Computation of Phase Transformation Paths in Steels by a Combination of the Partial- and Para-equilibrium Thermodynamic Approximations

Abstract : A model combining the partial‑equilibrium and para‑equilibrium thermodynamic approximations is presented. It accounts for fast diffusion of interstitial elements, such as carbon, and low diffusion of substitutional elements in the solid phases, while complete mixing is assumed for all elements in the liquid phase. These considerations are turned into classical mathematical expressions for the chemical potentials and the u‑fractions, to which mass conservation equations are added. The combination of the two models permits application to steels, dealing with partial‑equilibrium for solidification and para‑equilibrium for both the δ‑BCC to γ‑FCC peritectic transformation and the γ‑FCC to α‑BCC solid state transformation. The numerical scheme makes use of calls to Thermo‑Calc and the TQ‑interface for calculating thermodynamic equilibrium and accessing data from the TCFE6 database. Applications are given for a commercial steel. The results are discussed based on comparison with classical microsegregation models and experimental data.