ON THE DYNAMIC ORIGIN OF DISLOCATION-STRUCTURES IN DEFORMED SOLIDS

The formation of dislocation structures seems to be governed by two types of instability transitions. In the first type of transition the uniform distribution of dislocations stored in ductile solids becomes unstable, forming dipolar dislocation structures. Stored dislocations, mostly in the form of elongated dipolar loops, are swept by gliding dislocations or drifted' by stress gradients into dense regions (clusters, braids, veins, dipolar walls). When the dislocation density in the dense regions reaches a critical value, stored dislocations start to annihilate, causing dynamic recovery. The second type of instability transition is of non-linear continuum mechanics origin. In plastically deformed 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 (dislocation sheets, walls of misoriented cells, subgrain boundaries). The proposed continuum mechanics approach indicates that the observed plastic phenomena are the consequences of competition between the two instability processes. These processes can be understood as a trend towards minimizing the internal energy of the solid under dynamic conditions, where the synergetics of dislocations and the applied and internal stresses play a decisive role.