Application of the thermodynamic extremal principle to diffusion-controlled phase transformations in Fe-C-X alloys: Modeling and applications

Diffusion-controlled phase transformations are of singular importance in controlling microstructures and mechanical properties but are difficult to model and calculate for Fe-C-X alloys because of the large difference in the diffusivities of the interstitial element C and the substitutional element X. In this work, the thermodynamic extremal principle was applied to propose a modified quasi-sharp-interface model that integrates trans-interface diffusion from the product phase to the interface, trans-interface diffusion from the interface to the parent phase, interface migration and bulk diffusion of C and X. Applications to isothermal and cyclic phase transformations showed that the model allows the arbitrary setting of the initial conditions. For isothermal phase transformations, three different kinds of characteristic phase transformation kinetics (i.e., a gradual transition from non-partition to partition, a sharp transition from non-partition to partition and solely partition) were found, and their dependence on the interface properties, the grain size and the isothermal temperature was discussed. For both isothermal and cyclic phase transformations, the important roles of trans-interface diffusion and interface migration were highlighted.