First principles investigation of carbon-screw dislocation interactions in body-centered cubic metals

Using ab initio density functional theory calculations, we investigate the effect of interstitial carbon solutes on screw dislocations in non-magnetic body-centered cubic transition metals from group 5 (V, Nb, Ta) and group 6 (Mo, W). The two groups are found to display different solute-dislocation interaction behaviors. Group 6 shows a core reconstruction similar to that previously reported in Fe(C): the dislocation adopts a hard-core configuration with the carbon atoms at the center of regular trigonal prisms formed by the metal atoms. The solute-dislocation interaction energies are strongly attractive, ranging from -1.3 to -1.9 eV depending on the metal and the carbon-carbon distance. By way of contrast, the configuration of lowest energy in group 5 consists of the dislocation in its easy core and the carbon atom in a fifth nearest neighbor octahedral site. The configuration is attractive, but less than in group 6. We show that this group dependence is consistent with the carbon local environment in the stable stoichiometric carbide structures, namely cubic NaCl-type for group 5 and hexagonal WC-type for group 6: in both cases the carbon atoms are at the center of octahedra and prisms respectively.