Influence of stacking-fault energy on microstructural characteristics of ultrafine-grain copper and copper–zinc alloys

Experiments were conducted on samples of pure Cu and two Cu–Zn alloys to evaluate the influence of the stacking-fault energy (SFE) on microstructural development when processing using high-pressure torsion (HPT). Transmission electron microscopy, X-ray diffraction and hardness measurements were used for microstructural evaluation and the results show consistency between these techniques. Grain sizes in the nanometer range were formed at the edges of the HPT disks, larger submicrometer grains were formed in the disk centers and the measured grain sizes decreased with decreasing SFE. There was negligible twinning in pure Cu but the densities of dislocations and twins increased with increasing Zn content and thus with decreasing SFE. The values of the Vickers microhardness were lower in the centers of the disks for the two Cu–Zn alloy and this is consistent with the low SFE and slow rates of recovery.