thermodynamics of the Fe-Mn-C system from solid state EMF measurements

Abstract

The partial molar free energy of mixing of Mn, G2M in austenite and in various Mn-C and Fe-Mn-C phase mixtures was measured at temperatures of 690 to 1060°C with solid CaF2 electrolyte galvanic cells. The results for the Fe-Mn system can be represented by {ie2217-01} where T denotes temperature in K and y1 andy2 are the Fe- and Mn-to-metal atom ratios, respectively. The course ofGM2 in its dependence on the carbon atom fraction in the Mn-C system shows a low stability of Mn15C4 and Mn3C with respect to decomposition into neighboring carbides. Partial molar free energies of mixing of Fe and Mn in the metal (austenite or α-Mn(+Fe))-carbide(M3C, M23C6, or ε-M4C) phase mixtures are given by {ie2217-02} and {ie2217- 03} {ie2217- 04} whereyi denote thei-to-metal atom ratios(i = 1, 2, 3) in the carbide-saturated metal phase. The partial molar free energy of mixing of carbon, G^M ₃ in the same two-phase regions was calculated. The results for the austenite-cementite, two-phase, region confirm published gas carburization data but tend to be more negative in value and have a greater curvature in dependence on Mn content. The curvature shows that interaction terms higher than first order must be used to represent G^M3 over the composition rangey2 = 0 to 0.545. Evaluation of GM₃ over closed paths of integration in the ternary system demonstrates a self-consistency of the phase relations with the emf data. The derived free energy charge for the reaction of mixing of the binary cementite compounds, {ie2217-05} where M denotes the mixture (Fe, Mn), is more negative than that of an ideal solution and itsy2 dependence is asymmetric being more negative in Mn-rich than in Fe-Rich cementite.