Microstructural evolution and mechanical behavior of copper processed by low strain amplitude multi-directional forging

Experiments were performed to analyze the microstructural evolution and mechanical behavior of commercial purity copper (99.8%) processed by up to 48 cycles of multi-directional forging (MDF) using a low strain amplitude of similar to 0.075 (total accumulated strain epsilon approximate to 10.8). Parabolic work-hardening concomitantly with increasing dislocation densities was observed up to epsilon approximate to 2, followed by a practically constant flow stress due to dynamic recovery. The average grain size was reduced from 30.5 mu m in the annealed metal down to 4.1 mu m for epsilon approximate to 7.2; the fraction of sub-micrometric grains reached similar to 12% for epsilon approximate to 10.8. The microstructural changes were attributed to the fragmentation of the original grains by dislocation structures having low misorientation angles which gradually evolved into arrays of high-angle grain boundaries with increasing numbers of MDF cycles. The Cu samples subjected to 48 cycles of MDF displayed limited dynamic recrystallization, exhibiting basically dislocation cells and sub-grains with an average size of 0.6 mu m. It is demonstrated that low strain amplitude MDF delays the kinetics of grain refinement in copper compared with MDF using higher strain amplitudes.