Deformation twinning in polycrystalline copper at room temperature and low strain rate

Deformation twins were widely observed in polycrystalline Cu with grain sizes varying from micrometers to nanometers during the process of equal channel angular pressing at room temperature and low strain rate (~10^-2 s^-1). The microstructures of deformation twins were characterized by a transmission electron microscope (TEM) and a high-resolution TEM. It was found that deformation twinning in coarse-grained Cu occurred mainly in shear bands and their intersections as a result of the very high local stress resulted from the severe plastic deformation, and followed the well known pole mechanism. With a decrease in the grain size down to submicrometer (<1 (mu)m) and nanometer (<100 nm) dimensions, twinning was observed to take place via partial dislocation emission from grain boundaries and grain boundary junctions, which is different from the pole mechanism operating in coarse-grained Cu. These observations are consistent with the predictions of recent molecular dynamic simulations for nanocrystalline face-centered cubic materials. The deformation conditions required for twinning and the formation mechanism of deformation twins varying with grain size in Cu are discussed.