Dislocation arrangements in cyclically deformed copper single crystals

Detailed transmission electron microscopy (TEM) studies of dislocation arrangements in single-crystal copper samples, fatigued at a constant plastic shear strain amplitude of 0.15%, revealed clear differences between single- and multiple-slip-oriented crystals. In single-slip-oriented samples the dislocation arrangement consisted of loop patches and persistent slip bands (PSBs) with a ladder structure but, in addition, some cell formation also appeared in the PSBs. In multiple-slip-oriented crystals the dislocation matrix arrangement consisted of channel and wall layers lying nearly perpendicular to the loading axis. Ladder-type PSBs were also observed in these crystals, but they were normally separated from the matrix by cells and, in addition, PSBs seemed to have developed through cell formation. The resistance of PSBs to deformation can also locally vary because the proportional amounts of cells and ladder-like structures were not constant. On the basis of the present findings, PSBs should be considered only as structural elements into which the deformation concentrates during fatigue, regardless of their exact dislocation arrangement, which may contain ladder-type structures, cells and other possible dislocation configurations. As the role of point defects may also be significant in the initiation and, in particular, to the propagation of fatigue cracks, TEM observations were supplemented by small-angle neutron scattering and positron annihilation measurements which gave additional information about the point defect agglomerations.