Boundary Structure Research Articles

Mechanistic understanding of enhanced thermal stability of twinned copper nanowires

Thermal instability problems of nanostructured materials, including grain coarsening and recrystallization, can be alleviated by introducing nanoscale coherent twin boundary (CTB) structures. However, the detailed interaction mechanism between CTB and grain boundaries (GBs) during microstructural transformation has not been well elucidated. To gain insight, we investigate the migration behavior of CTB-intercepted GBs in twinned copper nanowires at elevated temperatures through transmission electron microscopic observations and molecular dynamics simulations. The results show that numerous partial dislocations are conveyed to the CTB-intercepted GB through the densely distributed CTBs and further transferred to the nanowire surface along the GB. The growth of the twinned region along the diminishing GB is driven by minimizing the excess energy stored in dislocation defects and GBs. This CTB-mediated GB migration in twinned copper nanowires can be deactivated by surface passivation. This study will help to elucidate how the thermal stability of nanostructured metals is influenced by the twin structure and surface passivation.

Tailoring deformation-induced martensitic transformation through cellular engineering in laser powder bed fusion processed 316L stainless steel

The microstructural effect of 316L stainless steel (SS) fabricated by laser powder bed fusion on deformation-induced martensitic transformation (DIMT) was investigated. Both opposing effects, i.e., disturbing/promoting DIMT were observed. While the cellular structure and low-angle grain boundaries reduce the DIMT rate by disturbing shear band formation, the high dislocation density can activate direct BCC martensite nucleation and accelerate the DIMT. This observation suggests that controlling the two competing effects is the key to cellular engineering using additive manufacturing.

Lithium stabilization of amorphous ZrO2

Small modular reactors (SMRs) are a key option to aid the worldwide net zero targets for carbon emissions. Some pressurised water reactors aim to operate with a boron-free coolant chemistry for simplification in plant design. In the absence of boron, Li has been found to accelerate the corrosion of the zirconium-based alloy fuel cladding under certain conditions and concentrations within pressurised water reactors (PWRs). The cause of the accelerated corrosion has yet to be identified. This work identifies the potential for amorphous ZrO2 phase stabilization by lithium that will potentially have an impact on the passive oxide's grain boundary structure. Higher lithium concentrations are found to stabilise the amorphous phase's stability before re-crystallisation to higher temperatures. Chemical assessment has also shown the lithium phase to be soluble in water, indicating a potential mechanism for lithium to alter grain boundaries and increase the pathways for further oxidation of the underlying zirconium material.

Compression Eliminates Charge Traps by Stabilizing Perovskite Grain Boundary Structures: An Ab Initio Analysis with Machine Learning Force Field.

Grain boundaries (GBs) play an important role in determining the optoelectronic properties of perovskites, requiring an atomistic understanding of the underlying mechanisms. Strain engineering has recently been employed in perovskite solar cells, providing a novel perspective on the role of perovskite GBs. Here, we theoretically investigate the impact of axial strain on the geometric and electronic properties of a common CsPbBr3 GB. We develop a machine learning force field and perform ab initio calculations to analyze the behavior of GB models with different axial strains on a nanosecond time scale. Our results demonstrate that compressing the GB efficiently suppresses structural fluctuations and eliminates trap states originating from large-scale distortions. The GB becomes more amorphous under compressive strain, which makes the relationship between the electronic structure and axial strain nonmonotonic. These results can help clarify the conflicts in perovskite GB experiments.

ПОЛУЧЕНИЕ И ИССЛЕДОВАНИЕ ПОЛИМЕРНЫХ НАНОЧАСТИЦ

Currently, there is an increasing interest in the study of polymers in a nanodisperse state. Since polymer materials, as a rule, are heterogeneous systems with a sufficiently variable surface, a correct assessment of their chemical structure, physical properties and morphology is of great scientific and practical importance. Polymer nanoparticles (PNPs) have been successfully studied for a long time as nanoscale colloidal particles obtained from polymers. Research on HDPE is most intensively conducted in the field of nanomedicine, and the possible application of HDPE in the field of optoelectronics, photonics, gadgets, sensors, sensors, and new nanocomposites is of great interest to researchers. An independent task is to compare the properties of the HDPE with the properties of a bulk polymer, including the features of its supramolecular structure. Previous studies of the supramolecular structure of PDFs have suggested that charge transfer in the polymer during resistive switching occurs along the grids of intergranular boundaries. This paper presents the results of an experimental study of polymer nanoparticles on different substrates by atomic force microscopy. The main metrological parameters of polymer nanoparticles were determined, the results were compared with the available data on the supramolecular structure of polymer films. The possibility of using atomic force microscopy techniques to visualize polymer nanoparticles on dif- ferent substrates has been demonstrated. Using the AFM method with a conductive probe, it was found that in some cases, during the aggregation of nanoparticles on a metal substrate, charge transfer is realized along separate nanometer-sized sections. The conducting channels in the current image have the form of separate points with a height corresponding to the locally flowing current. It was found that the arrangement of the observed channels correlates well with the model of conductivity along the grain boundaries of the supramolecular structure of the polymer. The issues of interpretation of current images of polymer nanoparticles and the physical model of charge transfer in polymer nanostructures are discussed. The work was carried out with the support of the Mirror Labora- tories project of the National Research University Higher School of Economics and an intra-university grant from the Bashkir State Pedagogical University named after M. Akmulla.

Race-conscious professional teaching standards: Where do the states stand?

Education policymakers have long sought to reduce persistent achievement disparities between students of color and White students with varying levels of success. Understanding the different needs and obstacles faced by students and families of color is important given educating all individuals for our future U.S. society is a priority. Educational policy should reflect the assumption that race matters and continues to impact educational opportunity. This paper argues that race-conscious professional teaching standards could extend the structural boundaries of teacher practice when working with racially, culturally, and linguistically diverse students. Using discourse analysis to analyze the deeper meanings of selected states’ teaching standards in different sociopolitical contexts, this paper describes the challenges and opportunities for infusing race-conscious perspectives in teaching standards. Implications for how states’ teaching policy language actively creates and builds teaching and learning environments are discussed.

Structure of singularities in the nonlinear nerve conduction problem

We give a characterization of the singular points of the free boundary \partial \{u>0\} for viscosity solutions of the nonlinear equation F(D^{2} u)=-\chi_{\{u>0\}}, where F is a fully nonlinear elliptic operator and \chi is the characteristic function. This equation models the propagation of a nerve impulse along an axon. We analyze the structure of the free boundary \partial\{u>0\} near the singular points where u and \nabla u vanish simultaneously. Our method uses the stratification approach developed in Dipierro and the author’s 2018 paper. In particular, when n=2 we show that near a flat singular free boundary point, \partial\{u>0\} is a union of four C^{1} arcs tangential to a pair of crossing lines.

Spatial 3D correlation of flux pinning with porosity distribution in YBa2Cu3O7−δ using tensorial neutron tomography

Engineering devices from High Temperature Superconductors (HTS) for practical applications such as in transport and medical imaging requires an understanding of their critical current density (JC) distribution and how the material properties affect this. JC describes the maximum gradient of flux that can be found in a superconductor sustained by vortex pinning which is the preferential positioning of vortex in defective regions of weaker superconductivity. Local correlation of imperfections with high pinning has required destructive methods such as slicing then scanning with local magnetic probes. We describe the first case of non-destructive spatial correlation of flux pinning and consequently JC with bulk imperfections at different temperatures in top-seeded melt grown (TSMG) YBa2Cu3O7−δ chosen for its high pinning in addition to a rich structure of pores, twins and grain boundaries. This is facilitated by a combination of polarized neutron tomography to image trapped magnetic fields in a range around the material critical temperature and conventional neutron tomography to characterize potential pinning objects. The results indicate that there is indeed preferential trapping in the porous regions independent of the resolvable pore size. Polarized optical microscopy data suggests that the observed phenomenon is attributable to twin boundaries and crack defects located at the pore interface.

Microstructure and mechanical properties of Al-Zn-Mg-Cu alloy joints welded by ultrasonic spot welding with soft specification

The high-strength lightweight Al-Zn-Mg-Cu alloy sheets were welded using ultrasonic spot welding with soft specification. The peak tensile shear failure load reached 7.3 kN, which satisfies the requirements of AWS D17.2 for spot-welded specimens. This is due to the fact that the material undergoes sufficient plastic deformation during the welding process with soft specification, thereby improving metallurgical bonding. Moreover, the dynamic recrystallization resulting from plastic deformation makes the grain size of the interface significantly refined. The grains with a size of 10 μm in the base metal are transformed into recrystallized grains with a size of 2 μm at most, which is in favor of improving the mechanical properties of the joint. Finally, even under soft specification conditions, excessive welding time can lead to macroscopic damage to the joint, initiation of cracks, and destruction of the grain boundary structure.

Stoichiometry of ZnO polycrystalline layers. Localization of vacancies

Nonstoichiometric ZnO1-x phases with oxygen deficiency at grain boundaries (GBs) in polycrystalline ZnO layers are a typical damaged layer formed during the synthesis or subsequent storage. In this case, the processes of adsorption-desorption of gases are the basic phenomenon used in the analysis of gas composition. MGB layers also play an important role in creating transparent electrodes for transparent electronics and optoelectronics devices. This makes it relevant to study the generalization of research data on non-stechiometric structures in various applications. Purpose: the research carried out is devoted to the analysis of the formation processes of non-stoichiometric ZnO1-x surface phases in gas sensors, transparent electrodes, ceramic targets for magnetron sputtering, and in light-emitting structures. Research is aimed at finding ways to control and optimizing the parameters of non-stoichiometric ZnO1-x phases. The structural features, electrical, optical, and emissive properties of the layers were studied depending on the stoichiometry of the material. The issues of transformation of the structure of grain boundaries in ZnO-based polycrystalline layers depending on the formation conditions and external influences are considered. The relationship between the mechanisms of carrier transport and the processes of formation of nonstoichiometric phases on the MG has been studied. The dependences of the luminescence spectra of ZnO layers on the density of oxygen vacancies were traced. The regularities of the formation of energy structures of magnetic grains in ZnO layers are considered. Establishment of patterns of formation and characteristics of non-stoichiometric phases on the MGB during the formation of various modifications of transparent electrodes. Study of ways to optimize the parameters of surface non-stoichiometric layers.

Model manifold of transmission loss from a Pekeris waveguide using VGS parameters

In geoacoustic inversion, selecting an appropriate seabed parametrization, especially with an unknown number of sediment layers, is a challenge that is compounded by potential bias when establishing bounds in the parameter search space. One approach to addressing these issues is rooted in the techniques of Information Geometry. Information Geometry informs model selection and parameterization by quantifying which model parameters are informed by observational data. This paper provides an information geometric analysis of the Pekeris waveguide, where the acoustic properties of the lower half-space are derived from the viscous grain-shearing (VGS) model. Specifically, we consider single frequency transmission loss (TL) across a wide range of VGS parameters. By exploring the limits and boundaries of the geometric manifolds, particularly as parameters approach both low and high extremes, this approach provides indications of parameter hierarchies and correlations. Results will include slices of the model manifold and an evaluation of the boundary structure, providing insight into the relative impact of VGS parameters and the delineation of limiting regions. In doing so, this paper seeks to inform model selection and parameterization in geoacoustic inversion studies. [Work supported by the Office of Naval Research. Grant N00014-21-S-B001.]

Magnetochiral tunneling in paramagnetic Co1/3NbS2

Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co1/3NbS2, the magnetization induced by the tunneling current is shown to become detectable by its coupling to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up an avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals.

An approach to improve transferability of adversarial examples

Adversarial examples (AE) have become a huge threat to the security and robustness of deep neural networks (DNN). The transferability of AE reveals the vulnerability of DNN even in a black-box situation. Due to the different structure and decision boundary of DNN models, the transferability of an AE generated on one model is limited to deceive another model with a different representation ability. To this end, we propose a dynamic ensemble attack (DEA), which generates AE on several models with an Elastic Soft Attention Layer (ESAL) to boost the transferability of AE. The EASL allocates the weights of models according to the distance from the benign image to the perturbed image in the feature space and the predicted probability of the models. Compared to the ensemble attack with equal weights, DEA breaks structural of the picture and expands the distance between AE and the benign example in the feature space. The experimental results on two benchmark datasets validate that DEA has superior transferability to the traditional ensemble attack. DEA could effectively improve the black-box attack success rate. We also conduct a visual analysis of the attacking effect of AE during generation and validate the advantage of DEA.

A Fourier neural operator-based lightweight machine learning framework for topology optimization

The combination of machine learning and topology optimization has preliminarily mitigated the enormous computational costs incurred during optimization processes. Its powerful design capabilities are warmly welcomed by many scholars, but this strategy still faces many problems, such as unclear structural boundaries and high dependence on big data. Here, we propose a new machine learning topology optimization paradigm that learns the mapping relationship between the initial and optimized Level set functions in the infinite-dimensional function space under the Fourier neural operator to truly parameterize the structural model and its mechanical information, thereby achieving purely data-driven topology optimization. A base model trained under the proposed approach could obtain a topological structure with clear and smooth structural boundaries within 0.4 seconds, making it faster than traditional methods by tens of times. In addition, a lightweight retraining strategy under the base model was proposed. Four additional numerical examples were used to demonstrate that this strategy requires only 100 or 200 new samples and 3-6 seconds of retraining time to attain prediction capabilities under different boundary conditions, different meshes (resolution), various initial level set functions, and multiple loads. In addition, we discussed all the experiments and found that most of the mechanical properties of the results were close to the ground truths, while some results were even better than those of the traditional method. Although we only used the maximum structural stiffness as an example, this method can be extended to different objective functions.

Dislocation descriptors of low and high angle grain boundaries with convolutional neural networks

Multiscale modeling of plasticity in polycrystalline metals is a long-standing challenge in part because of the lack of an accepted grain boundary descriptor, which has hindered the bridging of scales between atomistic simulations and meso-scale discrete dislocation dynamics (DDD) models. While grain boundary dislocations (GBDs) for low angle grain boundaries can be ascertained by Burgers circuit analyses, the dislocation structures of high angle grain boundaries have remained elusive because of overlapping dislocation core fields. Here, we use convolutional neural networks (CNNs) to establish the locations of GBDs responsible for the misorientations of <001> symmetrical-tilt Cu grain boundaries, from the local atomistic stress fields modeled with molecular dynamics (MD) simulations. We achieve accurate CNN predictions of GBDs with sub-angstrom resolution across an extensive set of low- to high-tilt-angle grain boundaries through a training dataset, generated from superposing continuum-representations of the MD stress fields of single dislocations that consider the contributions of both the Volterra and dislocation core fields. The approach paves the way for dislocation representations of the atomistic grain boundary structures modeled by MD simulations or density functional theory (DFT) calculations in mesoscale DDD models to elucidate multiscale plasticity effects.

Structure and Charge Carrier Separation Promotion Effects of Antiphase Boundaries in Cesium Lead Bromide.

Defects in lead halide perovskites (LHPs) may have a significant impact on charge carrier separation, but the roles of the defects are not fully understood. Here, using aberration-corrected scanning transmission electron microscopy (STEM), different types of antiphase boundaries (APBs) are discovered in CsPbBr3 platelets. APBs with a displacement vector of 1/4[111] are characterized by double layers of CsBr layers at the (110) or (001) planes, while APBs at the (112) planes are formed through edge sharing of PbBr6 ̵octahedra. Significant lattice distortions are determined at (001) and (110) APBs on the basis of quantitative analyses of STEM images. Density functional theory calculations demonstrate that all three types of APBs can induce band offsets at their valence bands and conduction bands. The APBs are intended to promote the separation of photogenerated charge carriers in LHPs. These findings provide a crystal engineering technique for enhancing the optoelectronic properties of LHPs by controlling defects.

Interplay between high-temperature creep deformation behavior and nanoscale precipitate evolution in a newly designed Fe–Ni-based superalloy

In an endeavor to achieve the critical goals of carbon neutrality, an explorative study was conducted to scrutinize the precipitate evolution and high-temperature creep behavior of an innovative and cost-effective Fe–Ni-based superalloy that was meticulously engineered for the next-generation ultra-supercritical (USC) power plants. The creep experiments conducted at temperatures and stresses of 650 °C/280 MPa (17,179 h), 675 °C/250 MPa (10,271 h), and 700 °C/250 MPa (2378 h), coupled with proficient data fitting were employed to accurately assess the long-term service performance of the superalloy under real-world operational environments. Though it can withstand theoretically an intense stress of 130 MPa at 700 °C for approximately 30 years as calculated by the Larson-Miller equation, its performance was not without a degree of perceived vulnerability. This was primarily traced to the conspicuous coarsening behavior of the precipitate-free zones (PFZs)/discontinuous coarsening zones (DCZs) formed in the grain boundaries (GBs) on a longer time scale. It had a significant diminishing impact on the overall strength of the GBs, resulting in a recorded creep life that was considerably shorter than initially projected. Moreover, a synergistic mechanism between PFZs/DCZs with TiC carbides was advanced to interpret the intergranular fracturing dynamic, which was postulated due to the evidence gathered from electron backscatter diffraction (EBSD), highlighting that the banded distribution of TiC carbides circumambient to GBs engendered considerable stress aggregation. Conversely, the plentiful supply of coincidence site lattice (CSL) boundaries disrupted the continuous network structure of random high-angle grain boundaries (HAGBs), thereby creating an effective barricade against the proliferation of cracks. These empirical findings paved the way for the proposition of an enhanced blueprint and the industrial utilization of such.

Late Triassic to Early Jurassic anatectic magmatism in the Chiapas Massif Complex, southern Mexico: New evidence for the onset and mode of crustal extension in western equatorial Pangea

During the Triassic, continental breakup started in western equatorial Pangea, while active continental margins developed along North and South America’s Pacific borders. Besides many uncertainties about how peri-Gondwanan terrains of Mexico evolved during and after Pangea’s amalgamation, the apparent absence of Late Triassic magmatism has been an enigmatic feature of Mexico’s geology. Here, we present the first documented occurrences of Late Triassic igneous rocks and a coeval high-grade metamorphic event in southern Mexico, as well as evidence for subsequent Early Jurassic ductile deformation and magmatism. Samples come from the Chiapas Massif Complex (CMC), a crystalline basement of the southern Maya Block. Field evidence suggests that garnet-bearing leucogranitic stocks intruded a former structural boundary between different Precambrian-Paleozoic basement units. High-precision Sm-Nd garnet-whole-rock ages (ID-TIMS partial dissolution techniques) and U-Pb geochronology on zircon rims (LA-ICPMS) revealed Norian igneous crystallization ages for the leucogranites (∼221–213 Ma). Isotope and geochemical data indicate that these granites originated from crustal reworking by anatexis. A garnet-bearing leucosome from a migmatite yielded a slightly younger Sm-Nd age (∼204 Ma), indicating cooling soon after the peak conditions of the anatectic event. Additionally, monazite from a mylonitized leucogranite yielded an Early Jurassic U-Pb age (∼179 Ma), indicating ductile deformation. Nearly coeval zircon ages from an undeformed pink granite and a nearby volcaniclastic sequence (∼184–189 Ma) further imply contrasting erosion levels during the Early Jurassic. The new data support models for regional stretching of eastern and southern Mexico, where crustal extension during the gravitational collapse of a collisional orogen in paleo-northern Mexico led to continental rifting and the breakup of Pangea. In the CMC, crustal extension is reflected by Norian anatectic crustal melts and migmatites, as well as Early Jurassic plutonic and extrusive magmatism that are localized in certain fault-bounded blocks with different erosion levels compared to adjacent blocks.

Effects of scanning speed on the microstructure, hardness and corrosion properties of high-speed laser cladding Fe-based stainless coatings

Fe-based coatings have been prepared by utilizing laser cladding with the scanning speed ranging from 1.57 to 20.4 m/min. The microstructure and corrosion properties have been further investigated to evaluate the effects of scanning speed. The results show that, both coating's thickness and dilution rate gradually decrease with the increase of scanning speed. However, the deposition efficiency on the surface can be greatly improved by around 4 times. Under high scanning speed, the Fe-based coatings show the similar microstructure feature, comprising with dendrites and inter-dendritic structures. But the coating's microstructure can be significantly refined with the secondary dendritic arm spacing less than 1 μm. Furthermore, the gap of Cr concentration between the two structures can be effectively reduced, which shrinks the difference in corrosion resistance. On the other hand, the Fe-based coating's hardness gradually decreases resulting from the retained austenite and ferrite under high scanning speed. Compared to Q235 steel, the cladded Fe-based coatings show superior corrosion resistance with much higher corrosion potential and lower corrosion current density. Pitting dominates the corrosion forms of high-speed laser Fe-based coatings, which initially takes place on the boundaries of different structures. With the increase of scanning speed, the coating's corrosion resistance increases firstly and then decreases, and the coating cladded with the speed of 15.7 m/min shows the best corrosion resistance. Higher average Cr content and more uniform Cr distribution account for the better corrosion resistance. However, excessive microstructure refinement plays a negative role in the coating's corrosion resistance owing to more grain boundaries.

A metamaterial-assisted coda wave interferometry method with nonlinear guided waves for local incipient damage monitoring in complex structures

Nonlinear guided waves exhibit high sensitivity to material microstructural changes, thus attracting increasing attention for incipient damage monitoring applications. However, conventional nonlinear guided-wave-based methods suffer from two major deficiencies which hinder their applications: (1) mostly relying on the first arrivals of wave signals, they apply to limited inspection areas in simple structures in order to avoid wave reflections from structural discontinuities or boundaries; (2) they are prone to numerous deceptive nonlinear sources in the measurement system which might overwhelm damage-induced signal components. To tackle these challenges, we propose a metamaterial-assisted coda wave interferometry (CWI) method using second harmonic Lamb waves, applicable to the monitoring of local incipient damage in complex structures. Embracing the metamaterial concept, a so-called meta-screen is designed, whose geometry and layout can be flexibly tailored to target specific inspection zones in a structure. Capitalizing on its customized bandgap features, the proposed meta-screen allows for the passing of fundamental waves while preventing the second harmonic components generated by deceptive nonlinear sources from penetrating into the inspection area. Through numerical analyses on a plate with a rib stiffener, the efficacy of the meta-screen and the influence of occasional disturbance and regular pollution are evaluated. Experimental validations on an adhesive structure also confirm the superior sensitivity of the nonlinear coda waves to incipient damage, which is further enhanced by the deployment of the meta-screen alongside improved robustness against deceptive nonlinear sources outside the inspection area. The proposed metamaterial-assisted CWI method with second harmonic Lamb waves holds great promise for local incipient damage monitoring of complex structures.