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

Abstract 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.