Serrated grain boundaries in hot-worked aluminum alloys at high strains

The Al-5Mg and Al-Mg2 Si (6015) alloys were deformed by torsion in the 300–560 °C range and 10−3 to 1 s−1 to strains from 0.5 to 9.0. The specimens were examined on tangential sections, that is, normal to a radius and close to the surface being ground so that the transition from gauge to shoulder could be examined. They were electropolished and anodized for examination in polarized light. The original large grains in the shoulder were observed to be transformed into thin elongated grains, wound around the axis. The serrations become more prominent as the temperature rises and strain rate diminishes, since their wavelength depends on the subgrain size. They are most prominent as strain increases into the 0.5–1.0 range, while the aspect ratio of the grains is in the 2–4 range. They are still present in the very elongated grains at high strains and lead to pinching off when the grain thickness decreases to about three times the subgrain diameter. This gives rise to almost equiaxed grains containing substructure without discontinuous dynamic recrystallization.