In situ atomistic observation of grain boundary migration subjected to defect interaction
Graphical Abstract
Introduction
Grain growth is a common dynamic phenomenon under thermal [1] or mechanical [2] stimulations, which critically affects the microstructural stability of polycrystalline materials [3]. Numerous theoretical and experimental investigations have rationalized grain growth by a series of mechanisms, including grain boundary (GB) migration [4,5], GB diffusion [6], atomic shuffling [7] and grain rotation/coalescence [8,9]. During GB migration, pre-existing defects such as solutes, vacancies and dislocations are frequently encountered (e.g. recrystallization after severe plastic deformation and deformation/cycling induced grain growth), which prominently influence the GB mobility via solute dragging/pinning [10], vacancy production/annihilation [11] or dislocation interaction [12]. These defect-influenced GB behaviours unambiguously affect the mechanical properties of polycrystalline materials in service. For example, frequent interactions between dislocations and GBs predominantly govern the intergranular deformation behaviours during the fatigue of metallic materials [13]. In nanocrystalline materials, where GB motion becomes more pronounced (e.g., grain coarsening), the interactions between lattice defects and mobile GBs critically govern their stability [14,15]. Unfortunately, the atomistic mechanism of GB deformation under the interactions with lattice defects remains largely unclear due to outstanding mechanistic complexity and technical limitations.
Both experimental and theoretical work have contributed to our current knowledge in terms of the interactions between incoming lattice dislocations and static GBs. The absorption, transmission or reflection mechanisms were proposed for most cases, leaving residual dislocations on the GB [16], [17], [18], [19], [20], [21], [22], [23]. To predict different slip modes transecting with GBs, two common geometric criteria have been developed and widely adopted. The Lee–Robertson–Birnbaum (LRB) criterion [24] incorporates three determinant factors including slip orientations (i.e., the angle between slip vectors κ and the angle between slip plane traces on the GB θ), residual dislocations and the resolved shear stress, where LRB = cos(θ)cos(κ). The other criterion named as m’ factor is defined as a function of angles between slip vectors (κ) and slip plane normals (ψ), where m’ = cos(ψ)cos(κ) [25,26]. In either criterion, a large geometric factor indicates better slip continuity across the GB, which favors slip transmission. Recent atomistic simulations quantitatively characterized the distinct reactions between a wide range of GBs and lattice dislocations with pure edge [27], screw [28] and mixed [29,30] characters, which contributed to a set of modified LRB criteria. However, we have noticed that the atomistic mechanism of interaction between authentic full dislocation (with compact core structure) and GB is largely lacking, due to the irreversible propensity of dislocation dissociation in current simulation studies [31], especially in face-centred cubic (FCC) metals. Moreover, the dynamic interaction between moving GBs and lattice defects (i.e., GB migration subjected to the interaction of gliding dislocations) should be fundamentally different from that between incoming dislocations and static GBs reported in all previous studies. Typically, the dynamic proceeding of GB disconnections during GB migration [32,33] may substantially influence the GB-lattice defects interactions, which remains yet unexplored. Therefore, experimental investigations are highly demanded for a mechanistic insight into the structural evolution during GB migration subjected to lattice defect interactions.
Here, we reveal the atomistic mechanism of shear-coupled GB migration under the intersection of different lattice defects in gold (Au) nanocrystals, by conducting the integrated state-of-the-art in situ nanomechanical testing and molecular dynamics (MD) simulation. Using custom-fabricated Au bicrystals with representative low energy Σ11 (113) symmetrical tilt GBs (STGBs) [34] as an example, we demonstrate that the disconnection-mediated STGB migration is undisturbed by the intersecting lattice defects, including full dislocations, stacking faults (SFs) and nanotwins, although different interaction dynamics occur during deformation. Upon migration, the Σ11 (113) GB can accommodate the 60° full lattice dislocations via the sequential core absorption and SF emission, which leaves glissile residual disconnections on the GB. Atomistic observations further demonstrate a similar accommodation mechanism for SFs intersecting a migrating Σ11 (113) GB. In contrast, the impinging nanotwins induce a facet on the GB, which can facilitate the nucleation of GB disconnections and the twin expansion in accordance with the GB migration. These findings provide systematic experimental evidence for the geometric criteria regarding GB-dislocation interaction and develop an unprecedented atomistic understanding on the GB dynamics under the influence of lattice defects, which would ultimately inspire the future investigations of GB-dominated plasticity under real circumstances.
Section snippets
In situ nanofabrication and nanomechanical testing
In situ nanofabrication and nanomechanical testing of Au bicrystals with tilt GBs were conducted using a PicoFemto® transmission electron microscope (TEM) electrical holder from Zeptools Co. inside a FEI Titan G2 60-300 Cs-corrected TEM. For in situ nanofabrication, two high purity Au rods (99.99 wt.%, ordered from Alfa Aesar Inc.) with a diameter 0.25 mm were cut by a wire cutter to obtain clean fracture surfaces with numerous nanoscale single crystalline tips; then, the freshly-fractured Au
Accommodation of full dislocations at the GB
Nanoscale Au bicrystal containing a Σ11 (113) STGB (with a deviation Δθ of 1.5° from the perfect Σ11 coincidence and a slanted angle of 7° with respect to the horizontal plane) was fabricated, where a single-layer (denoted as S) and a double-layer (denoted as D) disconnections pre-existed on the GB with Burgers vectors of 1/22[41] and 1/22[2], respectively, as shown in Fig. 1a. In addition, four 60° full dislocations (indicated by #1-#4) with the same Burgers vector of 1/2[01]1 existed
Discussion
As a characteristic low energy GB in FCC metals [34], Σ11 (113) STGB has been theoretically acknowledged as the strong obstruction to dislocation motion due to the inherent high energy barrier for slip transmission [19] and the common dislocation pile-up at this specific GB under different loading conditions were proposed based on simulation works [27,29]. Perfect and vicinal Σ11 GBs also showed strong propensity for dislocation absorption under different loading conditions and temperatures [45,
Conclusions
By conducting integrated in situ TEM nanomechanical testing and MD simulations, we have uncovered the atomistic mechanisms of shear-coupled migration of Σ11(113) STGBs under the influence of common lattice defects including full dislocations, SFs and nanotwins. Significant contributions of this study are summarized as follows:
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A new experimental method was introduced, which addressed the longstanding challenge of realizing the real-time atomistic observation of stress-induced GB-defect
Declaration of Competing Interest
None
Acknowledgements
J.W. Wang acknowledges the support of Basic Science Center Program for Multiphase Evolution in Hypergravity of the National Natural Science Foundation of China (51988101), the National Natural Science Foundation of China (51771172 and 51701179) and the Innovation Fund of the Zhejiang Kechuang New Materials Research Institute (ZKN-18-Z02). C. Deng acknowledges the use of computing resources provided by WestGrid and Compute/Calcul Canada. X.H. An acknowledges the support of Australia Research
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These authors contributed equally to this work.