Microstructural aspects of strain localization in AlMg alloys

The literature contains a number of continuum plasticity models describing the onset of strain localization but little definitive work which describes the microstructural transitions which accompany localization. In the present study a range of metallographic methods have been used in order to observe the progression of localization from events within single grains to the spatial organization of these events across the entire sample. The deformation mode used was cold rolling and observations were made using a variety of orientations relative to the rolling direction. It is concluded that in the AlMg system, localization begins by a structural instability in the accumulated dislocation substructure and later becomes organized into macroscopic bands due to local stress concentrations. The structure of the macroscopic bands is complex. They contain some high angle boundaries suggesting that they form due to the rapid and cooperative action of a number of slip systems over distances of the order of 0.2 μm. The bands cross grain boundaries and are organized in a cooperative sense because they represent local softening events. Thus, the shear bands in AlMg appear to form without texture softening or the need for the precursor of a lamellar structure. They involve a dramatic local change in the process of dislocation accumulation which is essentially a form a local dynamic recovery. The events become spatially organized to form macroscopic bands inclined at approx. 35° to the rolling plane as required by continuum plasticity.