DISLOCATION PATTERN-FORMATION AND STRAIN-HARDENING IN SOLIDS

At the microscale plastically deformed solid behaves as a strongly non linear system governed by instability transitions. According to the mo del proposed for cell forming materials the stored dislocations, mostl y in the form of dipolar loops, are arranged into a characteristic pat tern of tangles, veins. or walls giving rise to strain hardening. Anni hilation of stored dislocations in the high density regions changes th e wavelength and profile of the pattern. The process tends to reach a steady state, in which the generation and annihilations of stored disl ocations are balanced. The geometry of stabilized cell walls derived f or single, double and multi slip corresponds well to observed deformat ion microstructures. However, the dislocation cell formation may be se riously interfered by geometrical instability. The internal bending ty pe instability causes transformation of cell walls into subgrain bound aries or formation of dislocation grids. The second type of geometrica l instability triggers strain localization into shear bands or persist ent slip bands destroying the original dislocation structure and formi ng the new one.