Cold rolling of bulk nanocrystalline copper

By means of the electrodeposition technique, a bulk nanocrystalline (nc) copper sample was prepared with a high purity and a high density. The nc Cu sample was found to exhibit extreme extensibility under rolling at room temperature (Science, 287 (2000) 1463). In this paper, the microstructure evolution of the nc Cu during the cold-rolling process was examined by means of X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was found in the initial stage of cold rolling that the microstrain and grain boundary energy increase substantially, while the crystallite size remains unchanged during the whole process. When the degree of deformation exceeds 1000%, the microstrain and the grain boundary energy tend to saturated values, implying that this deformation process is dominated by grain boundary activity rather than lattice dislocation. This phenomenon agrees well with the observed mechanical behavior of the nc Cu sample.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.