Microstructural evolution and nanostructure formation in copper during dynamic plastic deformation at cryogenic temperatures

The microstructural evolution and formation mechanism of nanostructures in bulk pure Cu samples induced by dynamic plastic deformation (DPD) at high strain rates and cryogenic temperatures were investigated using transmission electron microscopic characterization. Three different mechanisms were identified for the plastic deformation and microstructural refinement, including dislocation manipulation and rearrangement, deformation twinning forming nanoscale twin/matrix (T/M) lamellae in bundles, and shear banding in the T/M lamellae. An increasing tendency of deformation twinning and shear banding was observed at higher strains. For strain ε = 2.1, a mixed nanostructure is formed in the DPD Cu bulk sample with nanoscale T/M lamellae making up about 33% of the volume and nano-sized grains making up about 67%. The nanograins can be classified into three types in terms of their origin: (i) nanograins derived from fragmentation of nanoscale T/M lamellae with an average transverse size of about 47 nm; (ii) nanograins in shear bands with an average transverse size of about 75 nm; and (iii) nanograins derived from dislocation cells with an average transverse size of about 121 nm. The high density of deformation twins induced by high strain rates and cryogenic temperatures in DPD, distinct from that in conventional severe plastic deformation, plays a crucial role in formation of the nano-sized grains.