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Role of disclinations and rotational modes of plastic deformation in
fine-grained materials is discussed. First, we consider disclination models of
generation and development of misorientation bands in severely deformed metals
and alloys. The models predict the existence of the critical external shear
stress, above which nucleation of misorientation bands takes place. The further
analysis demonstrates two main regimes of misorientation band development:
stable and unstable propagation, and allows to find another critical stress
that controls the transition between these two regimes. We quote also some
results of computer simulations of 2D dynamics of dislocations in the stress
field of a dipole of partial wedge disclinations to elucidate the
micromechanisms of misorientation band propagation. Second, theoretical models
of grain boundary disclination motion in fine-grained materials are considered.
This motion leads to changes in misorientations across the grain boundaries and
may explain the rotation of grain crystalline lattice as a whole. It is
demonstrated that motion of grain boundary disclinations may occur in
fine-grained materials through emission of pairs of lattice dislocations into
the adjacent grains or through climb of grain boundary dislocations. We also
consider a model of crossover from grain boundary sliding to rotational
deformation which is realized by the transformation of a pile-up of gliding
grain boundary dislocations stopped by a triple junction of grain boundaries,
into two walls of climbing grain boundary dislocations (treated as the dipoles
of partial wedge disclinations). The conditions necessary for such a
transformation are determined and discussed.
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