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Microstructural features, such as grain size and orientation, are crucial to the mechanical properties of materials. Therefore, understanding the structural evolution of grains, including processes like grain growth, recrystallization, and plastic deformation, is essential for material design and performance optimization. Grains often exhibit near-rigid-body rotation, particularly in nanocrystalline materials. The atomic-scale mechanisms behind grain rotation remain debated, including the roles of grain boundary migration and dislocation motion. To gain a deeper understanding of the mechanisms driving grain rotation, researchers from the University of California, the University of Hong Kong, and the City University of Hong Kong employed multi-scale in-situ scanning transmission electron microscopy (STEM) to investigate the atomic-scale processes of grain rotation in nanocrystalline materials, coupled with atomistic simulations to interpret the experimental observations.
A multi-scale in-situ scanning transmission electron microscopy (STEM) approach was used to investigate the mechanisms of grain rotation in nanocrystalline materials. Initially, in-situ high-resolution high-angle annular dark-field STEM (HAADF-STEM) was employed to quantitatively correlate each grain boundary migration event with disconnection motion during annealing, with imaging conducted at the atomic scale. Subsequently, in-situ four-dimensional STEM (4D-STEM) was utilized for micro-scale observations to verify the correlation between grain rotation and grain growth or shrinkage. Finally, atomistic simulations were performed to aid in the interpretation of the experimental observations.
Key Findings
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Fig. 1. In-situ observation of concurrent grain boundary (GB) migration and grain rotation. (A) HAADF-STEM image of a region in the initial Pt sample containing a Σ11 (113) symmetric tilt GB (green dashed line) and others (yellow dashed lines). Three grains on the [111] zone axis are marked as G1, G2, and G3. The inset shows the Fast Fourier Transform (FFT) pattern of the image, with diffraction spots of G1, G2, and G3 marked in red, blue, and pink, respectively. (B) Final state of the same region, with all GBs marked by solid lines and their initial positions by dashed lines. (C) Temporal evolution of the orientations of G1 and G2 in (A). Scattered points represent measured data, and curves are data processed using a Gaussian low-pass filter. Three grain rotation events are marked by black and gray dashed lines, with detailed atomic processes shown in (D). (D1 and D2) Sequential images of GB migration during the first grain rotation event marked in (C). GB locations before and after migration are marked by dashed and solid lines, respectively. Insets show GB structures in the green squares. Scale bars, 3 nm. (D3) Atomic displacement fields near the Σ11 GB in the green squares in (D1) and (D2). Atomic positions before and after GB migration are shown by white dashed and solid circles, respectively. Leftward (rightward) atomic displacements are marked by blue (red) arrows. (D4 to D6) Sequential images of GB migration between (D1) and (D2), with GB locations and atomic displacements marked.![]()
Fig. 2. Atomic-scale observations of disconnection motion and grain rotation happening concurrently. (A) HAADF-STEM image of the final state of the Pt sample, with GBs marked by white lines. Three grains on the [110] zone axis are marked as G1, G2, and G3. A local area in G2 is marked as A4. The right image shows the FFT pattern, with diffraction spots of G1, G2, and G3 marked in blue, red, and green, respectively. (B) Temporal evolution of the orientations of G1, G2, G3, and A4 in (A). (C), (E), and (G) Sequential HAADF-STEM images of grain rotation at the moment indicated by the gray dashed line in (B). GB locations are marked by white lines. Disconnection motion is indicated by white arrows, and grain rotation by red arrows. (D), (F), and (H) Atomic structures in the green squares in (C), (E), and (G), respectively, showing disconnection dynamics. Two types of disconnections are marked by "⊥" symbols. Two sets [(I and J) and (M and N)] of sequential images of disconnection motion in the black dashed square in (A) coincide with two rotation events of A4. The direction of disconnection glide is indicated by white arrows. GB locations before and after disconnection motions are labeled by yellow dashed and white solid lines, respectively. Grain rotation is indicated by red arrows. (K and O) Atomic displacements in the green squares in [(I) and (J)] and [(M) and (N)], showing reversible disconnection motions. Yellow dashed and white solid lines mark GB locations before and after disconnection motions. (L and P) Average atomic displacements parallel to GB, plotted against the distance from GB. Error bars correspond to the standard deviation of atomic displacements in the same row of atoms. Gray lines show the results of fitting the data to theory. In (A), (C), (E), (G), (I), (J), (M), and (N), scale bars are 3 nm. In (D), (F), (H), (K), and (O), white dashed and solid circles mark atomic positions before and after disconnection motions, respectively. Leftward (rightward) atomic displacements are marked by blue (red) arrows.![]()
Fig. 3. Micro-scale observations of concurrent grain growth and grain rotation, highlighting the correlation between these two processes. (A to D) Four sequential 4D-STEM orientation mappings of the same area of the Pt sample after annealing for 0, 5, 10, and 15 min, respectively. Color shows grain orientation based on the inverse pole figure [inset of (D)]. Color saturation changes according to indexing confidence. Scale bars, 100 nm. (E) Probability distribution for grain rotation rate, |ω|. The inset shows the schematic of concurrent rotation and growth of the shaded heptagon grain. (F) Probability distributions of grain growth rate |r| for all grains, P(|r|), and for fast-rotating grains (|ω| > 4°/min), Prot/(|r|), are indicated by gray and red bars, respectively. The ratio, Prot/s(|r|)/P(|r|), plotted as blue points, represents the probability of fast rotation occurring at grain growth rate |r|. (G) Probability distributions of grain size for all grains and for fast-rotating grains (|ω| > 4°/min) are indicated by gray and red bars, respectively. The ratio, Prot/(r)/P(r), plotted as blue points, represents the probability of fast rotation occurring on grains with size r. (H) Probability distributions of grain roundness for all grains and for fast-rotating grains are indicated by gray and red bars, respectively. The ratio, Prot/(x)/P(x), plotted as blue points, represents the probability of fast rotation occurring on grains with roundness x. Blue curves are guides for the eyes.
Authors
The first author of this work is Tian Yuan from the University of California. Prof. Han Jian from City University of Hong Kong, Prof. David J. Srolovitz from the University of Hong Kong, and Prof. Pan Xiao-Qing from the University of California are the corresponding authors of this paper.
Y. Tian, X. Gong, M. Xu, C. Qiu, Y. Han, Y. Bi, L.V. Estrada, E. Boltynjuk, H. Hahn, J. Han, D.J. Srolovitz, X. Pan, Grain rotation mechanisms in nanocrystalline materials: Multiscale observations in Pt thin films, Science 386(6717) (2024) 49-54. DOI: https://doi.org/10.1126/science.adk6384
Editor: Dr. Jun-Jing He
