[SY-I4] A new thermodynamic model for the austenite-ferrite massive transformation in Fe-C-X alloys.
Predictions of the ferrite to austenite or austenite to ferrite transformation kinetics during heating and/or cooling of ternary Fe-C-X alloys is challenging because diffusion coefficients of interstitial and substitutional elements are very different in both phases. The model described in this paper, is based on the prediction of (i) concentration profiles for all elements and (ii) interface velocity, that corresponds to the minimization of the Gibbs energy (i.e. phase field approach). The total energy of the system is given via a dynamic coupling with Thermocalc database.
The ferrite/austenite interface is assumed to have a finite width. This allows to deal with a unique diffusion profile for all species. Element fluxes are derived from their chemical potential gradient. Another advantage of such an approach is the possibility of introducing a potential depth within the interface, leading to solute atoms segregation, that is believed to control transformation kinetics in ternary systems.
During the transformation, it is observed that the model automatically switches between different equilibrium conditions (from out-of-equilibrium transformation to para-equilibrium, local equilibrium with and without partitioning until ortho-equilibrium). To our knowledge, it constitutes a real improvement in phase transformation modeling. This model has been calibrated on decarburization experimental kinetics reported in the literature in various Fe-C-X systems. This model shows an interface motion slowing down with the addition of a potential depth at the interface in agreement with experimental observations. By extension, the model has been used on isothermal and non-isothermal heating for complex Dual Phase steels and compared with experiments.
The ferrite/austenite interface is assumed to have a finite width. This allows to deal with a unique diffusion profile for all species. Element fluxes are derived from their chemical potential gradient. Another advantage of such an approach is the possibility of introducing a potential depth within the interface, leading to solute atoms segregation, that is believed to control transformation kinetics in ternary systems.
During the transformation, it is observed that the model automatically switches between different equilibrium conditions (from out-of-equilibrium transformation to para-equilibrium, local equilibrium with and without partitioning until ortho-equilibrium). To our knowledge, it constitutes a real improvement in phase transformation modeling. This model has been calibrated on decarburization experimental kinetics reported in the literature in various Fe-C-X systems. This model shows an interface motion slowing down with the addition of a potential depth at the interface in agreement with experimental observations. By extension, the model has been used on isothermal and non-isothermal heating for complex Dual Phase steels and compared with experiments.