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. I t consists of volume elements, typically in slab-like parallel sequences wi th alternating average lattice orientation, which are caused by the local s imultaneous operation of fewer slip systems (most often one or two) than wo uld be required for homologous deformation. A quantitative relationship bet ween average band width, band length and flow stress at the time of band fo rmation is derived This derivation is based on the LEDS hypothesis which fo llows from the second law of thermodynamics, i.e. "Among all microstructure s that are in equilibrium with the applied stresses and are in principle ac cessible 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 c ontrolling the size and morphology of the bands are (i) the elastic energy reduction due to flow stress decrease on account of spatially separating th e operation of equally stressed slip systems, balanced against the stored e nergy increase through (ii) the deformation band boundaries, and (iii) the stress concentrations at the ends of the bands and/or where the band bounda ries are not straight. The result is shown to be in accord with all pertine nt observations that were located in the literature. It is further shown th at 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 differ ence, thus providing very strong support for the LEDS hypothesis. Some qual itative considerations are presented which explain the general features inc luding the temperature dependence of the banding. (C) 1999 Acta Metallurgic a Inc. Published by Elsevier Science Ltd. All rights reserved.