Disclination Models of Misorientation Band Generation and Propagation

The generation and propagation of misorientation bands in polycrystalline metals under large deformation are theoretically investigated and discussed. Disclination models are proposed to describe the generation of misorientation bands at grain boundary kinks and junctions. The models consider edge disclination dipole and quadrupole configurations and predict the value for the critical external shear stress τ<INF>g</INF>, above which nucleation of misorientation bands takes place. The numerical estimates for τ<INF>g</INF> give values of G/1000 – G/400 (G being the shear modulus), which correspond to the level of the deforming stress observed in the materials with fine grain structure. The critical stress τ<INF>g</INF> is shown to be strongly dependent on the geometry and strengths of initial disclinations at grain boundary faults. The further development of the disclination structure at an isolated grain boundary fault demonstrates two main regimes of misorientation band development: stable and unstable propagation. The transition between the two regimes is controlled by another critical value for external stress τ<INF>p</INF> which is several times higher than τ<INF>g</INF>. To better understand the cooperative behavior of dislocations and disclinations in metals at late stages of plastic deformation, computer simulation of dislocation–disclination interaction has also been performed, using a 2D dislocation–disclination dynamics code. Some preliminary results of these calculations are reported and discussed.