Force The Shear Deformation Behavior Of Pure Copper Under High Strain Rate

Hat-shaped specimens, prepared along the normal direction (ND) of the annealed and cold-rolled copper sheet, were dynamically compressed by means of the Split-Hopkinson pressure bar (SHPB). The micro structure and microtexture evolution in adiabatic shear bands (ASBs) at high strain rate with different strains were studied using optical microscope (OM), transmission electronic microscope (TEM) and electron backscatter diffraction (EBSD). Furthermore, the anisotropy of dynamic forced shear behaviors in cold-rolled copper sheer were also investigated by compressing the hat-shaped samples with the axes along the0°(RD-0°),45°(RD-45°) and90°(RD-90°) directions from rolling direction lying in the rolling plane. The results show:The microstructure evolution in the ASBs of copper includes the stretched dislocation cells, elongated subgrains, the refinement and multiplication of subgrains from the fragmentation, subgrain rotation leading to the formation of ultrafine grains and re-elongated of the dynamic recrystallization (DRX) grains. Based on the observation results, a modified rotational dynamic recrystallization mechanism is proposed to explain the microstructure evolution in the ASBs of copper. The microtexture characterization reveals that an obvious preferred orientation, in which <110> directions align with shear direction, develops in the shear localization area. In addition, the crystals tend to have their slip plane parallel to the shear plane and slip direction parallel to the shear direction during the dynamic deformation. The calculated results for the grain growth and the dislocation density changes show that the ultrafine grains in the ASBs did not grow significantly during the cooling stage, while the dislocation density decreases rapidly. However, the final dislocation density is also much larger than the boundary of stress-free.Since the higher work-hardening level, the cold-rolled copper has a higher adiabatic shear sensibility than the annealed. The critical shear strains of the evolution stages in cold-rolled copper are less and the dynamic forced shear behavior is also different comparing to annealed copper. At the same time, the texture in the cold-rolled sheet has an influence to the microtexture evolution during deformation at a limited strain. When the shear strain increase, the microtexture evolution in cold-rolled copper has a similar trend to the annealed.The forced shear response and the shear deformation behavior of cold-rolled copper sheet exhibits pronounced anisotropy. The yield strength, flow stress as well as the adiabatic shear sensitivity for the RD-90°direction sample is the maximum, then RD-45°and minimum for those in RD-0°. The difference of crystal orientation for different directions is the main cause of the anisotropy. Based on the shear stress-shear strain curves and adiabatic shear dissipation energy, the difference of adiabatic shear susceptibility for different directions was explained qualitatively.The author would like to thank the National Natural Science Foundation of China (NSFC, No.50871125) and the Science and Technology Foundation of Central South University (No.2010QZZD014) for the financial support.