Surface Deformation Features in Ultrafine-Grained Copper Cyclically Stressed at Different Temperatures

The surface deformation features in ultrafine-grained copper produced by equal channel angular (ECA) pressing, which was cyclically deformed at temperatures between room temperature and 573 K under a constant stress amplitude of 200 MPa, were investigated. It was found that the surface deformation features and damage behaviour are strongly dependent upon the testing temperature. For examples, large-scale shear bands (SBs) formed at room temperature, whereas finer and discontinuous SBs, instead of large-scale SBs, were found to become the dominant feature with increasing temperature (below recrystallization), resulting from the reduction in quantity and volume fraction of grain boundaries as a consequence of grain growth and the enhanced dislocation slip. When the temperature is above recrystallization, no clear SBs were observed and dislocation slip deformation within grains governed the plastic deformation of UFG copper, causing nucleation of cracks along slip bands in grains or along grain boundaries, in contrast to the nucleation along SBs at temperatures below recrystallization.