Deformation bands in ⟨120⟩ grains in coarse-grained aluminium

Coarse-grained aluminium, deformed in tension to a strain of 0.05, was examined in a scanning electron microscope by channelling contrast. Pronounced bands with a width typically of the order of 200 μm were found in some grains with an orientation close to [120]. When observed on surfaces close to [001], the boundaries between the bands were parallel to [010] and the neighbouring bands were rotated around [100] with respect to one another. Two slip systems in a critical relationship are equally stressed with a Schmid factor of 0.49 in grains with a [120] orientation, namely (a/2)[011](111) and (a/2)[011](111). The Schmid factor for the highest stressed secondary systems has a local minimum of 0.245 at [120]. The application of Frank's equation shows that the only boundaries without long-range stresses that can be formed by combination of the two sets of dislocations, (a/2)- [011](111) and (a/2)[011](111), allowing for glide only, are tilt boundaries with a normal [001] and a rotation axis along [110] and twist boundaries with a normal [100] and a rotation axis along [100]. A consideration of the boundary energies indicates that the twist boundary has a lower energy than the tilt boundary for a given amount of deformation. The observed structure thus agrees with that expected on the basis of a low energy structure argument.