Two mechanisms for deformation-induced grain growth in nanostructured metals were proposed, including grain rotation-induced grain coalescence and stress-coupled grain boundary migration. A study was reported in which significant grain growth occurred from an average grain size of 46 to 90nm during high pressure torsion of cryomilled nanocrystalline Cu powders. Careful microstructural examination ascertained that grain rotation-induced grain coalescence was mainly responsible for the grain growth during high pressure torsion. 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 grain boundary migration prevails at ultrafine grain sizes. In addition, de-twinning of the pre-existing deformation twins was observed during high pressure torsion of the cryomilled Cu powders. The mechanism of de-twinning for deformation twins was proposed to be similar to that for growth twins.

High-Pressure Torsion-Induced Grain Growth and Detwinning in Cryomilled Cu Powders. H.Wen, Y.Zhao, Y.Li, O.Ertorer, K.M.Nesterov, R.K.Islamgaliev, R.Z.Valiev, E.J.Lavernia: Philosophical Magazine, 2010, 90[34], 4541-50