Methods used to strengthen metals generally also cause a pronounced decrease in electrical conductivity, so that a tradeoff must be made between conductivity and mechanical strength. We synthesized pure copper samples with a high density of nanoscale growth twins. They showed a tensile strength about 10 times higher than that of conventional coarse-grained copper, while retaining an electrical conductivity comparable to that of pure copper. The ultrahigh strength originates from the effective blockage of dislocation motion by numerous coherent twin boundaries that possess an extremely low electrical resistivity, which is not the case for other types of grain boundaries.

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.

Strengthening materials traditionally involves the controlled creation of internal defects and boundaries so as to obstruct dislocation motion. Such strategies invariably compromise ductility, the ability of the material to deform, stretch, or change shape permanently without breaking. Here, we outline an approach to optimize strength and ductility by identifying three essential structural characteristics for boundaries: coherency with surrounding matrix, thermal and mechanical stability, and smallest feature size finer than 100 nanometers. We assess current understanding of strengthening and propose a methodology for engineering coherent, nanoscale internal boundaries, specifically those involving nanoscale twin boundaries. Additionally, we discuss perspectives on strengthening and preserving ductility, along with potential applications for improving failure tolerance, electrical conductivity, and resistance to electromigration.

Nano-grained (NG) metals are believed to be strong but intrinsically brittle: Free-standing NG metals usually exhibit a tensile uniform elongation of a few percent. When a NG copper film is confined by a coarse-grained (CG) copper substrate with a gradient grain-size transition, tensile plasticity can be achieved in the NG film where strain localization is suppressed. The gradient NG film exhibits a 10 times higher yield strength and a tensile plasticity comparable to that of the CG substrate and can sustain a tensile true strain exceeding 100% without cracking. A mechanically driven grain boundary migration process with a substantial concomitant grain growth dominates plastic deformation of the gradient NG structure. The extraordinary intrinsic plasticity of gradient NG structures offers their potential for use as advanced coatings of bulk materials.

In conventional metals, there is plenty of space for dislocations-line defects whose motion results in permanent material deformationto multiply, so that the metal strengths are controlled by dislocation interactions with grain boundaries 1,2 and other obstacles 3,4 . For nanostructured materials, in contrast, dislocation multiplication is severely confined by the nanometre-scale geometries so that continued plasticity can be expected to be source-controlled. Nanograined polycrystalline materials were found to be strong but brittle 5-9 , because both nucleation and motion of dislocations are effectively suppressed by the nanoscale crystallites. Here we report a dislocation-nucleation-controlled mechanism in nano-twinned metals 10,11 in which there are plenty of dislocation nucleation sites but dislocation motion is not confined. We show that dislocation nucleation governs the strength of such materials, resulting in their softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, occurring at a critical twin-boundary spacing for which strength is maximized. At this point, the classical Hall-Petch type of strengthening due to dislocation pile-up and cutting through twin planes switches to a dislocation-nucleationcontrolled softening mechanism with twin-boundary migration resulting from nucleation and motion of partial dislocations parallel to the twin planes. Most previous studies 12,13 did not consider a sufficient range of twin thickness and therefore missed this strength-softening regime. The simulations indicate that the critical twin-boundary spacing for the onset of softening in nano-twinned copper and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twinboundary spacing, and the higher the maximum strength of the material.Ultrafine-grained Cu with nanoscale thin twins embedded in individual grains has recently been synthesized, achieving a strength increase by a factor of 7 to 10 relative to conventional coarse-grained polycrystalline Cu, as well as considerable ductility and high electrical conductivity 10,11 . More interestingly, the strength of such nanotwinned Cu first increases as the twin-boundary spacing l decreases, reaching a maximal strength at l 5 15 nm, then decreases as l is further reduced 11 . The trend of increasing strength in nano-twinned ultrafine-grained Cu with decreasing l can be relatively well explained by the Hall-Petch effect because the twin planes can serve as barriers to dislocations gliding on inclined slip planes. However, the strength softening with a further decrease of l from 15 nm to 4 nm is intriguing.For nanocrystalline metals without nano-twin substructures, molecular dynamics simulations 6-9 have shown a strength softening mechanism as grain size is reduced to about 10 nm in Cu, which has been attributed to a transition from dislocation-mediated plastic deformat...

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