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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 deformation—to multiply, so that the metal strengths are controlled by dislocation interactions with grain boundaries1,2 and other obstacles3,4. 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. Nano-grained polycrystalline materials were found to be strong but brittle5,6,7,8,9, 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 metals10,11 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-nucleation-controlled softening mechanism with twin-boundary migration resulting from nucleation and motion of partial dislocations parallel to the twin planes. Most previous studies12,13 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 twin-boundary spacing, and the higher the maximum strength of the material.

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Figure 1: Stress–strain relations and flow stress from molecular dynamics simulations of nano-twinned Cu.
Figure 2: Simulated deformation patterns in nano-twinned samples with grain size d = 20 nm at 10% strain.
Figure 3: Dislocation structures in d = 20 nm sample.
Figure 4: Yield stress of nano-twinned Cu as a function of twin-boundary spacing at different grain sizes.

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Acknowledgements

X.L. and H.G. acknowledge financial support by the NSF through the MRSEC Program (award number DMR-0520651) and grant CMMI-0758535 at Brown University. The simulations reported were performed on NSF TeraGrid resources provided by NICS under MSS090036 (H.G.) and DMR090083 (Y.W.), with additional support from the Computational Mechanics Research Facility at Brown University and the Alabama Supercomputer Center. Helpful discussions with J. D. Embury, W. D. Nix, H. Mughrabi, M. A. Meyers and R. Taylor are gratefully acknowledged. L.L. and K.L. acknowledge financial support by the NSFC (grant numbers 50621091, 50725103 and 50890171) and the MOST of China (grant number 2005CB623604).

Author Contributions All authors contributed equally to this work. H.G., K.L. and L.L. conceived the project. X.L. and Y.W. performed molecular dynamics simulations. All authors analysed data, developed the model, discussed the results and wrote the paper.

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Correspondence to Yujie Wei or Huajian Gao.

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Supplementary information

Supplementary Information

This file contains Supplementary Figures S1-S9, Supplementary Discussions 1-4, Supplementary References and legends for Supplementary Movies 1-6. (PDF 4338 kb)

Supplementary Movie 1

This movie file contains an animation of nano-twinned Cu with grain size d=10nm and twin boundary spacing λ=0.63nm under uniaxial tension (see Supplementary Information file for full legend). (MOV 7750 kb)

Supplementary Movie 2

Animation of nano-twinned Cu with grain size d=10nm and twin boundary spacing λ=3.75nm under uniaxial tension (see Supplementary Information file for full legend). (MOV 7792 kb)

Supplementary Movie 3

This movie file contains an animation of nano-twinned Cu with grain size d=20nm and twin boundary spacing λ=0.83nm under uniaxial tension (see Supplementary Information file for full legend). (MOV 5299 kb)

Supplementary Movie 4

This movie file contains an animation of nano-twinned Cu with grain size d=20nm and twin boundary spacing λ=6.25nm under uniaxial tension (see Supplementary Information file for full legend). (MOV 5331 kb)

Supplementary Movie 5

This movie file contains an animation of evolution of dislocation structures within one grain of nano-twinned Cu with grain size d=20nm and twin boundary spacing λ=0.83nm under uniaxial tension (see Supplementary Information file for full legend). (MOV 6208 kb)

Supplementary Movie 6

This movie file contains an animation of evolution of dislocation structures within one grain of nano-twinned Cu with grain size d=20nm and twin boundary spacing λ=6.25nm under uniaxial tension (see Supplementary Information file for full legend). (MOV 4288 kb)

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Li, X., Wei, Y., Lu, L. et al. Dislocation nucleation governed softening and maximum strength in nano-twinned metals. Nature 464, 877–880 (2010). https://doi.org/10.1038/nature08929

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