Quantification of energy dissipation induced by Mn segregation and partitioning at migrating interfaces during ferrite transformation

Mn is the most commonly used substitutional alloying element in low-alloy steel, having a great impact on ferrite growth kinetics since its diffusivity is much lower than that of carbon as an interstitial element. It is known that Mn segregation and partitioning at the migrating ferrite/austenite interface causes energy dissipation during ferrite growth. However, the velocity dependence of energy dissipation arising from the Mn enrichment dictated by interface coherency has not been quantitatively clarified. In this study, the velocity dependence of energy dissipation was evaluated by interfacial carbon enrichment in austenite with Mn enrichment analysis at both 'incoherent' non Kurdjumov-Sachs (K-S) and 'partly coherent' K-S interfaces of grain boundary ferrite formed by isothermal transformation in an Fe-1.5Mn-0.11C alloy (mass%) and ferrite formed by decarburization in an Fe-2Mn-0.77C alloy at the same temperature of 973 K. The interfacial carbon content at the ferrite/austenite interface deviates from para-equilibrium condition and remains nearly constant at the value expected from negligible partitioning local equilibrium (NPLE) at both non K-S and K-S interfaces in both alloys, regardless of interface velocity. The amount of Mn enrichment near the interface is larger at the non K-S interfaces than at the K-S interfaces, and it increases as the growth rate decreases. The velocity dependences of energy dissipation and Mn enrichment are explained based on solute drag model considering the interaction between Mn and carbon.