Abstract
Molecular dynamics simulations were performed to demonstrate the synergistic effects of the extrinsic size (nanowire length) and intrinsic size (twin boundary spacing) on the failure manner, yield strength, ductility and deformation mechanism of the twinned nanowires containing high density coherent twin boundaries CTBs paralleled to the nanowires’ axis. The twinned nanowires show an intense extrinsic size effect, i.e., shorter is stronger and more ductile, and an intense intrinsic size effect, i.e., thinner is stronger. Notably, the strengthening effect degradation of CTBs in the twinned nanowires is observed with an increase in nanowire length: remarkable strengthening effect can be obtained for the short nanowires, but the strengthening effect becomes less pronounced for the long nanowires. The twinned nanowires fail via a ductile manner or via a brittle manner depending on the synergistic effect of the nanowire length and twin boundary spacing. By atomic-level observation of the plastic deformation, we found that the emission of a trailing 30° partial from the free surface controls the yield behavior of the twinned nanowires. We also found that the special zigzag extended dislocations are formed by the dislocation–CTBs interactions, and propagate to sustain the plastic deformation.
Highlights
Nanotwinned metallic materials, comprised of coherent twin boundaries (CTBs) with nanometerscale spacing, have been paid special attention since firstly reported by Lu et al [1] because of the unusual combination of ultra-high strength and high tensile ductility [2,3,4], along with considerable work hardening [4,5,6]
The synergistic effects of the extrinsic size and intrinsic size on the failure manner, strength, ductility, and the deformation mechanism of the twinned Cu nanowires were addressed by molecular dynamics (MD) simulations
Our simulation results reveal that the mechanical behaviors of the twinned Cu nanowires are significantly related to the synergistic effects of the nanowire length and twin boundary spacing, indicating strong intrinsic and extrinsic size effects on strength, ductility and failure manner
Summary
Nanotwinned metallic materials, comprised of coherent twin boundaries (CTBs) with nanometerscale spacing, have been paid special attention since firstly reported by Lu et al [1] because of the unusual combination of ultra-high strength (on the order of 1 GPa for the nanotwinned Cu vs. 0.25 GPa for the coarse-grained Cu) and high tensile ductility (up to 14% for the nanotwinnedCu) [2,3,4], along with considerable work hardening [4,5,6]. Engineering CTBs into the metals has become an attractive strategy of simultaneously strengthening and toughening metal. Likewise, when the abundant CTBs are engineered into the face-centered cubic (FCC) metals nanowires or nanopillars, the extreme-high strength and even the ideal strength can be achieved [14,15]. The desired strain-hardening capability in low stacking-fault energy metals such as Au and Ag [16,17,18] can be obtained by the CTBs, which is always missing in single crystal metal nanowires or nanopillars [19,20,21].
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