Thermo-Kinetic Understanding of the Correlation Between Austenite Reverse Transformation and Mechanical Properties for Medium Manganese Steel

How to describe the austenite reverse transformation (ART) has always been considered as a key problem of controlling microstructures and mechanical properties in high-strength steels. So far, numerous studies have been conducted, unfortunately, without fully considering diffusion of elements, interface migration, and interaction between trans-interface diffusion and interface migration, as well as synergy of thermodynamic and kinetic for interfacial migration. A more flexible modeling for the ART is herein developed using thermodynamic extremal principle, where the concept of trans-interface diffusion in two steps, i.e., from the parent phase to the interface and from the interface to the product phase, as well as the Gibbs energy balance approach, was introduced to predict the behavior of interface migration and element trans-interface diffusion within the migrating interface. Subsequently, the thermodynamic driving force ΔG and the effective kinetic energy barrier Q_eff for the ART were also analytically performed, as well as a unified expression for so-called generalized stability (GS). It is demonstrated that the higher driving force in the ART generally results in the increased yield strength, while the larger GS tends to yield improved uniform elongation, thus forming a correspondence between the thermo-kinetics trade-off and the strength-ductility trade-off. Applying a proposed criterion of high ΔG-high GS, the present model can be adopted to design the ART, which will produce the austenite microstructure with high strength and high plasticity, as evidenced by the current experiments.