Grain Boundary Area Research Articles

Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi1-xCaxFe1-yTiyO3-δ Thin-Film Solar Cells.

Designing solid-state perovskite oxide solar cells with large short circuit current (JSC) and open circuit voltage (VOC) has been a challenging problem. Epitaxial BiFeO3 (BFO) films are known to exhibit large VOC (>50 V). However, they exhibit low JSC (≪μA/cm2) under 1 Sun illumination. In this work, taking polycrystalline BiFeO3 thin films, we demonstrate that oxygen vacancies (VO) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high JSC and VOC simultaneously. While aliovalent substitution (Ca2+ at Bi3+ site) is used to control the lattice VO, Ca and Ti cosubstitution is used to bring out only GB-VO. Fluorine-doped tin oxide (FTO)/Bi1-xCaxFe1-yTiyO3-δ/Au devices are tested for photovoltaic characteristics. Introducing VO increases the photocurrent by four orders (JSC ∼ 3 mA/cm2). On the contrary, VOC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO3. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-VO. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in JSC. The cosubstitution, following a charge compensation mechanism, decreases the lattice VO concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve VOC (3-8 V) much larger than that of BiFeO3 (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice VO concentration dominates GB-VO concentration. Notwithstanding, both lattice and GB-VO contribute to the increase in JSC; the former weakens ferroelectricity, and as a consequence, undesirably, VOC is lowered well below 0.5 V. Using optimum JSC and VOC, we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.

Mössbauer Study of the Implantation of Fe-57 Ions into Metallic Ta and Mo and Stainless Steel

The effect of the implantation of 57Fe ions with an energy of 1 MeV and fluence of 5 × 1016 ion/cm2 on the properties of structural materials used in the nuclear industry (metallic Ta and Mo and 12Cr18N10T stainless steel) is studied using 57Fe Mossbauer spectroscopy and X-ray diffraction. The concentration of implanted Fe atoms is calculated using the SRIM program. The formation of two phases is found in Ta and Mo. The main phase in molybdenum (84%) is a solid solution of Fe substitution in Mo. The main phase in tantalum (78%) corresponds to Fe-cluster formation in a Ta matrix. Apparently, additional surface phases are related to the formation of Fe + Ta/Mo + vacancy clusters in the areas of grain boundaries, or are associated with areas of partial amorphization of the material.

Effect of cryogenic quenching on microstructure and microhardness of Ti-6Al-4V alloy

The state of the art of the current job is to investigate the effect of cryogenic quenching on mechanical properties of Ti-6Al-4V such as microhardness and microstructure. Cooling rate plays significant role in the re-formation of grains/grains boundaries. Three type of quenching viz. water, cryogenic and air were used in combination with annealing and solution treated samples. It is found that the application of the cryogenic quenching with annealing process achieved minimum microhardness with lowest percentage area of grain boundaries. However, the cryogenic quenching with solution treated sample showed maximum microhardness. This increment in the microhardness is due to the increased bimodal α, β grains and α-β interfaces in the cryogenic quenched solution treated samples. Therefore, the results of the current job showed that the mechanical properties of Ti-6Al-4V can be improved with the help of proper selection of heat treatment and quenching medium.

Effect of shot peening coverage on hydrogen embrittlement of a ferrite-pearlite steel

Effect of a wide range of shot peening (SP) coverage on hydrogen diffusion and hydrogen embrittlement (HE) of a steel consisting of ferrite and pearlite was investigated. Conventional SP (CSP) reduces diffusivity and HE because of increase of dislocation trapping sites. Although severe SP (SSP), which uses higher coverages than CSP, increases the area of ferrite grain boundaries as a result of grain refinement, it cannot suppress further the diffusion and HE, as it cannot induce a further evident increase in strong hydrogen trapping sites such as dislocations. However, when coverage is increased to so high that causes the breaking and refinement of pearlite phase, diffusion and HE is suppressed further, because the dispersed distribution of ultrafine pearlite particles within ferrite matrix provides more strong interface trapping sites. SP-induced change of trapping sites plays a more critical role in HE and diffusion, in comparison with induced residual compressive stresses.

On the lower limit of misorientation of grain boundaries in hot forging of AA7050

The goal of this work is to find a reasonable norm to measure the area of low angle grain boundaries (LABs) using EBSD data, which is useful for the metal forming community involved in developing appropriate constitutive models for dynamic recovery. Typically, LABs are defined as the grain boundaries with misorientation between 2° and 10°. However, it is found in this study that reasonable variations of area per volume of LABs (Slab) with strain level (ε), strain rate (έ) and temperature is not be observed for AA7050 with this definition. There is a competition among different processes, including sub-grain formation and coarsening, rotation of sub-grain/cell boundary and migration of LABs and high angle grain boundaries (HABs) during recovery and continuous dynamic recrystallisation. By analysing the evolution of Kernal Angle Misorientation (KAM), it is shown that recovery has more effect on the lower angle part (< 1°) of local misorientation. If the lower limit of LABs is shift to 0.5°, the evolution of Slab with ε and έ shows clear pattern and increasing trend of Slab with temperature is consistent with temperature effect on sub-grain formation process. With such definition, we can have a better understanding of the microstructure evolution in the dynamic restoration of AA7050.

Insight into impurity scavenging effect of gadolinium-doped ceria electrolytes

In this study, Ce0.89+xGd0.1−2xSrxEu0.01O2−δ (x = 0–0.04) ceramic electrolytes sintered at different temperatures were prepared and the impurity scavenging effect of Sr2+ dopants on their grain boundary (GB) electrical properties were investigated. The phase compositions, microstructures, and ionic conductivities of the samples were investigated using X-ray diffraction, scanning electron microscopy, and AC impedance spectroscopy, respectively. The incorporation of Sr2+ improved the overall GB conduction of the electrolytes by decreasing their GB area and space-charge potential. It hindered conduction to some extent by weakening the impurity dilution effect. The scavenging effect could be observed only when the sintering temperature was higher than the eutectic temperature of Si GB impurities. At low sintering temperatures, the Sr2+ dopants could not scavenge the GB impurities and increased the GB impurity coverage proportion. The findings of this study provide insights into the optimisation of the GB electrical properties of oxygen-ion conductors by selecting appropriate dopants and sintering temperatures to induce the GB impurity scavenging effect.

Enhancement of fracture forming limit by crystallographic texture reformation of AA1050 sheets in Single Point Incremental Forming

Single Point Incremental Forming (SPIF) process that results in the formation of sheet metal components leads to higher forming capabilities in comparison to conventional forming strategies. However, the formability in SPIF is critical and limited by the rapid/sudden failure in the sheet material (without material localization) for the components with high wall inclination angles. The state of stresses and strains plays a huge role in determining the forming limits in SPIF, which is complexly governed by various factors such as sheet thickness, lubrication, incremental step depth, tool rotational speed, part geometry, tool dimensions, and geometry, to name a few. The present work focuses on investigating formability of the sheets from its microstructure and crystallographic texture point of view. Preliminary experimentation led to the conclusion that SPIF of the original commercially supplied AA1050 H14 sheets results in early and premature failure. Microstructural investigation of the undeformed sheet showed that the commercially supplied sheets undergo strain hardening due to cold rolling and therefore, results in dislocated and disorientated microstructure and texture. The present work attempts to reform the microstructure of the sheet by preheating at the optimized temperature. Finite element analysis (FEA) of the process led to the observation that the reformed microstructures result in enhanced fracture forming limit strain and reduced stresses in SPIF. This may be attributed to the decrease in the number of dislocations and dislocated grains, and decrease in the surface area of grain boundary per grain with preheating. Improved fracture forming limits due to preheating were experimentally validated by utilizing digital image correlation technique (DIC) for acquiring true strain values subjected to fractured original and different preheated samples.

Grain-boundary corrosion of nickel-based alloy by synchrotron radiation technology

The spatial distribution and chemical speciation of tellurium (Te) at the grain boundary (GB) of nickel (Ni)-based alloys are crucial for investigating the tellurium corrosion mechanism. In this work, corrosion products along GBs were investigated by synchrotron-radiation-based microprobe techniques, including micro-X-ray fluorescence, micro-X-ray diffraction and micro-X-ray absorption near-edge structure (μ-XANES), for the first time. A gradually decreasing tendency in the concentration of tellurium from the GB area into the grains was identified, together with the presence of GB corrosion products in the form of nickel telluride (Ni3Te2), as well as nickel–tellurium solid solution. Moreover, both the µ-XANES results and density functional theory results confirmed that the impurity tellurium atoms in the nickel–tellurium solid solution tend to bond with their neighboring nickel atoms through weak covalent interactions, due to the hybrid state of tellurium 2p–nickel 4s. Such an unstable antibonding character from the nickel–tellurium combination is believed to play a key role in weakening the structural stability of nickel-based alloys. The efficient analysis of the tellurium corrosion mechanism based on these feasible synchrotron radiation characterizations will inspire more pertinent experimental investigations.

Microstructure and Mechanical Properties of a Fiber Welded Socket-Joint Made of Powder Metallurgy Molybdenum Alloy

Fiber welding of socket-joints made of nanostructured high-performance molybdenum alloy (NS Mo) was carried out to get a better understanding of the role of welding heat input. It was found that low heat input (i.e., high welding speed) resulted in significantly refined grains in the fusion zone (FZ) of fiber laser welded NS Mo joints. When welding heat input decreased from 3600 J/cm (i.e., 1.2 kW, 20 cm/min) to 250 J/cm (i.e., 2.5 kW, 600 cm/min), the tensile strength of welded joints increased from about 250 MPa to about 570 MPa. It was confirmed by energy spectrum analysis that the higher the welding heat input, the higher the oxygen contents at the grain boundary (GB) within the FZ. In addition, the most important reason for poor strength of welded joints of Mo alloys was reported as being that MoO2 was segregated on the grain boundary. Therefore, it was concluded that welding under low heat input (i.e., high welding speed) was able to reduce the segregation degree of MoO2 at the grain boundary by refining grains and increasing the total area of GBs, thus improving the strength of welded joints and reducing the proportion of the intergranular fracture zone in tensile fractures.

Grain boundary curvatures in polycrystalline SrTiO3: Dependence on grain size, topology, and crystallography

AbstractBy mapping grain orientations on parallel serial sections of a SrTiO3 ceramic, it was possible to reconstruct three‐dimensional orientation maps containing more than 3000 grains. The grain boundaries were approximated by a continuous mesh of triangles and mean curvatures were determined for each triangle. The integral mean curvatures of grain faces were determined for all grains. Small grains with fewer than 16 neighbors mostly have positive mean curvatures while larger grains with more than 16 neighbors mostly have negative mean curvatures. It is also possible to correlate the mean curvature of individual triangles with the crystallographic characteristics of the grain boundary. The mean curvature is lowest for grain boundaries with (100) orientations and highest for grain boundaries with (111) orientations. This trend is inversely correlated to the relative areas of grain boundaries and directly correlated to the relative grain boundary energy. The direct correlation between the energy and curvature is consistent with the expected behavior of grain boundaries made up of singular orientations. Furthermore, because both the relative energy and curvature of grain boundaries with (100) orientations are minima in the distributions, these boundaries also have the lowest driving force for migration.

The specific grain-boundary electrical resistivity of Ni

ABSTRACTThe purpose of the present study was to provide a reliable value for the specific grain-boundary resistivity ρSGBR of Ni metal. New results are presented on the room-temperature electrical resistivity of nanocrystalline (nc) Ni metal samples produced by electrodeposition with various grain sizes. These resistivity data were compared with previous reports on nc-Ni and all results were analysed according to the procedure of Andrews [Phys. Lett. 19, 558 (1965)] who found that the resistivity increment due to grain boundaries is proportional to the grain-boundary surface area per unit volume which is, on the other hand, inversely proportional to the grain size. It is pointed out that the grain size directly accessible by transmission electron microscopy imaging is the relevant parameter for the evaluation of ρSGBR whereas the crystallite size deduced from X-ray diffraction line broadening leads to an underestimation of ρSGBR because coherency-breaking intragrain defects not contributing significantly to the resistivity also cause a line broadening. From the evaluation of the nc-Ni resistivity data at room temperature, we find that 4.45·10−16 Ω·m2 < ρSGBR(Ni) < 6.2·10−16 Ω·m2 and our upper bound agrees exactly with the most recent calculated value in the literature.

In situ backscattered electron imaging study of the effect of annealing on the deformation behaviors of Ni electroformed from additive-free and saccharin-containing sulfamate solutions

The Ni samples were electroformed from additive-free (AF) and saccharin-containing (SC) sulfamate solutions, respectively. In situ backscattered electron (BSE) imaging, electron backscatter diffraction (EBSD), and electron-probe microanalysis (EPMA) were used to investigate the effect of annealing on the deformation behaviors of the AF and SC samples. The results indicate that columnar grains of the as-deposited AF sample had an approximated average width of 3 μm and an approximated aspect ratio of 8. The average width of columnar grains of the as-deposited SC sample was reduced to approximately 400 nm by the addition of saccharin to the electrolyte. A few very-large grains distributed in the matrix of the SC sample after annealing. No direct evidence indicated that S segregated at the grain boundaries before or after annealing. The average value of the total elongations of the SC samples decreased from 16% to 6% after annealing, whereas that of the AF samples increased from 18% to 50%. The dislocation recovery in grain-boundary areas of the annealed AF sample was reduced, which contributed to the appearance of microvoids at the triple junctions. The incompatibility deformation between very-large grains and fine grains contributed to the brittle fracture behavior of the annealed SC Ni.

Grain Boundary Complexions and Phase Transformations in Al- and Cu-Based Alloys

High-pressure torsion has been used to obtain the ultra-fine grained (UFG) state with a high specific area of grain boundaries (GBs) in Al-Zn, Al-Mg, Cu-Ag, Cu-Co, and Cu-Ni solid solutions with face-centered cubic (fcc) lattices. The UFG samples were heated in a differential scanning calorimeter (DSC). Small endothermic peaks in the DSC curves were observed in the one-phase solid-solution area of the respective phase diagrams, i.e., far away from the bulk solidus and solvus lines. A possible explanation of these endothermic peaks is based on the hypothesis of phase transformations between GB complexions. This hypothesis has been supported by observations with transmission electron microscopy and electron backscattering diffraction. The new lines of GB phase transformations have been constructed in the Al-Zn, Al-Mg, Cu-Ag, Cu-Co, and Cu-Ni bulk phase diagrams.

Significant improvement in reversibility of MWCNT-Sn compound composite electrode: Nanostructure effect of MWCNT-Sn compound composite on high initial reversible capacity

Using a simple wet method, a composite anode in which Sn, SnO, and SnO2 were inserted into a multiwall carbon nanotube (MWCNT), was fabricated. The synthesized composites had Sn, SnO, and SnO2 compounds in the MWCNT only, with these compounds not present outside the MWCNT. This is because the precursors penetrate the MWCNT through channel-like defects (CLDs) formed on the MWCNT walls by acid treatment; the Sn-compounds are formed from these precursors by post-annealing. The synthesized MWCNT-Sn compound composite as an anode showed high capacity retention efficiency during 85 charge and discharge cycles. The initial average discharge capacity after 5 cycles, and the average discharge capacity in the 75th - 85th cycles were 743 mAh/g and 660 mAh/g, respectively. The initial Coulombic efficiency was particularly high, reaching ∼96.3%. The dramatic improvement in the initial capacity retention characteristics, as well as the capacity retention during these subsequent cycles is attributed to four effects: (1) reduction of irreversible Li2O phase formation during charging due to low oxidation states of Sn such as zero and monovalent states; (2) easy reduction of the Li2O phase during discharge by a large SnLi2O grain-boundary area formed by small Sn compound particles; (3) suppression of the Sn compounds’ particle destruction due to the wrapping effect of the MWCNT; and (4) suppression of the increase in interfacial resistance between the current collector and the negative electrode slurry. These results provide a meaningful insight into the rational design of composites in which Sn, SnO, and SnO2 nanoparticles exist in the MWCNT only. This is significant because MWCNT is one of the anode materials for power supplies that can be used in various environments in which the application of metal Li is difficult.

Effect of preheating on the microstructure and properties of fiber laser welded girth joint of thin-walled nanostructured Mo alloy

Fiber laser girth welding of a thin-walled nanostructured Mo (NS-Mo) alloy tube was conducted. The microstructures, properties, and residual stresses of the welded girth joints achieved at different preheating temperatures were compared. Combining finite element simulation with experimental data, it was found that as the preheating temperature increased, the maximum welding residual tensile stress monotonically decreased while the tensile strength of the joints increased at first and then declined. At the preheating temperature of 673 K, the tensile strength reached a maximum, which was approximately 50% that of the base metal. The results showed that oxygen content was segregated at the grain boundary of fusion zone during welding and further produced Mo oxides. As the preheating temperature increased, vaporized Mo oxides might escaped from the molten pool, so the oxygen concentration at the grain boundary first decreased. However, when the preheating temperature increased over 773 K, the oxygen concentration rose due to the increased oxidation tendency of Mo at high temperatures and grains in the fusion zone coarsened greatly, while the total area of grain boundaries reduced.

Tension-compression asymmetry in plasticity of nanotwinned 3C-SiC nanocrystals

Encompassing nanoscale thin twins in metals induces diverse influences, either strengthening triggered by the lattice dislocation blockage effects or softening prompted by dislocation emission from coherent twin boundary (CTB)/grain boundary (GB) intersections as well as CTB migration; yet the deformation mechanism remains poorly understood in ceramic nanostructures possessing covalent bonds. Here, we report the results of uniaxial compressive and tensile stress loading of twin-free and nanotwinned nanocrystalline cubic silicon carbide (3C-SiC) ceramic attained by large-scale molecular dynamics simulations. We find a strong and unique tension-compression asymmetry in strength of nanocrystalline ceramics, much higher than that of metals. We demystify that strength and ductility behaviour do not correlate simply with the amount of dislocation density, voids, intergranular disordered phase, and total strain energy; instead, it arises from a complex interplay of the aforementioned features and structural characteristics, e.g., GB and triple junction area distribution along/normal to the direction of straining as well as the capability of strain accommodation by the GBs and CTBs, with the dominant role of the structural characteristics in nanotwinned samples. Our results also reveal that primarily intergranular crack propagation and fracture along the GBs transpires, and not along the CTBs, resulting from the high energy of the GBs. However, a high density of nanoscale twins in the 3C-SiC nanocrystals could give rise to the alternation of the crack path from intergranular to intragranular type induced by shear, which brings about the glide of Shockley partials along the CTBs and subsequent formation of CTB steps and twin plane migration.

Chemical Nanoanalyses at Grain Boundaries By Joint Use of Scanning Transmission Electron Microscopy and Atom Probe Tomography

Polycrystalline materials with grain boundaries (GBs), involving excess free energy because of their structural imperfection, can reduce their energy by the nanoscopic structural changes of the GBs via impurity segregation. Those local changes at GBs can stabilize non-equilibrium nanostructures, resulting in the drastic change in the macroscopic properties of those materials. The mechanism of GB segregation is, however, far from being understood due to difficulties in characterizing both crystallographic and chemical properties of the same GB at atomistic levels. We have therefore developed an analytical method to determine the impurity segregation ability on the same GB at the same nanoscopic location by a joint use of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) combined with ab-initio calculations, and discussed the segregation mechanism in terms of bond distortions at the GB. In the present work, we examined the GBs in Si ingots used for solar cells. They have serious impacts on the solar cell efficiency via the segregation of detrimental impurity atoms, such as oxygen and transition metals introduced inevitably during crystal growth and cell processing, depending on their structural condition at those GBs. Accordingly, precise understanding of the segregation mechanism of impurity atoms is one important issue to produce cost-effective solar cells by engineering the structural condition of impurity atoms segregating at GBs. Three-dimensional distribution of impurity atoms was systematically determined at the typical large-angle GBs (i.e., Σ3{111}, Σ5{013}, Σ9{221}, Σ9{114}, Σ9{111}/{115}, and Σ27{552}) [1-3] and small-angle GBs [3-5] by APT with a low impurity detection limit (0.01 at.% on a GB plane) simultaneously with a high spatial resolution (about 0.4 nm), and it was correlated with the atomic stresses around the GBs estimated by ab-initio calculations based on atomic-resolution scanning TEM data (for large-angle GBs [2]) and by calculations with the elastic theory based on dark-field TEM data (for small-angle GBs [4]). It was hypothesized that oxygen atoms segregate at the bond-centered positions under tensile stresses above about 2 GPa so as to attain more stable bonding network by reducing the local stresses [6]. The number of segregating atoms per unit GB area (N GB ) is in proportion to both the number of the stressed positions per unit GB area (n bc) and the average concentration of oxygen atoms around the GB ([Oi]) with N GB ~ 50n bc[Oi]. This indicates that the probability of oxygen atoms at the segregation positions would be, on average, fifty times larger than in bond-centered positions in defect-free regions. This nanoscopic finding may provide a general guidance to control compositions and band structures at GBs via oxygen segregation.

The investigation of strength and plasticity mechanism of low-temperature annealed ultrafine grained stainless steel

The properties of steel material can be greatly improved when its grain size becomes finer, so grain refinement is the most commonly used to improve the strength, ductility and impact on the toughness of steel at the same time. Compared to ordinary fine-grained steel, the average grain size of ultra-fine grained steel is less than 2 μm and it has different strength and plasticity mechanism. In this paper, effect of low-temperature annealing on the microstructure and properties of ultra-fine grained stainless steel in internal friction was studied by utilizing transmission electron microscope (TEM) and universal tensile testing machine. At the same time, the strength and plasticity mechanism of ultra-fine grained stainless steel was revealed by internal friction experiments. The results show that with the increase of annealing time and annealing temperature, both the tensile strength and yield strength decreases, while the elongation increases continuously. The mechanical properties of tested steel are more sensitive to the temperature. In the internal friction curve of tested steel, the peak of P1 which result from the interaction between solid solution atoms and dislocations increases first and then decreases, while the peak of P2 which result from the interaction between dislocation and carbide precipitates decreases first and then increases. The peak of P3 which result from the structure, area and movement of grain boundary decreases continuously. The strength and plasticity mechanism of tested steel at room temperature is the result of interaction between the grain boundary and dislocation.

Optical Properties of Multilayered Sol–Gel Zinc-Oxide Films

Study of structural, optical and photocatalytic properties of multilayered (1–8 layers) zinc oxide films deposited on glass substrates by sol-gel method showed, that after thermal treatment at 500°C they consist of random oriented hexagonal crystalline grains with size of 34–40 nm, forming larger particles with sizes of 100–150 nm, which do not depend on number of layers. With an increase in the number of layers, the intensity of exciton photoluminescence decreases by a factor of 10, the absorption of light in the visible and near IR ranges increases, and the efficiency of photocatalytic decomposition of the test organic dye rhodamine B increases by 10–12%. The observed changes are related to the increase in the total area of grain boundaries and to the change in the integral amount of oxygen vacancies and interstitial atoms as the number of layers increases, which makes it possible to control the properties of zinc oxide films for applications in optoelectronics, photovoltaics and photocatalysis.

Preferential Pt Nanocluster Seeding at Grain Boundary Dislocations in Polycrystalline Monolayer MoS2.

We show that Pt nanoclusters preferentially nucleate along the grain boundaries (GBs) in polycrystalline MoS2 monolayer films, with dislocations acting as the seed site. Atomic resolution studies by aberration-corrected annular dark-field scanning transmission electron microscopy reveal periodic spacing of Pt nanoclusters with dependence on GB tilt angles and random spacings for the antiphase boundaries ( i.e., 60°). Individual Pt atoms are imaged within the dislocation core sections of the GB region, with various positions observed, including both the substitutional sites of Mo and the hollow center of the octahedral ring. The evolution from single atoms or small few atom clusters to nanosized particles of Pt is examined at the atomic level to gain a deep understanding of the pathways of Pt seed nucleation and growth at the GB. Density functional theory calculations confirm the energetic advantage of trapping Pt at dislocations on both the antiphase boundary and the small-angle GB rather than on the pristine lattice. The selective decoration of GBs by Pt nanoparticles also has a beneficial use to easily identify GB areas during microscopic-scale observations and track long-range nanoscale variances of GBs with spatial detail not easy to achieve using other methods. We show that GBs have nanoscale meandering across micron-scale distances with no strong preference for specific lattice directions across macroscopic ranges.