Deformation structures in ball milled copper

The deformation structures of copper powder induced by ball milling were studied in detail by using high resolution electron microscopy. It was found that copper powder after 20 h of milling shows the formation of shear bands and a number of mechanical twins. The observed twins belong to two types, i.e. multiple twins and high-order ones. It is suggested that Venables' model on mechanical twins in face-center-cubic metals is reasonable in this case. The generation of mechanical twins could be explained as follows: the shear stress (P-max) induced by ball milling exceeds the critical shear stress for twinning (tau); the grain size decreases to a critical value below which twinning rather than slip is the preferred mode of deformation as well as the high strain rates induced by ball milling. The formation of subgrains was found to occur via two routes: they were formed both in the shear bands and at the tip of them or at the tip of the twin boundaries, or at the edge of the larger grains. The subgrains formed via the two ways are in nanometer scale (10-100 nm), and their orientations are completely random. The as-received nanograins contained high density of dislocations. The grain boundaries (GBs) of nanocrystalline (NC) Cu are usually ordered, curved and strained, and disordering, lattice distortion and nanovoids in local regions were observed too.