| Plastic flow of all bcc metals is controlled by the glide of 1/2⟨111⟩ screw dislocations since they possess non-planar cores and thus experience high Peierls stress. Atomistic studies at 0 K determine the Peierls stress and reveal that it is strongly dependent on non-glide stresses, i.e. components of the stress tensor other than the shear stress in the slip plane parallel to the Burgers vector. At finite temperatures the corresponding Peierls barrier is surmounted via the formation of pairs of kinks. Theoretical description of this thermally activated process requires knowledge of not only the height and shape of the barrier but also its intrinsic dependence on the applied stress tensor. This information is not obtainable from any experimental data and the atomistic studies at 0 K determine the Peierls stress but not the shape of the Peierls barrier.; In this Thesis we first show how the shape of the Peierls barrier and its dependence on the applied loading can be extracted from the data obtained in atomistic studies at 0 K. We consider the Peierls barrier as a two-dimensional periodic function of the position of the intersection of the dislocation line with the perpendicular {lcub}111{rcub} plane, with adjustable terms dependent on the shear stresses parallel and perpendicular to the slip direction. The functional forms of these terms are based on the effective yield criterion recently developed on the basis of atomistic modeling of the glide of screw dislocations at 0 K. The minimum energy path between two potential minima, and thus the corresponding activation barrier, is obtained using the Nudged Elastic Band method. The constructed Peierls barrier reproduces correctly both the well-known twinning-antitwinning asymmetry observed for pure shear parallel to the slip direction and the effect of shear stresses perpendicular to the slip direction. This advancement introduces for the first time the effect of both shear stresses parallel and perpendicular to the slip direction into the model of thermally activated dislocation motion. Based on this model we formulate a general yield criterion that includes not only the full stress tensor but also effects of temperature and strain rate. This approach forms a basis for multislip yield criteria and flow relations for continuum analyses in both single and polycrystals the results of which can be compared with experimental observations. |
