First-principles calculation of possible carbon positions over cementite lattice

The properties of iron-carbon alloys are largely determined by the formation of iron carbide. Therefore, knowledge about the structure of cementite plays an important role. The possible reason for changes in the nearest neighborhood of iron and carbon atoms in cementite during various heat treatments of steel could be the re-arrangement of carbon atoms over different interstitial sites of cementite iron sublattice. Crystal geometry analysis revealed four types of such sites:“normal” and “distorted” prismatic (NPS, DPS), and “normal” and “distorted” octahedral (NOS, DOS) sites. In this study, we tested this hypothesis using the density functional theory by full-potential method implemented in the WIEN2k program package. The simulated cementite supercells contained 16 and 32 atoms. We confirmed that carbon in NPS resulted in the system with the lowest energy and volume. Systems with all carbon atoms in DPS and one or all carbon atoms in DOS were mechanically unstable and transferred to conventional NPS structure during the system volume relaxation. In other cases, carbon atoms remained in NOS or DPS and yielded the increased structure total energy. The thermodynamic analysis showed that over 10 % of carbon atoms might move from NPS to other sites.