The depth-dependent gradient deformation bands in a sliding friction treated Al-Zn-Mg-Cu alloy

Surface sliding friction treatment of Al-Zn-Mg-Cu alloy generally introduces a large number of depth-dependent gradient deformation bands (DBs), which are approximately parallel to {111}Al and penetrated deeply into the coarse grain matrix. The DB segment adjacent to the ultrafine grain (UFG) layer is composed of a string of nanograins and generally shows a bamboo-like morphology. The intersected nanograin boundaries (INBs) and parallel DB boundaries (DBBs) are more often low angle dislocation boundaries. Compared with precipitates in the surrounding matrix, the precipitates at DBBs are much coarser, whereas those within DBs are smaller and scarcer. With increasing depth from the UFG layer, the INBs gradually disappear along with the decrease in the size differentiation between precipitates within DBs and at DBBs. Such deformation banding induced grain refinement and precipitate redistribution were fundamentally attributed to intense dislocation activities and possible dynamic recovery and recrystallization, which can be significantly affected by the gradient shear strain component as well as the randomly encountered coarse dispersoids and precipitates. Additionally, a variant selection phenomenon during mechanically induced precipitate dissolution process was pointed out, and the crystallographic orientation of equivalent variants with respect to the DBB plane was considered to be responsible for this orientation dependent precipitate dissolution. Deformation banding is of great benefit to work hardening, although there are probably some detrimental effects on mechanical properties.