Dislocation nucleation governed softening and maximum strength in nano-twinned metals

Abstract

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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