ON THE DYNAMIC ORIGIN OF DISLOCATION-STRUCTURES IN DEFORMED SOLIDS

The formation of dislocation structures seems to be governed by two ty pes of instability transitions. In the first type of transition the un iform distribution of dislocations stored in ductile solids becomes un stable, forming dipolar dislocation structures. Stored dislocations, m ostly in the form of elongated dipolar loops, are swept by gliding dis locations or drifted' by stress gradients into dense regions (clusters , braids, veins, dipolar walls). When the dislocation density in the d ense regions reaches a critical value, stored dislocations start to an nihilate, causing dynamic recovery. The second type of instability tra nsition is of non-linear continuum mechanics origin. In plastically de formed solids, this instability leads to the formation of a microshear band and to misorientation of the crystal lattice accompanied by the formation of geometrically necessary bipolar dislocation structures (d islocation sheets, walls of misoriented cells, subgrain boundaries). T he proposed continuum mechanics approach indicates that the observed p lastic phenomena are the consequences of competition between the two i nstability processes. These processes can be understood as a trend tow ards minimizing the internal energy of the solid under dynamic conditi ons, where the synergetics of dislocations and the applied and interna l stresses play a decisive role.