High-pressure torsion-induced grain growth and detwinning in cryomilled Cu powders

Two mechanisms for deformation-induced grain growth in nanostructured metals have been proposed, including grain rotation-induced grain coalescence and stress-coupled grain boundary (GB) migration. A study is reported in which significant grain growth occurred from an average grain size of 46?nm to 90?nm during high pressure torsion (HPT) of cryomilled nanocrystalline Cu powders. Careful microstructural examination ascertained that grain rotation-induced grain coalescence is mainly responsible for the grain growth during HPT. Furthermore, a grain size dependence of the grain growth mechanisms was uncovered: grain rotation and grain coalescence dominate at nanocrystalline grain sizes, whereas stress-coupled GB migration prevails at ultrafine grain sizes. In addition, detwinning of the preexisting deformation twins was observed during HPT of the cryomilled Cu powders. The mechanism of detwinning for deformation twins was proposed to be similar to that for growth twins.