  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 {111} 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.
