In Situ Scanning Transmission Electron Microscopy Observations of Fracture at the Atomic Scale

The formation, propagation, and structure of nanoscale cracks determine the failure mechanics of engineered materials. Herein, we have captured, with atomic resolution and in real time, unit cell-by-unit cell lattice-trapped cracking in two-dimensional (2D) rhenium disulfide (${\mathrm{ReS}}_2$) using in situ aberration corrected scanning transmission electron microscopy (STEM). Our real time observations of atomic configurations and corresponding strain fields in propagating cracks directly reveal the atomistic fracture mechanisms. The entirely brittle fracture with non-blunted crack tips as well as perfect healing of cracks have been observed. The mode I fracture toughness of 2D ${\mathrm{ReS}}_2$ is measured. Our experiments have bridged the linear elastic deformation zone and the ultimate nm-sized nonlinear deformation zone inside the crack tip. The dynamics of fracture has been explained by the atomic lattice trapping model. The direct visualization on the strain field in the ongoing crack tips and the gained insights of discrete bond breaking or healing in cracks will facilitate deeper insights into how atoms are able to withstand exceptionally large strains at the crack tips.

Figure 1

The mode I cracks in 2D ${\mathrm{ReS}}_2$ along the $\mathit{a}$ axis. (a) The monoclinic crystal structure model of 2D monolayer ${\mathrm{ReS}}_2$, plan view (upper) and side view (lower), with $\mathit{a}$,$\mathit{b}$ as basis vectors. (b) The generation of initial cracks by $e$ beam irradiation. Scale $\mathrm{bar}=100\mathrm{nm}$. (c) HAADF showing the mode I crack driven by postedge lattice switching. Scale $\mathrm{bar}=2\mathrm{nm}$. Inset shows the GPA strain analysis of this area for normal stress perpendicular to the crack direction. (d) Two HAADF snapshots of the crack tip zone of an ongoing crack. Crystal directions are highlighted by arrows. These directions apply to all of the HAADF images in Fig. 1. All images have been drift compensated and aligned to show the same positions of the specimen. False colors are applied on HAADF images. Brighter spots represent positions of Re atoms, while S atoms are barely visible. Scale $\mathrm{bar}=1\mathrm{nm}$. (e) In situ snapshot series of cracking and healing processes in 2D ${\mathrm{ReS}}_2$ by single steps (unit cell). Cracking in yellow and healing in red. Scale $\mathrm{bar}=1\mathrm{nm}$. (f) Evolution of a crack edge contour observed by in situ STEM with observation times marked, scale $\mathrm{bar}=2\mathrm{nm}$. (g) The inner most 16 Re atomic positions inside the crack tips extracted from 13 experimental images (black: crack heal; red: crack advance), an example HAADF image shown in the right side, and the DFT simulation results are shown as blue crosses. All sets of atomic position data are aligned using the upper right Re atom’s position as reference. Scale $\mathrm{bar}=0.2\mathrm{nm}$.

Figure 2

Atomic-scale strain analysis on the crack tip zones. (a) Two sequential snapshots of DFT simulated mode I cracks in ${\mathrm{ReS}}_2$. The ruptured Re-S bond is marked by red arrow. (b) Magnified HAADF image of one ${\mathrm{ReS}}_2$ cracked edge, white arrows indicate the position of dangling S atoms. Scale $\mathrm{bar}=0.3\mathrm{nm}$. Corresponding DFT results of ${\mathrm{ReS}}_2$ cracked edge shown on right side. (c) The discontinuity in STEM image caused by crack move when the electron beam is scanning closely to the crack tip. Two consecutive STEM snapshots of the same position in monolayer ${\mathrm{ReS}}_2$ sample showing the crack instantly move by 4 unit cells when the beam scanning reaches the yellow dashed line position. Scale $\mathrm{bar}=1\mathrm{nm}$. (d) Shear strain results (color encoded, discretized by half unit cell) of ${\mathrm{ReS}}_2$ Mode I crack and healing along the $\mathit{a}$ axis in our experiments. (e) The normal strain on the $x$ axis (shown in the inset) for mode I crack in $1L$ ${\mathrm{ReS}}_2$. The strain inside the 1 nm region deviate from the LEFM theory.

Figure 3

Cracks in bilayers. (a) The STEM snapshots of a crack in bilayer ($2L$) ${\mathrm{ReS}}_2$ which propagate by one unit cell. Scale $\mathrm{bar}=1\mathrm{nm}$. (b) Scheme for the bilayer ${\mathrm{ReS}}_2$ and the always synchronized cracking in the upper and lower layers. (c) The normal (tensile) strain ($e_{yy}$) distribution corresponding to (a). Scale $\mathrm{bar}=1\mathrm{nm}$. (d) The normal strain distribution ($e_{yy}$) for another $1L$ ${\mathrm{ReS}}_2$ sample, showing similar strain fields for $1L$ and $2L$ specimens. Scale $\mathrm{bar}=1\mathrm{nm}$.

Figure 4

The statistical results on the mode I crack and crack dynamics in $1L$ ${\mathrm{ReS}}_2$. (a) The histogram on the frequencies of loaded stress intensity ($K_I$) for our experimental mode I crack moves, crack advance, and crack heal cases are distinguished. $E$ is modulus of and $d$ is the lattice spacing perpendicular to cracking ($\mathit{y}$) direction for ${\mathrm{ReS}}_2$. (b) The histogram for frequencies of crack moving lengths (step) within one line scan of electron beam ($\sim 10\mathrm{ms}$), so it means the electron beam is close to the crack tip when crack moves. (c) The histogram for frequencies of crack moving lengths between two continuous snapshots of STEM images ($\sim 5\mathrm{s}$) when the electron beam is far away from the crack tip and no discontinuous lines are observed.