PATTERNS IN HEAVILY DEFORMED METALS

Ductile metals can sustain very high levels of plastic strain without fracture. At low or intermediate temperatures (deformation regime domi nated by crystallographic slip), deformation is accompanied by an incr easing storage of dislocation density concomitant with a strength rise that can amount to more than two orders of magnitude. Dislocations ac cumulate in characteristic heterogeneous cellular patterns whose avera ge size correlates with the interdislocation distance and with the inv erse of the strength. The patterns do not merely shrink as deformation proceeds but suffer qualitative changes attested by a marked transiti on in work hardening behaviour. Although the starting cellular disloca tion substructure, composed of thick, tangled walls is progressively s ubstituted by a neat arrangement of two-dimensional sub-boundaries, th e work hardening transition observed at large strains does not coincid e with any sudden cell to subgrain substructural transition and its un derlying substructural basis has not yet been unveiled. This and other unsolved questions about the substructures of heavily deformed metals will be addressed in the paper, namely the qualitative and quantitati ve observation of self-similarity of the substructures in a limited sc ale-range and its implications in the discussion on the physical proce ss of substructure building.