Shape effect in the drift diffusion of point defects into straight dislocations

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Abstract
Effects on point-defect drift diffusion in the strain fields of straight edge or screw dislocations, due to anisotropic diffusion arising from the anisotropy of the point defect in its saddle-point configuration, are investigated. Expressions for sink strength and bias that include the saddle-point shape effect are derived, both in the absence and presence of an externally applied stress. These are found to depend on instrinsic parameters such as the relaxation volume and the saddle-point shape of the point defects, and on extrinsic parameters such as temperature and the magnitude and direction of the externally applied stress with respect to the line direction and Burgers-vector direction of the dislocation. The theory is applied to fcc copper and bcc iron. It is found that the stress-induced bias differential for edge dislocations depends much more on the line direction than on the Burgers-vector direction. The origin of this strong line-direction dependence is shown to be unrelated to the dislocation strain field, but is solely a consequence of elastodiffusion and the translational symmetry of the dislocation line. Comparison is made with the stress-induced bias differential due to the usual stress-induced preferred absorption effect. It is found that the shape effect causes a bias differential that is more than an order of magnitude larger.