Kinetics of the Austenite-to-Ferrite Phase Transformation - From the Intrinsic to an Effective Interface Mobility
Recent studies indicate that the austenite(γ)-to-ferrite(α) transformation kinetics in low alloyed steels is solely controlled by the intrinsic mobility of the interface at least in the initial stages of ferrite growth. Then, diffusion processes in the interface significantly retard ferrite growth, so that bulk diffusion of the fast diffusing interstitial component carbon becomes relevant. Two series of dilatometer tests, one from a low to ultra-low carbon steel  (alloy A) and the other from an Fe-Mn steel  (alloy B), are considered. In case of alloy A the first stage of the transformation kinetics is apparently controlled by the intrinsic interface mobility, whereas in the second stage carbon diffusion in the interface and in the bulk material comes into play. The transition region can be modeled by an effective mobility, which depends on the interface velocity. In the second stage the interface mobility depends on the temperature only. In case of alloy B a hierarchical model allows for a direct estimation of the intrinsic mobility. The numerical results indicate that the interface mobility also changes from an intrinsic mobility at the initial stage of the transformation to an effective mobility due to solute drag during the transformation process.
T. Chandra, K. Tsuzaki, M. Militzer , C. Ravindran
E. Gamsjäger "Kinetics of the Austenite-to-Ferrite Phase Transformation - From the Intrinsic to an Effective Interface Mobility", Materials Science Forum, Vols. 539-543, pp. 2570-2575, 2007