Dislocation wall and cell structures and long-range internal stresses in deformed metal crystals

Abstract

Long-range internal stresses in deformed metals have traditionally been associated primarily with the existence of dislocation pile-ups. Recent experimental work has shown, however, that long-range internal stresses prevail also in crystals containing dislocation wall or cell structures which have so far been considered as energetically favourable dislocation patterns of negligible internal stress. These experimental results are interpreted in terms of a simple model in which the crystal is considered as a composite consisting of hard dislocation walls of high local dislocation density which are separated by soft regions of low local dislocation density. The model is developed for single-slip and multiple-slip deformation. The considerations show that, in crystals in which a heterogeneous dislocation distribution develops during deformation, substantial long-range internal stresses arise unavoidably as a natural consequence of the compatibility requirements in the stress-applied state. A discussion of the microscopic glide processes makes it evident that, in addition to the dislocations located in the walls and in the regions separating them, so-called “interface dislocations” must be considered also. The latter maintain compatibility and give rise to long-range internal stresses (in the absence of dislocation pile-ups). These long-range internal stresses aid the applied stress in the walls and oppose it between the walls. They can be evaluated quantitatively from experiment and are readily interpreted in terms of the model. Beyond that the composite concept leads to a simple description of the macroscopic flow stress in terms of local properties of the heterogeneous dislocation microstructure and provides a convenient distinction between micro- and macroplasticity in monotonie and cyclic deformation.