Influence of Interstitial Segregation on Grain Boundary Cohesion in Ferritic Steels

Interstitial solute segregation at grain boundaries (GBs) plays a decisive role in determining the mechanical properties of ferritic steels, yet the underlying mechanisms behind the contrasting embrittling behaviors of light elements such as B, C, and N remain elusive. Through first‐principles calculations, the segregation and embrittling potency of these elements at two representative GBs are systematically investigated: the open Σ5(310) and the compact Σ3(112). A dual mechanism governing GB cohesion is revealed: at open boundaries, chemical bonding dominates, with B providing the strongest cohesion enhancement and C also acting as a strengthening solute, whereas N is consistently embrittling; at compact boundaries, mechanical distortion induced by solute size becomes critical, so that C remains an efficient cohesion enhancer while the size penalty reduces the beneficial effect of B; N again shows the strongest embrittling tendency. This work offers fundamental insights for grain‐boundary segregation engineering in the design of high‐performance ferritic steels.