Microstructures and compression properties of copper processed by high-pressure torsion

Microstructures of commercial purity copper deformed by high-pressure torsion have been examined by means of transmission electron microscopy. Observations have been carried out in two perpendicular directions, i.e. the longitudinal section and the transverse section. Microstructural observations in the longitudinal section revealed a rather different morphology of the dislocation structures from those observed in the transverse section. In the transverse section, dislocation cell structure was formed at low strains. With increasing strain, the dislocation cells continually transformed into equiaxed subgrains and finally into equiaxed grains with high angle grain boundaries due to dynamic recrystallization processes. In the longitudinal section, at low strains, extended dislocation boundaries were observed. With increasing strain, a lamellar structure was observed with some dislocation cell structured across it. The lamellar boundary misorientation as well as the misorentation between the dislocation cells increasing with shear strain. At median strains, the dislocation cells have transformed into elongated subgrains. At large strain, the subgrains transformed into individual grains and the outline of the lamellar structure disappeared. Compression test results show that an increase in yield stress ( 385MPa) and strain rate sensitivity (0.021) as compared to coarse grain copper.