Atomic Structure Of Grain Boundaries Research Articles

Atomic-scale insights into grain boundary-mediated plasticity mechanisms in a magnesium alloy subjected to cyclic deformation

Sustaining grain boundary (GB) plasticity is important to prevent the premature GB cracking during plastic deformation of hexagonal-close-packed (HCP) materials. Here, we investigated atomic structures of GBs and their deformation modes in the form of motion, twin nucleation, dislocation emission, and dissociation in a HCP magnesium alloy subjected to cyclic deformation, according to atomic-resolution observations and crystallographic analyses. Besides migration and sliding, symmetric tilt GBs tended to move to form local facets, resulting in generation of asymmetric tilt low-angle GBs (LAGBs) parallel to {101¯2} lattice plane or serrated asymmetric tilt high-angle GBs (HAGBs). Crystallographic analyses indicated that local facets of the resultant asymmetric tilt GBs were often parallel to {101¯2} plane, basal and/or prismatic planes of adjacent grains. Interestingly, abundant {101¯2} twin embryos and recrysatllized nanograins nucleated from local facets of the asymmetric tilt GBs, accompanying with emission of arrays of basal 〈a〉 dislocations. A {101¯2} twin lamella along a tilt LAGB could be formed either by the coalescence of {101¯2} twin embryos, or by the combination of recrystallized nanograins, followed by {101¯2} twin nucleation from triple junctions of GBs and twin growth. Furthermore, the twin lamella could be formed through dissociation of one tilt LAGB into two tilt HAGBs, followed by {101¯2} twin nucleation from the HAGBs and triple junctions of GBs. Importantly, activation of GB deformation modes mentioned above can release local stress concentrations at GBs and sustain the ability of GB-mediated plasticity, playing important roles in plastic deformation and mechanical properties of HCP materials.

Incommensurate grain-boundary atomic structure

Grain-boundary atomic structures of crystalline materials have long been believed to be commensurate with the crystal periodicity of the adjacent crystals. In the present study, we experimentally observed a Σ9 grain-boundary atomic structure of a bcc crystal (Fe-3%Si). It is found that the Σ9 grain-boundary structure is largely reconstructed and forms a dense packing of icosahedral clusters in its core. Combining with the detailed theoretical calculations, the Σ9 grain-boundary atomic structure is discovered to be incommensurate with the adjacent crystal structures. The present findings shed new light on the study of stable grain-boundary atomic structures in crystalline materials.

Insights from symmetry: Improving machine-learned models for grain boundary segregation

Grain boundary (GB) segregation substantially alters structural and functional properties of metallic alloys, including strength, fracture mode, and corrosion resistance. A critical factor causing the variability in the segregation of solute atoms at GBs is the atomic structure of the boundary. GB segregation and its link with GB structure should thus be included in modern computational strategies for polycrystalline materials design. Here, we first propose an efficient and user-friendly Machine-Learning (ML) framework capable of predicting the segregation of different solute atoms at grain boundaries in the form of a segregation energy density, from the corresponding undecorated GB atomic structure. We then show that ML provides a fresh and promising perspective to address the long-standing issue of GB structure-segregation property relationships, in the form of two correlated atomic parameters quantifying the degree of structural and segregation symmetry respectively.

Analysis of grain-boundary segregation of hydrogen in bcc-Fe polycrystals via a nano-polycrystalline grain-boundary model

Hydrogen embrittlement caused by hydrogen segregation at grain boundaries (GBs) is the most serious issue in the development of high-strength steels, but the mechanisms behind this process are still not well understood. The GB segregation behavior of hydrogen in body-centered cubic (bcc)-Fe polycrystals was comprehensively analyzed based on the interatomic potentials derived from first-principles calculations. Considering that the atomic structure of GBs is almost independent of the grain size, the GBs in polycrystals were modeled as nano-polycrystalline GBs with random orientations. The segregation energies of hydrogen for ∼17 million interstitial sites in this GB model were calculated. From these segregation energies, the effective segregation energy for the polycrystalline GB at thermal equilibrium under various temperature and hydrogen content conditions were determined, and the validity of the calculation method was verified by comparing the results with experimental data. The relationship between the segregation energy of hydrogen at each segregation site and the surrounding local atomic environment was used to identify the major hydrogen segregation sites at the atomic level, and the changes in the crystal structure near the GB that dominated segregation were clarified. The effective segregation energies at the polycrystalline GBs were in the range of −0.48 to −0.42 eV, which are in good agreement with the experimentally reported binding energy of hydrogen at GBs of bcc-Fe polycrystals (−0.52 eV). The major hydrogen segregation sites were octahedral sites with Voronoi volumes larger than 7.0 Å3, and the segregation energy was mainly due to the uniaxially distorted crystal structure in the short-axis direction of octahedral sites. Our findings and the developed calculation method contribute to the understanding of the hydrogen segregation behavior and hydrogen embrittlement mechanism in polycrystalline metallic materials.

Atomic structures of grain boundaries for Si and Ge: A simulated annealing method with artificial-neural-network interatomic potentials

To accurately predict low-energy structures for symmetric tilt grain boundaries (GBs) in Si and Ge, artificial-neural-network (ANN) interatomic potentials are constructed and are combined with a simulated annealing (SA) method based on molecular dynamics simulations. The ANN-driven SA method is demonstrated to predict GB structures that are in good agreement with previous electron microscopy observations, without prior knowledge about their atomic configurations. Their GB energies also reasonably agree with density-functional-theory (DFT) calculations. By contrast, a conventional empirical potential fails to predict those GB structures. For misorientation angles 2θ≥93.37°, the lowest-energy structures are found to contain atomic configurations that cannot be reproduced by one repeat unit of the perfect crystal along the tilt axis. Such GB structures cannot be obtained using the γ-surface method, although it is most commonly used for exploring low-energy GB structures. These results highlight the importance of using simulation cells with multiple repeat units along the tilt axis and of performing the SA method with high-accuracy interatomic potentials transferable to GBs.

Controlling the Formation of Conductive Pathways in Memristive Devices

Resistive random-access memories are promising candidates for novel computer architectures such as in-memory computing, multilevel data storage, and neuromorphics. Their working principle is based on electrically stimulated materials changes that allow access to two (digital), multiple (multilevel), or quasi-continuous (analog) resistive states. However, the stochastic nature of forming and switching the conductive pathway involves complex atomistic defect configurations resulting in considerable variability. This paper reveals that the intricate interplay of 0D and 2D defects can be engineered to achieve reproducible and controlled low-voltage formation of conducting filaments.The author find that the orientation of grain boundaries in polycrystalline HfOx is directly related to the required forming voltage of the conducting filaments, unravelling a neglected origin of variability. Based on the realistic atomic structure of grain boundaries obtained from ultra-high resolution imaging combined with first-principles calculations including local strain, this paper shows how oxygen vacancy segregation energies and the associated electronic states in the vicinity of the Fermi level govern the formation of conductive pathways in memristive devices. These findings are applicable to non-amorphous valence change filamentary type memristive device. The results demonstrate that a fundamental atomistic understanding of defect chemistry is pivotal to design memristors as key element of future electronics.

Determining Atomic Structure of Grain Boundaries and Heterostructure Interfaces at Solid-State Electrode/Electrolyte Heterostructures in Lithium Based Oxides

Determining Atomic Structure of Grain Boundaries and Heterostructure Interfaces at Solid-State Electrode/Electrolyte Heterostructures in Lithium Based Oxides

Microstructural Engineering of Li7La3Zr2O12 Solid Electrolytes for Enhanced Stability in H2O/CO2

Solid-state batteries with nonflammable inorganic solid electrolytes provide a fundamental solution for resolving safety concerns. Moreover, mechanically robust solid electrolytes can physically block the penetration of Li dendrites, and thus, Li metal can be applied as an anode (negative electrode), boosting the energy density of the batteries. Garnet-type cubic Li7La3Zr2O12 (LLZO) has been considered as a promising candidate for solid electrolytes, due to its high Li+ conductivity and chemical/electrochemical compatibility with Li metal. When exposed to the ambient air, however, LLZO electrolytes are known to react with H2O and CO2 to form Li2CO3. Li2CO3 also grows along the grain boundaries, resulting in the significant degradation in Li+-conducting properties.In an effort to develop LLZO electrolytes with enhanced interfacial stability, here, we investigate the composition–microstructure–stability relationship of LLZO modified with aliovalent elements (Ta and Ga) under controlled H2O and CO2 atmospheres. Dense LLZO electrolytes doped with Ta (LLZTO) and Ta/Ga (Ga-LLZTO) are synthesized via a sol-gel process, followed by high-temperature sintering (1100 °C). Microstructural analyses show that the elemental doping has a crucial role in the grain growth behavior during the sintering process, thereby modifying the density, morphology, and atomic structure of grain boundaries. The interfacial stability of doped LLZO electrolytes is examined under accelerated durability test (ADT) conditions, where the concentration of O2/H2O/CO2 is precisely controlled to promote the interfacial reaction. During the ADTs, the degradation kinetics of Li+-conducting properties is quantitatively determined using in situ AC-impedance measurements combined with ex situ surface characterizations. The Li+ conductivity of Ga-LLZTO is almost retained during the ADT, while LLZTO shows only 27% of its initial conductivity value after 24 h of ADT. The significant degradation of LLZTO is caused by the formation of Li2CO3, which is further verified by Raman mapping and X-ray photoelectron spectroscopy analysis. On the other hand, the compositional and structural characterizations suggest that the enhanced interfacial stability of Ga-LLZTO is attributed to the reduced grain boundary density with enlarged grains and the segregation of H2O/CO2-tolerant Li–Ga–O species in the grain boundaries. The findings of this study would be essential in understanding the degradation of Li+-conducting properties and in developing highly conductive and stable LLZO solid electrolytes.

Revealing the Structure and Properties of Polycrystalline Components of the Solid Electrolyte Interface

The formation, structure and composition of the solid-electrolyte interphase (SEI) in lithium batteries have been extensively explored in the prior literature. It is commonly accepted that the SEI consists of two inorganic layer followed by organic layer. However, very little is known about the actual grain boundary (GB) structure of the inorganic layer and the ions transport through it. The present work seeks to understand the structure and stability of the GBs formed between various inorganic components of the SEI and mechanisms of Li diffusion through them. The density functional theory (DFT) calculations supported by the high-resolution transmission electron microscopy (HRTEM) measurements are employed to understand the SEI GB atomic structures and their chemo-mechanical properties.DFT calculations were performed using the Vienna Ab Initio Simulations Package (VASP) code1 employing the generalized-gradient approximation (GGA) using the PBE (Perdew, Burke, and Ernzerhof)2 functional to account for the exchange-correlation effects. The Nudged Elastic Band (NEB) method3 is used for the Li migration barriers calculations as implemented in VASP. To build the GB structures, the Atomistic Tool Kit (ATK)4 “Interface Builder” tool is used, which allows to analyze all possible interphases between two slabs. Cryogenic TEM (Cryo-TEM) images are recorded on a JEOL ARM200CF TEM, equipped with a Gatan Oneview camera operated at 200 kV.The HRTEM measurements revealed the polycrystalline structure of the SEI inorganic layer consisting mainly of LiF, Li2CO3, Li2O and LiOH components. In addition, different surface orientations (e.g., 111, 200, 002, etc.) of the aforementioned components are identified. Based upon the HRTEM results (cf. Fig. 1 left-hand side), the corresponding computational slabs have been built as shown on the right-hand side in Fig. 1. After the optimization of all structures, Li adatom diffusion inside grain and GB and mechanical properties were calculated.For each GB, several Li adatom positions with multi-atom bonding were identified. The activation energy of Li diffusion varies significantly depending upon the slab’s structure and the number of Li adatoms. The NEB calculations also reveal a non-symmetric transition state for all considered structures, which indicates a complex multi-atom hopping mechanism. The calculated elastic properties show the decrease of Young’s modulus with an increase of Li adatoms. Our combined experimental and modeling findings provide valuable insights into developing mitigation strategies towards a better understanding of the SEI components stability and Li migration through the SEI.Figure 1. Identification of grain and GB of the SEI busing HRTEM (left) and the corresponding built DFT slab (right). Lavender spheres denote Li atoms, red spheres are the oxygen atoms, grey spheres represent carbon atoms and green spheres are F atoms. Blue sphere in the GB denotes Li adatom.References G. Kresse, Phys. Rev. B, 59, 1758–1775 (1999).J. P. Perdew, A. Ruzsinszky, G. I. Csonka, O. A. Vydrov, G. E. Scuseria, L. A. Constantin, X. Zhou, K. Burke, Phys. Rev. Lett., 100, 136406 (2007).G. Henkelman, B. P. Uberuaga, H. Jónsson, and G. Henkelman, J. Chem. Phys, 113 9901 (2020). Synopsys QuantumWise A/S, 2017/2. Figure 1

Application of Grain Boundary Segregation Prediction Using a Nano-Polycrystalline Grain Boundary Model to Transition Metal Solute Elements: Prediction of Grain Boundary Segregation of Mn and Cr in bcc-Fe Polycrystals

A prediction method for grain boundary segregation using a nano-polycrystalline grain boundary model is applied to the grain boundary segregation of Cr and Mn in bcc-Fe polycrystals for which experimental results exist, and the validity of the prediction method is verified. In this prediction method, focusing on the fact that the atomic structure of grain boundaries is almost independent of grain size, grain boundaries of polycrystals with grain sizes of the order of micrometers are modeled as grain boundaries of nano-polycrystals, for which structural relaxation calculations by molecular dynamics calculations are possible. For this grain boundary model, the grain boundary segregation energy of each site is calculated exhaustively using the interatomic potential. In addition, the average amount of grain boundary segregation in the polycrystal is calculated from the obtained grain boundary segregation energy. With this prediction method, the average amounts of grain boundary segregation and segregation energies of Cr and Mn in bcc-Fe polycrystals can be calculated and compared with the experimental results. Calculated results for both elements reproduced the experimental results well, suggesting that this prediction method is also effective in predicting the grain boundary segregation of other solute elements.

Atomic Structures of Grain Boundaries for Si and Ge: A Simulated Annealing Method with Artificial-Neural-Network Interatomic Potentials

Atomic Structures of Grain Boundaries for Si and Ge: A Simulated Annealing Method with Artificial-Neural-Network Interatomic Potentials

Application of Grain Boundary Segregation Prediction Using a Nano-Polycrystalline Grain Boundary Model to Transition Metal Solute Elements: Prediction of Grain Boundary Segregation of Mn and Cr in bcc-Fe Polycrystals

A prediction method for grain boundary segregation using a nano-polycrystalline grain boundary model is applied to the grain boundary segregation of Cr and Mn in bcc-Fe polycrystals for which experimental results exist, and the validity of the prediction method is verified. In this prediction method, focusing on the fact that the atomic structure of grain boundaries is almost independent of grain size, grain boundaries of polycrystals with grain sizes of the order of micrometers are modeled as grain boundaries of nano-polycrystals, for which structural relaxation calculations by molecular dynamics calculations are possible. For this grain boundary model, the grain boundary segregation energy of each site is calculated exhaustively using the interatomic potential. In addition, the average amount of grain boundary segregation in the polycrystal is calculated from the obtained grain boundary segregation energy. With this prediction method, the average amounts of grain boundary segregation and segregation energies of Cr and Mn in bcc-Fe polycrystals can be calculated and compared with the experimental results. Calculated results for both elements reproduced the experimental results well, suggesting that this prediction method is also effective in predicting the grain boundary segregation of other solute elements.

Determination of Grain-Boundary Structure and Electrostatic Characteristics in a SrTiO3 Bicrystal by Four-Dimensional Electron Microscopy.

The grain boundary (GB) plays a critical role in a material’s properties and device performance. Therefore, the characterization of a GB’s atomic structure and electrostatic characteristics is a matter of great importance for materials science. Here, we report on the atomic structure and electrostatic analysis of a GB in a SrTiO3 bicrystal by four-dimensional scanning transmission electron microscopy (4D-STEM). We demonstrate that the Σ5 GB is Ti-rich and poor in Sr. We investigate possible effects on the variation in the atomic electrostatic field, including oxygen vacancies, Ti-valence change, and accumulation of cations. A negative charge resulting from a space-charge zone in SrTiO3 compensates a positive charge accumulated at the GB, which is in agreement with the double-Schottky-barrier model. It demonstrates the feasibility of characterizing the electrostatic properties at the nanometer scale by 4D-STEM, which provides comprehensive insights to understanding the GB structure and its concomitant effects on the electrostatic properties.

Atomic structure of grain boundaries in UO2 Bicrystals: A coupled high resolution transmission electron Microscopy/Atomistic simulation approach

In this work, high resolution transmission electron microscopy and numerical simulation at the atomic scale were coupled, for the first time to our knowledge, to study the atomic structure of two Σ5 symmetric tilt grain boundaries in uranium dioxide (UO2) bicrystals. We show that whereas the Σ5 (021)/[100] is fully symmetric, with the grain boundary plane forming a mirror symmetry plane, a rigid body translation exists between the crystals in the Σ5 (031)/[100]. We particularly believe that the excellent agreement between experimentally observed and simulated structures highlights the relevance of the coupled approach. Such result emphasize that the use of CRG and Yakub empirical potentials including a Morse contribution can be relied upon to model the behavior of UO2.

Resolving atomic-scale phase transformation and oxygen loss mechanism in ultrahigh-nickel layered cathodes for cobalt-free lithium-ion batteries

Summary Doped LiNiO2 has recently become one of the most promising cathode materials for its high specific energy, long cycle life, and reduced cobalt content. Despite this, the degradation mechanism of LiNiO2 and its derivatives still remains elusive. Here, by combining in situ electron microscopy and first-principles calculations, we elucidate the atomic-level chemomechanical degradation pathway of LiNiO2-derived cathodes. We uncover that the O1 phase formed at high voltages acts as a preferential site for rock-salt transformation via a two-step pathway involving cation mixing and shear along (003) planes. Moreover, electron tomography reveals that planar cracks nucleated simultaneously from particle interior and surface propagate along the [100] direction on (003) planes, accompanied by concurrent structural degradation in a discrete manner. Our results provide an in-depth understanding of the degradation mechanism of LiNiO2-derived cathodes, pointing out the concept that suppressing the O1 phase and oxygen loss is key to stabilizing LiNiO2 for developing next-generation high-energy cathode materials.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

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.

Influence of solute hydrogen on the interaction of screw dislocations with vicinal twin boundaries in nickel

The influence of solute H on the interaction between screw dislocations and vicinal twin boundaries in Ni bicrystals is investigated using molecular dynamics (MD) simulations. Several interaction mechanisms such as dislocation transmission, nucleation and reflection are reported for different glide planes and grain boundary (GB) structures. The presence of H tends to transform these interactions into ones involving dislocation absorption. This disorders the atomic structure of GBs and establishes a local stress state, which promotes the ultimate failure of GBs due to the formation of vacancies. These findings will deepen our understanding on the experimentally-observed H embrittlement in metallic materials.

Machine-Learning Informed Representations for Grain Boundary Structures

The atomic structure of grain boundaries plays a defining but poorly understood role in the properties they exhibit. Due to the complex nature of these structures, machine learning is a natural tool for extracting meaningful relationships and new physical insight. We apply a new structural representation, called the scattering transform, that uses wavelet-based convolutional neural networks to characterize the complete three-dimensional atomic structure of a grain boundary. The machine learning to predict GB energy, mobility, and shear coupling using the scattering transform representation is compared and contrasted with learning using a smooth overlap of atomic positions (SOAP) based representation. While predictions using the scattering transform are not as good as those of SOAP, other factors suggest that the scattering transform may yet play an important role in GB structure learning. These factors include the ability of the scattering transform to learn well on larger datasets, in a process similar to deep learning, as well as their ability to provide physically interpretable information about what aspects of the GB structure contribute to the learning through an inverse scattering transform.

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.