Evaluating the paradox of strength and ductility in ultrafine-grained oxygen-free copper processed by ECAP at room temperature

Oxygen-free copper of >99.95% purity was processed by equal-channel angular pressing at room temperature (RT) for up to 24 passes and then pulled to failure at RT using strain rates from 10⁻⁴ to 10⁻² s⁻¹. The results show that the microstrain increases with strain at the lower numbers of passes but decreases between 16 and 24 passes. Similar trends were found also for the dislocation density, the Vickers microhardness and the values of the measured yield stresses in tensile testing. X-ray diffraction measurements showed a minor increase in the crystallite size at the high strain imposed by processing through 24 passes. These results demonstrate the occurrence of dynamic recovery at the highest strain. In tensile testing at a strain rate of 10⁻³ s⁻¹ the results gave a yield stress of ~391 MPa and an elongation to failure of 52% which is consistent with an earlier report using Cu of much higher purity but not consistent with an earlier report using Cu of the same purity.