Thermodynamic analysis of the Fe-AI-C ternary system by incorporating ab initio energetic calculations into the CALPHAD

摘要

A thermodynamic analysis of the Fe-C-AI ternary system has been carried out covering a wide range of temperatures and composition. Special care was taken to the expression of the free energy for the ternary Perovskite carbide phase, Fe_3AIC (kappa) by considering the crystallographic similarity between the K phase and the L1_2 structure. The free energy was calculated using the (Fe,Al)_3(Fe,Al)_1(C,Va)_1 sublattice model, and the kappa and L1_ structures were treated as a continuous solution. Because of the lack of experimental information available, the thermodynamic properties of the E2_1 structure were evaluated using the Full Potential Linearized Augmented Plane Wave (FLAPW) method. The ab initio energetic calculations show that the stable E2_1 structure of the kappa phase is highly preferred compared with the metastable Fe_3AI-L1_2 structure. Contour plots of the charge density for the K phase indicate that the bonding between the Fe and C atoms forms in the Fe_3AIC-E2_1 structure, and that this interaction between the atoms enhances the energetic stability of the kappa phase. According to our phase diagram calculations, the kappa phase is in equilibrium with the fee Fe, B2-type inter-metallic compound, as well as the graphite phase. This finding is in good agreement with previous experimental results.