A re-examination of the thermodynamics of the proeutectoid ferrite transformation in Fe-C alloys

Abstract

Three models of the statistical thermodynamics of interstitial solid solutions have been used to reevaluate the thermodynamics of the proeutectoid ferrite reaction. The models of Kaufman, Radcliffe and Cohen and of Lacher, Fowler and Guggenheim, which were em-ployed in a previous study of this type, together with the model recently developed by McLellan and Dunn are used in conjunction with the extensive experimental data of Ban-ya, Elliott and Chipman, of Lobo and Geiger and of Dunn and McLellan on the activities of carbon in austenite and ferrite. Application of the McLellan and Dunn model and that of Lacher, Fowler and Guggenheim to carbon in austenite yields activities of carbon which are numerically indistinguishable and activities of iron which are mathematically identi-cal. However, the new activity data have revealed important differences between the pres-ent calculations and those of Aaronson, Domian and Pound. An average carbon-carbon repulsion energy in austenite of 1925 cal/mole (8054 J/mole) was determined from the CO/CO₂ data of Ban-yaet al. However, the C-C interaction energy in ferrite was found to be opposite in sign but exhibited erratic variations with temperature despite the large amount of activity data available. The γ/(α + γ) phase boundary calculated from the new data differs significantly, at lower temperatures, from the best curves reported by Aaron-sonet al. The calculateda/(α +γ) phase boundary also differs appreciably from the pre-vious results and exhibits only limited agreement with the experimentally determined phase boundary. Calculation of the free energy change associated with the proeutectoid ferrite reaction andT ₀- composition curves differs little from previous results; internal agreement among the new sets of curves, however, is much improved.