Atomistic simulation of ideal shear strength, point defects, and screw dislocations in bcc transition metals: Mo as a prototype

Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have studied ideal shear strength, point defects, and screw dislocations in the prototype bcc transition metal molybdenum (Mo). Many-body angular forces, which are important to the structural and mechanical properties of such central transition metals with partially filled d bands, are accounted for in the present theory through explicit three- and four-ion potentials. For the ideal shear strength of Mo, our computed results agree well with those predicted by full electronic-structure calculations. For point defects in Mo, our calculated vacancy-formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations have also been investigated. The [110] split dumbbell interstitial is found to have the lowest formation energy, in agreement with the configuration found by x-ray diffuse scattering measurements. In ascending order, the sequence of energetically stable interstitials is predicted to be [110] split dumbbell, crowdion, [111] split dumbbell, tetrahedral site, [001] split dumbbell, and octahedral site. In addition, the migration paths for the [110] dumbbell self-interstitial have been studied. The migration energies are found to be 3-15 times higher than previous theoretical estimates obtained using simple radial-force Finnis-Sinclair potentials. Finally, the atomic structure and energetics of [lll] screw dislocations in Mo have been investigated. We have found that the so-called ''easy'' core configuration has a lower formation energy than the ''hard'' one, consistent with previous theoretical studies. The former has a distinctive threefold symmetry with a spread out of the dislocation core along the [112] directions, an effect which is driven by the strong angular forces present in these metals.