Investigation of the growth kinetics of γ → α in Fe-C-X alloys with a thick interface model

A thick interface model is proposed aiming at predicting the whole spectrum of conditions at an austenite/ferrite interface during ferrite growth in ternary Fe-C-X alloys, from full equilibrium to paraequilibrium, with intermediate cases as the most interesting feature. It is shown that the different kinetics can be obtained by tuning only the ratio between the diffusivities inside the thick interface and in bulk austenite. The predictions of the model for the kinetic transitions in different Fe-C-Ni alloys with a single value of this ratio are in good agreement with available measurements. In particular, sudden growth stops observed in alloys with high substitutional species contents are explained by the transition from paraequilibirium to local equilibrium non partitioning growth modes.