Spatial distribution of defects in ultra-fine grained copper prepared by high-pressure torsion

Abstract

A study of ultra-fine grained copper produced by high-pressure torsion is presented. Positron annihilation spectroscopies (slow positron implantation, lifetime and Doppler broadening spectroscopies), transmission electron microscopy, X-ray diffraction spectroscopy, and microhardness measurements have been employed in order to identify defects and their spatial distribution, i.e. lateral distribution and depth profile, in the specimens. Two types of defects could be identified: dislocations in the distorted regions along grain boundaries and microvoids of size of 3–4 vacancies inside the grains. No change in grain size as a function of the distance from the center of the sample disk was observed. The mean coherent domain size close to the surface was found to be 80 ± 20 nm and it slightly increases with depth. The concentration of microvoids has been found to decrease with depth and it slightly varies with the distance from the center of the disk, being lowest in the center. No position dependence of the concentration of dislocations was observed.