Determination of internal stresses in cyclically deformed copper single crystals using convergent-beam electron diffraction and dislocation dipole separation measurements

Single crystals of copper were cyclically deformed, in single slip, to pres aturation at 298 K. The dislocation substructure was analyzed using convent ional bright-field and dark-field transmission electron microscopy with par ticular attention directed towards the dislocation dipole spacing. It was f ound that, in both metals, the dipole spacing and statistical distribution of spacings were independent of the location in the heterogeneous substruct ure, which consisted of dense dipole bundles (or veins) and channels with r elatively low dislocation density. Furthermore, the stress to separate the dipoles with largest spacing (upper-bound separation for stable dipoles) ca n be used to calculate the stress at the dipole location. This stress is wi thin a factor of about two of the applied stress in both channels and veins . The stress necessary to pass dislocations through the dense veins was cal culated to also be within a factor of about two of the applied stress. Conv ergent-beam electron diffraction (CBED) experiments were also performed at several locations very near the dipole bundles and within the channels. The lattice parameter measurements also suggested an absence of long-range int ernal stresses. The observations and calculations suggest a uniform state o f stress throughout the heterogeneous dislocation substructure, without the presence of significant internal stresses. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.