Overview no. 131 - "Regular" deformation bands (DBs) and the LEDS hypothesis

Regular deformation banding is distinguished from several other types of bands caused by plastic deformation. It is observed mainly in homogeneous f.c.c. and b.c.c. metals. It is common in wavy glide materials up to about T-M/2 deformation temperature, but is much less prevalent in planar glide. It consists of volume elements, typically in slab-like parallel sequences with alternating average lattice orientation, which are caused by the local simultaneous operation of fewer slip systems (most often one or two) than would be required for homologous deformation. A quantitative relationship between average band width, band length and flow stress at the time of band formation is derived This derivation is based on the LEDS hypothesis which follows from the second law of thermodynamics, i.e. "Among all microstructures that are in equilibrium with the applied stresses and are in principle accessible to the dislocations, those are formed which minimize the energy of the system composed of the deforming material and the applied tractions". In applying the LEDS hypothesis to DBs, the critical energy contributions controlling the size and morphology of the bands are (i) the elastic energy reduction due to flow stress decrease on account of spatially separating the operation of equally stressed slip systems, balanced against the stored energy increase through (ii) the deformation band boundaries, and (iii) the stress concentrations at the ends of the bands and/or where the band boundaries are not straight. The result is shown to be in accord with all pertinent observations that were located in the literature. It is further shown that the relative free energy reduction between a structure with and without regular deformation banding is typically quite small, namely ranging about 0.01 degrees C and on occasion less than 10(-3)degrees C temperature difference, thus providing very strong support for the LEDS hypothesis. Some qualitative considerations are presented which explain the general features including the temperature dependence of the banding. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.