Structural Unit Model Research Articles

Linkage Effect Induced by Hierarchical Architecture in Magnetic MXene-based Microwave Absorber.

Hierarchical architecture engineering is desirable in integrating the physical-chemical behaviors and macroscopic properties of materials, which present great potential for developing multifunctional microwave absorption materials. However, the intrinsic mechanisms and correlation conditions among cellular units have not been revealed, which are insufficient to maximize the fusion of superior microwave absorption (MA) and derived multifunctionality. Herein, based on three models (disordered structure, porous structure, lamellar structure) of structural units, a range of MXene-aerogels with variable constructions are fabricated by a top-down ice template method. The aerogel with lamellar structure with a density of only 0.015g cm-3 exhibits the best MA performance (minimum reflection loss: -53.87dB, effective absorption bandwidth:6.84GHz) at a 6wt.% filling ratio, which is preferred over alternative aerogels with variable configurations. This work elucidates the relationship between the hierarchical architecture and the superior MA performance. Further, the MXene/CoNi Composite aerogel with lamellar structure exhibits >90% compression stretch after 1000 cycles, excellent compressive properties, and elasticity, as well as high hydrophobicity and thermal insulation properties, broadening the versatility of MXene-based aerogel applications. In short, through precise microstructure design, this work provides a conceptually novel strategy to realize the integration of electromagnetic stealth, thermal insulation, and load-bearing capability simultaneously.

Small-data-based machine learning interatomic potentials for graphene grain boundaries enabled by structural unit model

Small-data-based machine learning interatomic potentials for graphene grain boundaries enabled by structural unit model

Ab initio phonon transport across grain boundaries in graphene using machine learning based on small dataset

Establishing the structure-property relationship for grain boundaries (GBs) is critical for developing next-generation functional materials but has been severely hampered due to its extremely large configurational space. Atomistic simulations with low computational cost and high predictive power are strongly desirable, but the conventional simulations using empirical interatomic potentials and density functional theory suffer from the lack of predictive power and high computational cost, respectively. A machine learning interatomic potential (MLIP) recently emerged but often requires extensive size of the training dataset, making it a less feasible approach. Here, we demonstrate that an MLIP trained with a rationally designed small training dataset can predict thermal transport across GBs in graphene with ab initio accuracy at an affordable computational cost. We employed a rational approach based on the structural unit model to find a small set of GBs that can represent the entire configurational space and thus can serve as a cost-effective training dataset for the MLIP. Only 5 GBs were found to be enough to represent the entire configurational space of graphene GBs. Using the atomistic Green's function approach and the MLIP, we revealed that the structure-thermal resistance relation in graphene does not follow the common understanding that large dislocation density causes larger thermal resistance. In fact, thermal resistance is nearly independent of dislocation density at room temperature and is higher when the dislocation density is small at sub-room temperature. We explain this intriguing behavior with the buckling near a GB causing a strong scattering of flexural phonon modes. In this paper, we show that a machine learning technique combined with conventional wisdom (e.g., structural unit model) can extend the recent success of ab initio thermal transport simulation, which has been mostly limited to single crystals, to complex yet practically important polycrystals with GBs.

Study on hydrogen-affected interaction between dislocation and grain boundary by MD simulation

Hydrogen-affected dislocation motion and hydrogen-induced intergranular fracture play key roles in hydrogen embrittlement. The quantitative characterization of H-affected interaction between dislocation and grain boundary (GB) is of great significance to understand the underlying physics of hydrogen embrittlement, which is systematically studied here by hybrid Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The structural features of GBs are depicted through the structural unit model (SUM) and grain boundary dislocation model (GBDM). The mechanisms of interactions between dislocation and H-free/H-segregated GB arethoroughly investigated and classified. For the cases without H segregation at GB, dislocation nucleation from GB and dislocation gliding on GB are the fundamental mechanisms governing the dislocation-GB interaction. In contrast, for the cases with H segregation, the dislocation-GB interaction mechanism is changed owing to H-inhibited GB dislocation emission, dislocation transmission across GB and dislocation gliding on GB. Due to dislocation pile-up promoted by H segregation, crack initiation is facilitated at the H-segregated GB. These results can provide essential information not only for understanding H-induced intergranular fracture but also for developing an up-scaled discrete dislocation dynamics (DDD) simulation framework.

Methods for flexibility determination of bolted joints: empirical formula review

In this work, a comparative analysis of existing methods for determining the compliance of mechanical joints such as ‘composite-composite’ and ‘composite-metal’ is performed. Polymer composite materials are widely used in modern aircraft industry. As a result, it is necessary to take into account the characteristics of joints of composite aggregates with composite and metallic ones. Considering the compliance of connections in the global finite element models of structural units of aircraft allows increasing the accuracy of calculations. The use of empirical formulae can significantly reduce time and labour costs in calculating the compliance of bolted connections for use in global finite element models. In this paper we review and analyse the existing empirical dependencies. Calculation of rigidity and compliance of single-shear ‘composite-composite’ and ‘composite-metal’ joints by finite element method for small, medium and large membrane thicknesses is carried out, and the results are compared with the calculations using empirical formulae. As a result of the analysis for medium and small thicknesses it is proposed to determine the value of bolted joint flexibility by the empirical formula Boeing 1, and for large thicknesses of connecting membranes it is proposed to use empirical formula Huth.

3D arrangement of atomic polyhedra in tilt grain boundaries

Polycrystalline materials have been used throughout the recorded history, and their macroscopic properties strongly depend on the defects in their crystalline structures. Unique properties of the grain boundaries (GBs) should result from nearby nano-scale space, leading to the questions regarding the type of atomic polyhedra that pack the GB region as well as the universal set of structural units. It is essential to reveal the underlying mathematical framework for the polyhedral arrangement of GB structures and identify the origin of structure-property relationships in polycrystalline materials, which is a fundamental aspect but important for designing high-performance materials. Herein, 3D atomic structures of [001], [110], and [111] symmetrical tilt GBs are analyzed in the face-centered cubic (fcc) crystals, where the polyhedral units are rigorously defined in contrast to the polygonal arrangement within the 2D structural unit model. It is concluded that the GB region can only be packed by the bulk or GB-type polyhedral units with minor variations. In this study, the 3D arrangement of atomic polyhedra around the GBs is quantitatively determined and systematically described by numerical analysis. Moreover, the GB hierarchy directly follows the distribution of rational numbers that is represented by the modified Farey diagram, which represents the 3D atomic structure of the symmetrical tilt GBs in an fcc crystal. The definition of singular GB is proposed as the GB packed by at the most two types of polyhedral units, and any GB can be packed by the polyhedral units that form the singular GBs.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

Summary Grain boundaries (GBs) determine the properties of polycrystals, and tailoring the GB structure offers a promising method for the discovery and engineering of new materials. However, GB structures are far from well understood because of their structural complexity and limitations of conventional projection imaging methods. Here, we decipher three-dimensional atomic structure and crystallography of GBs in nanometals using atomic-resolution electron tomography. Unlike conventional descriptions, whereby they are either straight or curved planar planes with one-dimensional translational symmetry, we show that the high-angle GBs completely lose translational symmetry due to undulated curvature related to configurations of structural units. Moreover, we directly visualize kinks and jogs at the single-atom scale in dislocation-type GBs and investigate their mobilities. Our findings bring new insights to the conventional wisdom of GBs and show the importance of developing methods to include the non-planar nature of GBs to statistically evaluate the behavior of GBs in modeling studies.

University Structural Units Rankings: Models and Solutions

Formation of strategic management tools in Russian universities, including mechanisms for ranking the activities of structural units is an important task of the innovative development of universities.The article discusses the rating models of structural units operating in Russian universities. Based on the analysis of real practices, the university-wide models for ranking educational and scientific units are distinguished. A complex-structured rating model of all structural divisions of the university is characterized by the example of one of the leading universities. The article highlights the general characteristics of ranking models used by domestic universities. To a large extent, they concentrate around the methods of assessing the activities of organizational units that have developed in the strategic management system. They consider the achievement of indicators of the Universities Effectiveness Monitoring as the target goals to increase their competitiveness.

Relationship between carbon segregation and the carbide precipitation along grain boundary based on the structural unit model

Intragranular embrittlement is key to the weakening of properties of ultra-supercritical applied materials as HR3C heat-resistant steel in service and is considered to be related to the structure of grain boundary and the precipitation of carbides. In this work, the structural unit model was applied to study the structure of grain boundary and carbide formation with molecular dynamics simulation, and then, the precipitation tendency of carbides was analyzed through the structural similarity between grain boundary and carbide by using radial distribution function of the metal atom. At the same time, an electron backscatter diffraction study on serviced and treated HR3C heat-resistant steel was used for experimental exploration. Results show that the structural similarity between grain boundary and carbide depends on the grain boundary angle, wherein the structural similarity between grain boundary and carbide is higher for the grain boundary angle of 20–50°, while it is lower at the grain boundary angle close to 60° or smaller than 20°. The structure of Cr23C6 and Cr7C3 carbides is more similar to the local structure of grain boundary than that of Cr3C2 carbide. The grain boundary angle study on HR3C heat-resistant steel shows that the high content of 20–50° grain boundary indeed increases the precipitation of carbides, which is proposed to reduce the proportion of 20–50° grain boundaries by a proper procedure of cold deformation + heat treatment as 20% + 900 °C/6 h.

Adsorption of different PAM structural units on kaolinite (0 0 1) surface: Density functional theory study

The objectives of this study are to explore the adsorption mechanisms for how the polyacrylamide (PAM) interact with kaolinite (0 0 1) surface. Three structural unit models (P-AM, P-AA, and P-DAC) were developed from the structure of PAM. The adsorption energies were calculated with density functional theory (DFT) to determine the optimal adsorption system of PAM structural units on the kaolinite (0 0 1) surface. From the Mulliken population analysis, electron density difference, electronic density of states, and electron localization function of the optimal adsorption system, the adsorption mechanism of each adsorption system was derived and their differences were understood. The results show that these reactive sites of the three PAM structural units are O atoms. P-AM and P-AA can be adsorbed on the kaolinite (0 0 1) surface by forming hydrogen bonds. The interaction of kaolinite (0 0 1) surface using P-DAC is driven by both hydrogen bonds and electrostatic attraction, electrostatic attraction as a prevailing role is determined simultaneously. The strength of the hydrogen bonds of the P-AM adsorbed on the kaolinite (0 0 1) surface is higher than that of P-DAC and P-AA. However, the order of the adsorption interaction of the PAM structural units adsorbed onto the kaolinite (0 0 1) surface is P-DAC > P-AM > P-AA.

Atomic-scale structural characterization of grain boundaries in epitaxial Ge/Si microcrystals by HAADF-STEM

The atomic structure of grain boundaries in Ge micro-crystals grown on Si pillars for the fabrication of a monolithically integrated X-ray detector was studied by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three different boundaries are found in Ge: Σ3{111} coherent twin boundaries, Σ3{112} incoherent twin boundaries, and Σ9{122} and Σ27{552} grain boundaries. They are described using the structural unit models containing single columns. Remarkably, we find for the first time a Σ3{112} incoherent twin boundary exhibiting two different atomic structures; one symmetric and one asymmetric. Their co-occurrence is explained by the presence of a small step in the boundary plane and the introduction of dislocations. Likewise, the atomic structure of junctions formed by the interaction of twin boundaries which result in Σ9{122} and Σ27{552} grain boundaries is also revealed. Geometrical phase analysis is applied to map the strain fields at two triple junctions and to uncover the position of the dislocations.

Misorientation dependence of atomic structure and energy of symmetric tilt boundaries in magnesium

ABSTRACTTwelve symmetric tilt boundaries in magnesium spanning misorientation range 78.29° ≤ θ ≤ 145.85° are investigated with molecular dynamics simulation at 0 K, using an embedded-atom method. Three favoured boundaries are identified in this range: 78.29°, 116.88° and 145.85°. Boundary structures intermediate between the first two boundaries are predicted by the structural unit model, which cannot be applied, however, to the misorientation range 116.88° < θ < 145.85°. A sharp transition between the structural units of the 116.88° and 145.85° boundaries is observed within the narrow misorientation range 135° < θ < 138° at or close to the energy maximum. The transition is obviously caused by avoidance of long-range compatibility stresses between these units.

Quantifying and connecting atomic and crystallographic grain boundary structure using local environment representation and dimensionality reduction techniques

In this work, the relationship between grain boundary crystallography and grain boundary atomic structure is examined, using [100] - symmetric tilt grain boundaries in nickel. The structural unit model is used as a benchmark to evaluate the atomic structure description capacities of an emerging structural descriptor, the local environment representation, which itself is a refinement of the Smooth Overlap of Atomic Positions (SOAP) descriptor. We show that the local environment representation encodes both the information of the structural unit model and additional information, such as distortion in the structural units and the arrangement of the structural units at the interface. The use of the local environment representation permits the use of a visualization tool known as SPRING to represent structural similarities between grain boundaries. With the SPRING representation, we produce objective evidence of a relationship between crystallography and atomic structure, at least for [100] - symmetric tilt grain boundaries.

Fast Fourier transform-based micromechanics of interfacial line defects in crystalline materials

Spectral methods using Fast Fourier Transform (FFT) algorithms have recently seen a surge in interest in the mechanics community. The present contribution addresses the critical question of determining local mechanical fields using the FFT method in the presence of interfacial defects. Precisely, the present work introduces a numerical approach based on intrinsic discrete Fourier transforms for the simultaneous treatment of material discontinuities arising from the presence of disclinations, i.e., rotational discontinuities, and inhomogeneities. A centered finite difference scheme for differential rules are first used for numerically solving the Poisson-type equations in the Fourier space to get the incompatible elastic fields due to disclinations and dislocations. Second, centered finite differences on a rotated grid are chosen for the computation of the modified Fourier-Green’s operator in the Lippmann–Schwinger–Dyson type equation for heterogeneous media. Elastic fields of disclination dipole distributions interacting with inhomogeneities of varying stiffnesses, grain boundaries seen as DSUM (Disclination Structural Unit Model), grain boundary disconnection defects and phase boundary “terraces” in anisotropic bi-materials are numerically computed as applications of the method.

Grain boundary phases in bcc metals.

We report a computational discovery of novel grain boundary structures and multiple grain boundary phases in elemental body-centered cubic (bcc) metals represented by tungsten, tantalum and molybdenum. While grain boundary structures created by the γ-surface method as a union of two perfect half crystals have been studied extensively, it is known that the method has limitations and does not always predict the correct ground states. Herein, we use a newly developed computational tool, based on evolutionary algorithms, to perform a grand-canonical search of high-angle symmetric tilt and twist boundaries, and we find new ground states and multiple phases that cannot be described using the conventional structural unit model. We use molecular dynamics (MD) simulations to demonstrate that the new structures can coexist at finite temperature in a closed system, confirming that these are examples of different grain boundary phases. The new ground state is confirmed by first-principles calculations.

Correlation of grain boundary extra free volume with vacancy and solute segregation at grain boundaries: a case study for Al

Grain boundary extra free volume (GB EFV) can be considered as fundamental microstructural parameter for polycrystalline or nano-crystalline materials. Here, we present a systematic first principles study on a group of representative symmetric tilt grain boundaries of Al with various EFVs subjected to vacancy formation and Mg segregation. All grain boundaries were constructed using the coincident site lattice (CSL) and the structural unit (SU) models. It was found that the SU model is superior to the CSL in describing FCC-Al GBs, the same as we previously revealed for BCC-Fe. The predicted relation between GB misorientation angle and EFV, and the predicted EFV criteria for a stable GB, both agree with available experimental observations. Vacancy formation and Mg segregation show stronger preference to those GBs with high EFV values, due to the resultant high levels of atomic disorder. These findings not only provide a new, atomistic perspective on the significance of EFV, but also suggest a viable means of predicting GB properties based on direct experimental characterisation of GB EFVs.

A tool for automatic recognition of [110] tilt grain boundaries in zincblende-type crystals

The local atomic structure of [110] tilt grain boundaries (GBs) formed in ∼100 nm-sized GaAs nanocrystals, which crystallize in the non-centrosymmetric zincblende-type structure with face-centred cubic lattice symmetry, was imaged and analysed by means of high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The nanocrystals were grown by metal–organic vapour phase epitaxy on top of (001) Si nanotips embedded in an oxide matrix. This paper introduces an automatic analysis method and corresponding processing tool for the identification of the GBs. The method comprises (i) extraction of crystallographic parameters,i.e.misorientation angles and transformation matrices for the different crystal parts (grains/twins) observed by HAADF-STEM, and (ii) determination of their common plane(s) by modelling all possible intersections of the corresponding three-dimensional reciprocal lattices. The structural unit model is also used to characterize the GB structures and to validate the data obtained by the developed algorithm.

A Description of Preferential Sites for Vacancy Formation on Grain Boundaries in Copper by Structural Unit Model

The preferential sites for vacancies on a series of symmetric tilt grain boundaries in copper have been investigated by molecular dynamics simulation. The regularity of preferential sites for vacancies on these boundaries can be described by the structural unit model. This is essential because of the correspondence between the geometries of the structural units and the local stress field. The vacancies are energetically preferred at the sites with relatively large tensile stress, and these sites are the corner sites of the structural units. Moreover, these preferential sites are mainly related to the structural unit types irrespective of which grain boundary that the structure units locate in. Therefore, the preferential sites for vacancies on various grain boundaries formed by combinations of certain structural units can be readily described and predicted by the structural unit model.

The grain-boundary structural unit model redux

Properties of grain boundaries (GBs) and their underlying structures are key to understanding polycrystalline material phenomena. The most widely used model for GB structure is the structural unit model (SUM), introduced ∼ 50 years ago. The SUM represents GB structure as a combination of structural units (SUs); this combination evolves systematically with GB misorientation. Despite its successes, many observations suggest the SUM does not completely describe the GB structure; its utility for predicting GB properties is limited. There has been a growing realization that, even for fixed misorientation, multiple stable/metastable structures are common (corresponding to different microscopic degrees of freedom). We generalize the SUM by considering the effect of such metastable structures. While the SUM can describe GB structure evolution between a pair of delimiting boundaries, there will be many such evolutionary paths, corresponding to SUs associated with the metastable structure of the delimiting boundaries. The equilibrium GB energy vs. misorientation does not necessarily correspond to one of these paths, but will have contributions from many. Recognizing this, we propose a new approach to predict GB structure and energy, allowing for accurate determination of the GB energy vs. misorientation based on a very small number of atomistic simulations. For example, we predict the GB energy vs. misorientation for [100] and [111] symmetric tilt boundaries in BCC tungsten over the entire misorientation range to a mean error of <2 % based on atomistic simulations at only three or four misorientations. Our approach allows for the trade-off between computational cost and prediction accuracy.

Geodynamic simulation of ore-bearing geological structural units by the example of the Strel’tsovka uranium ore field

Information on designing a 3D integrated model of the deflected mode (DM) of rock massif near the Strel’tsovka uranium ore field (SUOF) in the southeastern Transbaikal region is presented in the paper. This information is based on the contemporary stresses estimated by geostructural and tectonophysical techniques and by studying the seismotectonic deformation of the Earth’s surface using the data on earthquake source mechanisms and GPS geodesy focused on the recognition of active faults. A combination of the results of geostructural, geophysical, geotectonic, and petrophysical research, as well as original maps of faulting and the arrangement of seismic dislocations and seismotectonic regimes (stress tensors), allowed us to design models of the structure, properties, and rheological links of the medium and to determine the boundary conditions for numerical tectonophysical simulation using the method of terminal elements. The computed 2D and 3D models of the state of the rock massif have been integrated into 3D GIS created on the basis of the ArcGIS 10 platform with an ArcGIS 3D-Analyst module. The simulation results have been corroborated by in situ observations on a regional scale (the Klichka seismodislocation, active from the middle Pliocene to date) and on a local scale (heterogeneously strained rock massif at the Antei uranium deposit). The development of a regional geodynamic model of geological structural units makes it possible to carry out procedures to ensure the safety of mining operations under complex geomechanical conditions that can expose the operating mines and mines under construction, by the Argun Mining and Chemical Production Association (PAO PPGKhO) on a common methodical and geoinformational platform, to the hazards of explosions, as well as to use the simulation results aimed at finding new orebodies to assess the flanks and deep levels of the ore field.