Accurate atomistic simulation of (a/2) ⟨111⟩ screw dislocations and other defects in bcc tantalum

The fundamental atomic-level properties of (a/2) [111] screw dislocations and other defects in bcc Ta have been simulated by means of new quantum-based multi-ion interatomic potentials derived from the model generalized pseudopotential theory (MGPT). The potentials have been validated in detail using a combination of experimental data and ab-initio electronic structure calculations on ideal shear strength, vacancy and self-interstitial formation and migration energies, grain-boundary atomic structure and generalized stacking-fault energy (gamma) surfaces. Robust and accurate two- and three-dimensional Green's function (GF) techniques have been used to relax dynamically the boundary forces during the dislocation simulations. The GF techniques have been implemented in combination with a spatial domain decomposition strategy, resulting in a parallel MGPT atomistic simulation code that increases computational performance by two orders of magnitude. Our dislocation simulations predict a degenerate core structure with threefold symmetry for Ta, but one that is nearly isotropic and only weakly polarized at ambient pressure. The degenerate nature of the core structure leads to possible antiphase defects (APDs) on the dislocation line as well as multiple possible dislocation kinks and kink pairs. The APD and kink energetics are elaborated in detail in the low-stress limit. In this limit, the calculated stress-dependent activation enthalpy for the lowest-energy kink pair agrees well with that currently used in mesoscale dislocation dynamics simulations to model the temperature-dependent single-crystal yield stress. In the high-stress limit, the calculated Peierls stress displays a strong orientation dependence under pure shear and uniaxial loading conditions, with an antitwinning-twinning ratio of 2.29 for pure shear {211}- [111] loading.