Severe plastic deformation-produced gradient nanostructured copper with a strengthening-softening transition

Low-excess energy twin boundary can effectively stabilize the conventional grain boundary. It has been reported that deformation-activated nanotwins in nanograined metals produced by severe plastic deformation techniques can significantly enhance mechanical-thermal stability. However, fabrication, structural evolution, and the effect of grain size and twin thickness on the mechanical stability of nanograined-nanotwinned metals, where both the grain size and twin thickness reach the nanometer scale (especially grain size is lower than 40 nm), remain unclear. In this study, a gradient nanostructured layer containing a nanograined-nanotwinned sub-layer region and an extremely refined twin-free nanograined top surface layer with grain size as small as ~10 nm is achieved on copper by using an ultrahigh-strain rate single point diamond turning technique. High-resolution transmission electron microscope observations, atomistic molecular dynamic simulations, and nanoindetation tests were performed to reveal the size-dependent mechanisms of grain refinement and hardness along the gradient direction. The propensity of deformation multifold twinning is increased firstly in large-size nanograins and then decreased once grain size is below ~48 nm, finally replaced by detwinning to form extremely fine twin-free nanograins at the topmost surface layer. In other words, both the zero-macrostrain-induced deformation multifold twinning and symmetry-breaking-based detwinning processes can continuously refine nanograins along the gradient direction. Critical grain sizes for deformation multifold twinning and detwinning are discussed. Interestingly, a Hall-Petch strengthening-softening transition is discovered at a critical grain size of ~30 nm in the gradient nanostructured layer. The softening mechanisms are elucidated to be attributed to the twin thickness effect on deformation mode in nanograined-nanotwinned structures and the pure grain boundary-mediated plasticity in extremely fine twin-free nanograins. A series of critical twin thicknesses for softening in nanograins with different grain sizes are discussed; that is, the smaller the grain size is, the smaller the critical twin thickness will be. This study offers the potential for understanding and developing stable nanostructured metals.