The equilibrium core structures of isolated (a/2)<111> screw dislocations were calculated by using a first-principles pseudopotential plane-wave method within the local-density approximation of density functional theory. The long-range strain field of the dislocation was treated by using a variation of a newly developed lattice Green’s function boundary condition method. This method permitted the dislocation to be contained in a very small simulation cell without impairing the accuracy of the final core configuration. Super-cells of 168, 270 or 504 atoms were used to evaluate the local screw and edge displacements of the (a/2)<111> screw dislocation. The results were compared with previous results of atomistic and dipole array calculations. It was found that isolated screw dislocations were evenly spread over 3 conjugate (110) planes. The twinning/anti-twinning anisotropy was calculated by applying a pure glide stress to the (112) plane. In these simulations, the dislocation always moved on a {110} plane. The lattice frictional stresses required to move a straight screw dislocation on the (112) plane in the anti-twinning and twinning senses were estimated to be equal to 0.025μ and 0.0125μ, respectively. These values were in good agreement with atomistic simulations, and experimental measurements, of slip asymmetry.

Ab initio Simulation of Isolated Screw Dislocations in BCC Mo and Ta. C.Woodward, S.I.Rao: Philosophical Magazine A, 2001, 81[5], 1305-16

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