Strengthening Mechanisms in Ultrafine-Grained and Sub-grained High-Purity Aluminum

This study investigates the effect of grain/sub-grain size, boundary misorientation, and dislocation density on mechanical properties of nanostructured aluminum. A fully recrystallized high-purity aluminum was deformed to different strains from low to ultrahigh strains by a combination of conventional cold rolling and accumulative roll-bonding, followed by annealing for recovery and structural coarsening, to produce sub-grained samples dominated by low-angle boundaries and ultrafine-grained samples dominated by high-angle boundaries. The ultrafine-grained samples showed unusual discontinuous yielding and had a very high strength, which was positively deviated from the extrapolation of the Hall–Petch curve in coarse grains. On the other hand, sub-grained samples showed continuous yielding, and the strength was lower than that of ultrafine-grained samples at the same structural size. It is suggested that in the ultrafine-grained samples, due to lack of dislocation sources in the grains, extremely high stress is required for yielding, which is responsible for the unexpected discontinuous yielding and extra Hall–Petch strengthening. On the other hand, in the sub-grained samples, dislocations in the low-angle dislocation boundaries may act as active dislocation sources, leading to a lower yield stress.