The microstructures of cryogenically ball-milled Cu were investigated by high-resolution electron microscopy. It was found that the grain-size reduction was a dislocation-controlled continuous process which consisted of the formation of small-angle grain boundaries, a gradual increase in misorientations as a result of accumulation of more dislocations and, finally, the formation of large-angle grain boundaries. The grain boundaries were generally curved, wavy or faceted, and heavily strained, which were typical characteristics of nano-structured materials. In addition, extrinsic dislocations

were found in many grain boundaries, indicating that most were in a high-energy non-equilibrium configuration, which was consistent with observations in equal-channel angular pressing processed Cu, Ni, and Al-Mg, repetitive corrugation and straightening processed Cu and room-temperature ball-milled Cu. These results support a still-disputed concept that grain boundaries in nano-structured metals processed by severe plastic deformation were mostly in non-equilibrium states.

Grain Boundary Structure of Nanocrystalline Cu Processed by Cryomilling. J.Y.Huang, X.Z.Liao, Y.T.Zhu, F.Zhou, E.J.Lavernia: Philosophical Magazine, 2003, 83[12], 1407-19