Orientation-Dependent Interfacial Stability and Elastic Anisotropy of α-Fe/Fe3C: A First-Principles Study

Cementite (Fe3C) plays a key role in determining the mechanical performance of pearlitic and heat-resistant steels, where both its intrinsic elastic properties and interfacial bonding with ferrite are critical. In this study, first-principles calculations based on density functional theory were employed to systematically investigate the elastic anisotropy of orthorhombic Fe3C and the interfacial stability of α-Fe/Fe3C with three typical orientation relationships (Isaichev, Bagaryatsky, and Pitsch–Petch). The calculated elastic constants of Fe3C satisfy the mechanical stability criteria and reveal pronounced anisotropic behavior, indicating strong directional dependence of its deformation resistance. Interfacial models with optimized lattice matching demonstrate that the Isaichev orientation exhibits the lowest lattice mismatch (~1.71%), the smallest interface energy (0.55 J/m2), and the highest work of adhesion (4.29 J/m2), suggesting superior thermodynamic stability and interfacial bonding strength. In contrast, the Bagaryatsky and Pitsch–Petch interfaces exhibit larger lattice mismatch and more pronounced local interfacial distortion, resulting in higher interface energies and reduced interfacial stability. These findings provide atomic-scale insights into the structure–property relationship of Fe3C and highlight the critical role of orientation relationships in governing interface stability. The results offer theoretical guidance for interface engineering and microstructure optimization in high-performance steels.