Equal channel angular pressing (ECAP) was conducted at room
temperature to a high strain level of ~24 in high purity copper. Tensile testing,
Transition Electron Microscopy (TEM) and Electron backscatter diffraction (EBSD)
were used to characterize the microstructure and property evolution with the increase
of ECAP strain. It was found that tensile yield strength and the stored energy
increases upon ECAP processing until a peak reached at 8~12 passes of ECAP, and
their saturation was observed at higher ECAP passes. Continuous recrystallization
phenomenon in microstructure was observed, where dislocation free crystallites with
large misorientation to their surrounding matrix and smaller than the nuclei for
discontinuous recrystallization were observed embodied in the matrix of deformed
structure with high dislocation density. A two-step process was observed for the
formation of these small crystallites, first the condensing of dislocation tangles into a
narrow boundary, mostly low angle boundary; And second local migration (in
sub-micrometer range) of short grain boundaries, in strong contrast to the dramatic
migration of long large angle grain boundaries during discontinuous recrystallization
to swallow the deformed matrix, was observed leading to vanish of small subgrains.
The significance of self-annealing at room temperature in high purity copper processed by high-pressure torsion
