A Combined DFT and MD Study on Interface Stability in Ferrite-Cementite Systems

Understanding the atomic structure and energetic stability of ferrite-cementite interfaces is essential for optimizing the mechanical performance of steels, especially under extreme conditions such as those encountered in nuclear fusion environments. In this work, we combine Classical Molecular Dynamics (MD) and Density Functional Theory (DFT) to systematically investigate the stability of ferrite-cementite interfaces within the Bagaryatskii Orientation Relationship. Three interface orientations and several cementite terminations are considered to identify the most stable configurations. MD simulations reveal that the (010)||(11-2) and (001)||(1-10) orientations are energetically favourable for selected terminations, and these predictions are validated and refined by subsequent DFT calculations. A key result of our study is the destabilizing effect of interfacial carbon atoms, which increase the interface energy and decrease the Griffith energy, indicating a reduced resistance to fracture. This finding contrasts with earlier reports suggesting a stabilizing role for carbon. Our analysis of the electronic structure shows that Fe-C bonding at the interface perturbs the metallic environment of interfacial Fe atoms. This bonding response explains the observed variations in magnetic moment and helps rationalize the trends in interface energy. We also establish correlations between interface energy, magnetic perturbation, and a bond-based descriptor quantifying new and broken bonds. These insights provide a physically grounded, predictive framework for the design and optimization of ferrite-cementite interfaces in advanced steels.