Triple Junctions as Dislocation-Like Defects: The Role of Grain Boundary Crystallography Revealed by Experiment and Atomistic Simulation

TL;DR

The paper investigates a grain boundary triple junction in a {111}-textured Al thin film to reveal its dislocation-like defect character. By integrating atomic-resolution STEM with GRIP-driven GB structure searches, EAM-based atomistic simulations, and Burgers-circuit analysis, the authors extract the Burgers vector content and quantify the line energy of the junction. They demonstrate that the triple junction has dislocation-like character with a Burgers vector set by the microscopic DOFs of the joining GBs, and that the line energy follows a logarithmic dependence on radius, scaling roughly as $\lambda \propto b^2$; surprisingly, the experimentally observed junction does not minimize $b$ or $\lambda$, indicating slow kinetics and strong GB-network coupling. This work provides a framework to connect GB DOFs to triple junction energetics, highlighting how junctions interact with dislocations and influence GB network evolution and plastic response in polycrystalline materials.

Abstract

Grain boundary networks and their evolution are strongly influenced by triple junctions. The defect nature of these line defects significantly affects the properties of the network, but they have not been fully characterized to date. Here, we use scanning transmission electron microscopy combined with atomistic computer simulations to investigate a triple junction at the atomic scale in an Al thin film with {111} texture. Using sampling methods, we were able to construct the same junction structure as in the experiment within a computer model. We present a technique to calculate the Burgers vector of the triple junction. This allows us to connect the junction's dislocation character to the microscopic degrees of freedom of the joining grain boundaries. The junction line energy in the computer model can then be calculated using an embedded atom method potential. It follows the same laws as a bulk dislocation. Finally, we discovered a range of possible triple junctions for the observed grain boundaries, which vary in the magnitude of their Burgers vector. Interestingly, the experimentally observed junction is not the one with the smallest possible Burgers vector and energy. This suggests that the kinetics of transforming the junction line are likely too slow to be driven by the small energy contribution of the triple junction.

Figures (9)

• Figure 1: HAADF-STEM image of a GB triple junction in Al. The crystallographic $\langle112\rangle$ directions of the three grains were indexed and used to determine the GB planes. We found that the GBs are three symmetric $[11\overline{1}]$ tilt GBs---$\Sigma13$, $\Sigma39$, and $\Sigma3$---with the indicated planes. The table shows the misorientation values as indexed. Note that the cubic crystal has a threefold symmetry around $[11\overline{1}]$ and angles ±120 are crystallographically equivalent. We find that the misorientations add up to 0, meaning that this triple junction has no disclination content.
• Figure 2: Atomic structure of the $\Sigma3$$[11\overline{1}]$$\{112\}$ tilt GB. (a) Simulated structure. On the left, we show the same viewing direction as in the STEM image. The three images in the center show slices of the three nonequivalent $(11\overline{1})$ planes A, B, and C as marked in the sideview on the right. The gray lines in the slices represent the next-neighbor bonds inside the plane, highlighting that the fcc structure in this $\Sigma3$ GB is only disturbed on the A plane. Color coding of the atoms highlights the (arbitrary) atomic motifs, with the dark blue atoms representing the center of the GB. An excerpt of the STEM image (b) is compared with a STEM image simulation (c). Here, we applied a stress $\tau_{31}$ to the latter to achieve a better match. This stress is likely to also occur in the real sample, see text and Supplemental Fig. \ref{['fig:suppl:S3-shear']}.
• Figure 3: Atomic structure of the $\Sigma39$$[11\overline{1}]$$\{257\}$ tilt GB. (a) Simulated structure. On the left, we show the same viewing direction as in the STEM image. The three images in the center show slices of the three nonequivalent $(11\overline{1})$ planes A, B, and C as marked in the sideview on the right. The gray lines in the slices represent the next-neighbor bonds inside the plane. Color coding of the atoms highlights the (arbitrary) atomic motifs. An excerpt of the STEM image (b) is compared with a STEM image simulation (c).
• Figure 4: Atomic structure of the $\Sigma13$$[11\overline{1}]$$\{134\}$ tilt GB. (a) Simulated structure. On the left, we show the same viewing direction as in the STEM image. The three images in the center show slices of the three nonequivalent $(11\overline{1})$ planes A, B, and C as marked in the sideview on the right. The gray lines in the slices represent the next-neighbor bonds inside the plane. Color coding of the atoms highlights the (arbitrary) atomic motifs. An excerpt of the STEM image (b) is compared with a STEM image simulation (c).
• Figure 5: Comparison of the experimental junction with the computer model. (a) Excerpt of the experimental STEM image, with false colors to highlight motifs. (b) STEM image simulation of the computer model at 300K with the same false color highlights. (c) Snapshot of the model structure at 0K.