By using multi-ion interatomic potentials that were derived from first-principles generalized pseudopotential theory, a study was made of defects in this typical body-centered cubic transition metal. Many-body angular forces, which were important with regard to the mechanical properties of a transition metal with a partially filled d band, were accounted for by using explicit 3-ion and 4-ion potentials. It was found that the calculated vacancy formation and activation energies were in excellent agreement with experimental data. The energies of 6 self-interstitial configurations were also investigated. The <110> split dumb-bell interstitial was predicted to have the lowest formation energy; in agreement with the configuration that was indicated by X-ray diffuse scattering measurements. The sequence of energetically stable interstitials, in ascending order, was predicted to be the <110> split dumb-bell, the crowdion, the <111> split dumb-bell, the tetrahedral site, the <001> split dumb-bell, and the octahedral site. The migration paths of the <110> dumb-bell self-interstitial were also studied. The migration energies were found to be 3 to 15 times higher than the theoretical estimates that had previously been obtained by using simple radial-force Finnis-Sinclair potentials. The atomic structures and energies of <111> screw dislocations were investigated, and it was found that the so-called easy core configuration had a lower formation energy than the so-called hard one. This was consistent with the predictions of previous theoretical studies. The former configuration had a clear 3-fold symmetry, with spreading of the dislocation core along <112> directions. It was concluded that this effect was caused by the strong angular forces in the metal.

W.Xu, J.A.Moriarty: Physical Review B, 1996, 54[10], 6941-51