A general mixed-mode model for the austenite-to-ferrite transformation kinetics in Fe-C-M alloys

Using the Gibbs energy balance (GEB) concept, a concise GEB model assuming a planar interface is proposed to study the austenite-to-ferrite transformation kinetics in Fe C M alloys (where M is a substitutional alloying element). Compared with the binary mixed-mode model, the newly developed GEB model is a more general mixed-mode model, and can physically describe the mixed-mode kinetics during the austenite-to-ferrite transformation in binary, ternary and even higher alloy systems. The GEB model can also predict kinetic transitions during the austenite-to-ferrite transformation, and the classical paraequilibrium and Local equilibrium are found to be its two specific cases. Furthermore, the GEB model provides a physical understanding of the effect of solute drag on the effective interface mobility. Experiments in Fe-C-Mn and Fe-C-Ni alloys indicate that the transformation stasis phenomenon can also occur during the austenite-to-ferrite transformation. The degree of incomplete transformation increases with increasing Mn or Ni concentration, while Mn has a stronger effect on the degree of incomplete transformation than Ni does due to its higher partitioning coefficient. The GEB model describes well the austenite-to-ferrite transformation stasis phenomenon. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.