Motion of a screw dislocation in a two-dimensional Peierls potential

Motion of a screw dislocation in a crystal lattice occurs by kink pair formation in a two-dimensional Peierls potential. Employing the line-tension model of a dislocation, the saddle-point configuration of a nonplanar kink pair in three-dimensional space and the associated activation energy are calculated. The method is applied to a 〈111〉 screw dislocation in a bcc lattice assuming a threefold periodic sinusoidal Peierls potential in the (111) plane. In this potential the dislocation can nucleate a kink pair from one stable straight configuration towards any of the three adjacent ones. Depending on the direction of the applied stress the relative frequencies of the three activation processes determine the temperature dependence of the critical flow stress and the direction of total slip. The calculations reproduce well the plastic anisotropy of bcc transition metals.