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Segregated light elements at grain boundaries in Niobium and Molybdenum
The influence of the interstitial impurities B, C, N, O, and H on the interfacial cohesion and structure of grain boundaries in body-centered cubic transition metals is investigated by means of the local-density-functional theory (LDFT), using the mixed-basis pseudopotential approach. \Sigma 5 (310) [001] symmetrical tilt grain boundaries (STGB) in Nb and Mo are chosen for this theoretical case study. The analysis of interface energies and geometric translation states, site-projected densities of states and bonding electron densities yields the following systematic trends: B and C form angle-dependent covalent like bonds with the nearest host-metal neighbors, strengthen the interfacial cohesion by inducing covalent metal-metal bonds across the interface, and favor a mirror-symmetric configuration of the STGB, while N and O as well as H form isotropic polarlike bonds, cause interfacial embrittlement, and break the mirror symmetry of the STGB. The ab initio LDFT results for energetical trends concerning cohesion of boundaries and embrittlement by segregation support the qualitative validity of empirical models of Cottrell and of Rice and Wang for bcc transition metals. In addition, structural trends are derived, which relate the geometrical translation state of the STGB to the type of bonding of the impurity with the host metal at the interface.