No 2, Vol. 4, 2003 


M.Yu. Gutkin and I.A. Ovid'ko

Institute of Problems of Mechanical Engineering, Russian Academy of Sciences,
Bolshoj 61, Vasil. Ostrov, St. Petersburg 199178, Russia


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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