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(a) Grain structure of sample I. The 120 nm × 120 nm simulation cell has 25 randomly oriented grains and 545 000 atoms. Atoms are colored according to their coordination numbers. The armchair direction that forms the smallest angle (mis-orientation angle) with the horizontal axis is marked by a red line for each grain. Histograms of grain size and mis-orientation angles are shown in (b) and (c), respectively. A histogram of relative angles between adjacent grains is shown in (d). The average grain size is 25 nm.
Details of GB microstructure. (a) A typical GB structure consisting of pentagon and heptagon pairs (5–7 pairs). (b)–(d) showing a variety of defects such as vacancies and nano-voids segregated at the GB triple junctions. The fully coordinated C-atoms are shown in green color while the pentagon and heptagon pairs are in red. Atoms with coordination of 2 or less are in brown color. Relative angles between adjacent grains are marked by the cross lines. The scale bar is 2 nm.
Uniaxial tensile stress-strain curves of graphene samples at T = 300 K and a strain rate of 1 × 109 s−1. The resulting Young's modules and Poisson ratio are 0.85 ± 0.01 TPa and 0.12 ± 0.02, respectively, for the NC samples.
(a)–(d) Snapshots of the microstructure to show the unzipping mechanism leading to brittle intergranular fracture. No significant out-of-plane displacement is observable. The arrow indicates the unzipping direction. (e)–(h) show the perforation mechanisms of separate nano-voids that cause the spontaneous initiation of crack in sample II. The color rendering scheme is the same as in Fig. 2 . The scale bar is 1 nm.
The evolution of atomic-level stress component in the loading direction. At the initial state of zero applied strain, the residual stress in the sample is fairly small. The atomic-level stress at the GB triple junction where crack nucleates (pointed by an arrow) is found to have the highest local stress at a strain of 0.157. The stress near the GB reduces to almost zero once the crack tip passes through. The scale bars shows the different scales used in these graphs.
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