Ab initio modelling of screw dislocations in body-centered cubic transition metals

We performed electronic structure ab initio calculations based on density functional theory (DFT) to study the <111> screw dislocation properties in body-centered cubic transition metals (V, Nb, Ta, Mo, W and Fe). In all investigated elements, the nondegenerate easy core is the minimum energy configuration and the split core configuration has a high energy near or above that of the hard core, contrary to interatomic potential predictions. A strong group dependence of the core energy of the easy dislocation is also evidenced, related to the position of the Fermi level with respect to the minimum of the pseudo-gap of the electronic density of states. Our work also reveals an atypical behavior in Fe, with a low relative energy at the hard core position, close to that of the saddle configuration between easy cores, resulting in a flat Peierls potential around the hard core configuration, at variance with other elements. From these DFT calculations, the two-dimensional energetic landscape in the {111} plane (Peierls potential) is constructed and we investigated several properties of dislocation glide and in particular, the kink-pair formation enthalpy, as well as the dependence of the Peierls stress on crystal orientation. We proposed a simple modification to the Schmid law that takes account of the non-straight trajectory of the dislocation and that qualitatively explains why the twinning/anti-twinning asymmetry is less pronounced in Fe than in other body-centered cubic metals. (author)