The strength of polycrystalline materials increases with decreasing grain size. Below a critical size, smaller grains might lead to softening, as suggested by atomistic simulations. The strongest size should arise at a transition in deformation mechanism from lattice dislocation activities to grain boundary–related processes. We investigated the maximum strength of nanotwinned copper samples with different twin thicknesses. We found that the strength increases with decreasing twin thickness, reaching a maximum at 15 nanometers, followed by a softening at smaller values that is accompanied by enhanced strain hardening and tensile ductility. The strongest twin thickness originates from a transition in the yielding mechanism from the slip transfer across twin boundaries to the activity of preexisting easy dislocation sources.

Stronger copper through twin power: Materials with structural gradients often have unique combinations of properties. Gradient-structured materials are found in nature and can be engineered. Cheng et al. made a structural gradient by introducing gradients of crystallographic twins into copper. This strategy creates bundles of dislocations in the crystal interiors, which makes the metal stronger than any of the individual components. This method offers promise for developing high-performance metals. Science , this issue p. eaau1925

Strength by cyclic torsion: For most alloys, conventional or high entropy, increasing strength comes at the cost of poor ductility. Although there are many strategies to break this inverse relationship, Pan et al . now show that cyclic torsion on a high-entropy alloy enhances strength without degrading ductility (see the Perspective by Yeh). Cyclic torsion creates a gradient of dislocations and low-angle grain boundaries from the surface to the interior that organize into tiny stacking faults and twin when straining begins. These structures allow for the good ductility while simultaneously helping to work harden the alloy. —BG

Coarse-grained materials are widely accepted to display the highest strain hardening and the best tensile ductility. We experimentally report an attractive strain hardening rate throughout the deformation stage at 77 kelvin in a stable single-phase alloy with gradient dislocation cells that even surpasses its coarse-grained counterparts. Contrary to conventional understanding, the exceptional strain hardening arises from a distinctive dynamic structural refinement mechanism facilitated by the emission and motion of massive multiorientational tiny stacking faults (planar defects), which are fundamentally distinct from the traditional linear dislocation–mediated deformation. The dominance of atomic-scale planar deformation faulting in plastic deformation introduces a different approach for strengthening and hardening metallic materials, offering promising properties and potential applications.

Significance: The emergence of heterogeneous nanostructured metals offers exciting opportunities for achieving extraordinary mechanical properties. Gradient nanotwinned Cu is a prominent class of heterogeneous nanostructured metals, as it exhibits a superior extra strength compared with nongradient counterparts. However, the mechanistic origin of the extra strength remains elusive. At a more fundamental level, the strengthening effects of plastically inhomogeneous deformation in heterogeneous nanostructured metals are not well understood. Here, we use a combination of controlled material processing, back-stress measurement, dislocation microstructure characterization, and strain gradient plasticity modeling to unravel the origin of the extra strength in gradient nanotwinned Cu. The combined experimental and modeling framework may be applied to accelerate the rational design of heterogeneous nanostructured metals with enhanced mechanical performance.