Ab Initio Simulation of the Energy of the α-Fe/Fe₃C Interphase Boundary with Bagaryatsky Orientation Relationships

Pearlite is one of the fundamental structural components of carbon and low-alloy steels. In pearlite, the orientation relationships of Bagaryatsky, Isaychev, and Pitsch can be observed between the body-centered cubic ferritic and rhombic cementite Fe₃C phases. In low-temperature pearlite, which exhibits the highest strength, the first two predominate, and they are closely related, sometimes indistinguishable in experiments. In this study, ab initio simulation using density functional theory in the WIEN2k software package is conducted to investigate the structures and energies of coherent α-Fe/Fe₃C interphase boundaries. The supercells undergo structural and volume optimization. Calculations of the interphase boundary surface energy yield values of 0.383 and 0.594 J/m² for the Bagaryatsky and Isaychev orientation relationships, respectively. These results align well with existing experimental values and outcomes from other molecular dynamics and ab initio calculations. The difference in the surface energy may play a significant role in low-temperature pearlite with thin plates of ferrite and cementite and a large interphase-boundary area per unit volume.