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

Abstract

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 ∼0.075 (total accumulated strain ε ≈ 10.8). Parabolic work-hardening concomitantly with increasing dislocation densities was observed up to ε ≈ 2, followed by a practically constant flow stress due to dynamic recovery. The average grain size was reduced from 30.5 μm in the annealed metal down to 4.1 μm for ε ≈ 7.2; the fraction of sub-micrometric grains reached ∼12% for ε ≈ 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 μm. It is demonstrated that low strain amplitude MDF delays the kinetics of grain refinement in copper compared with MDF using higher strain amplitudes.