Ferrite formation in Fe-C alloys during austenite decomposition under non-equilibrium interface conditions

The growth of ferrite during the decomposition of supersaturated austenite in binary Fe-C alloys is studied using a numerical model that allows non-equilibrium conditions at the moving α-γ-interface. A numerical method is presented to account for non-equilibrium interface conditions during diffusional growth. In this model, the driving force for interface migration is given by the difference of the chemical potentials of the austenitic and the ferritic Fe-lattice at the interface. Carbon rearrangement, resulting from the movement of the interface, affects the driving force for interface migration during the transformation, thus establishing a mixed mode of growth control. The mode of control is predicted to vary from essentially a diffusion controlled mode at very low undercooling towards essentially an interface controlled mode at deep undercoolings. The results from this model are in good agreement with several sets of experimental growth rates and the carbon concentration profiles of partially transformed Fe-C alloys.