Gradient microstructure, recrystallization and mechanical properties of copper processed by high pressure surface rolling

The microstructure, hardness and tensile properties have been studied in copper processed by high pressure surface rolling (HPSR) both in the as-deformed condition and after subsequent annealing at 150 °C. It is found that a gradient structure with significant differences in the scale of microstructural features is formed by HPSR. The deformed microstructure varies from nano- and ultrafine-scale structures with a large fraction of high angle boundaries near the surface to lightly deformed grains at depths of 1–3 mm below the surface. Tensile tests of 1-mm-thick specimens demonstrate that the as-deformed material has a high strength and a low uniform elongation. Annealing at 150 °C results in partial recrystallization, which creates new through-thickness gradients. Except for the topmost layer and several bands in the adjacent layer, recrystallization is more pronounced close to the surface. The fraction recrystallized is at least 80% at depths of 60–300 µm after annealing for 960 min. The fraction recrystallized in the subsurface decreases with increasing depth, and the deformed layer at depths greater than 500 µm remains largely non-recrystallized after annealing. This partially recrystallized condition demonstrates an improved combination of strength and ductility.