Grain Boundary Structure Research Articles

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.

Structure and chemical composition of grain boundaries in the magnetic semiconductor GaSb<Mn>

The structure and chemical composition of grain boundaries in GaSb<Mn> magnetic semiconductors have been investigated. We determined that quenching of the GaSb melt with 2% Mn results in the formation of a textured polycrystal (111). The grain boundaries of the texture are formed by split 60 degree dislocations with <110> dislocation lines. Microinclusions based on the ferromagnetic compound MnSb are located on the stacking faults of split dislocations. The chemical compositions of microinclusions differ, but their average composition is close to Mn1.1Sb. The synthesized GaSb<Mn> is a soft ferromagnet with a coercive force of 10 Oe and a magnetic state approaching superparamagnetic

Study of Micro-Upsetting by Finite Element Simulation Based on Crystal Plasticity and Grain Boundary Strengthening Theories

The properties of individual grains affect the mechanical behaviors and response of materials in micro-scaled deformation, viz., microforming, and there are unknown phenomena and deformation behaviors existing and limiting the wide application of microforming due to size effect. In this paper, a composite model combining crystal plasticity and grain boundary strengthening theories was developed for numerical investigation into the effect of grain boundaries on the plastic deformation of copper micro-upsetting. By comparing the results with and without grain-boundary structure, it is revealed that grain boundaries, which act as the barriers of crystal slip, result in the enhanced flow stress and the discontinuous distribution of stress and strain. The grain size effect is also considered in this research, and the results show the coarse-grained material reduces the flow stress and enhances the inhomogeneous deformation.

Superhardness Induced by Grain Boundary Vertical Sliding in (001)-textured ZrB[formula omitted] and TiB[formula omitted] Nano Films

Superhardness up to 50 and 60 GPa has been achieved recently in orientationally (001)-textured ZrB2 and TiB2 films composed of nano columnar grains, which almost double those in common TMB2 (TM=Zr, Ti). But the atomic origin, especially the role of shear stresses under indentation on grain boundaries (GBs), remains largely unknown. In this work, we report the identification of the experimentally observed GBs as Σ13 [001] tilt GB by first-principles calculations. The calculated formation energies of the GBs and TM vacancies on the GBs explain well the experimental results. Under shear stresses beneath indentors, the GB motion transforms the accumulated shear stresses parallel to (001) plane into a vertical sliding of the nano grains in [001] direction, while maintaining the GB structure unchanged. This special mode of shear plastic deformation correlates their hardness to the compressive strengths in [001] direction, instead of to the shear strengths on (001) plane as in bulk crystals, with the calculated compressive strengths well accounting for the measured superhardness in these (001)-textured nano ultra-high temperature ceramics.

Size-Dependent Solute Segregation at Symmetric Tilt Grain Boundaries in α-Fe: A Quasiparticle Approach Study.

In the present work, atomistic modeling based on the quasiparticle approach (QA) was performed to establish general trends in the segregation of solutes with different atomic size at symmetric 〈100〉 tilt grain boundaries (GBs) in α-Fe. Three types of solute atoms X1, X2 and X3 were considered, with atomic radii smaller (X1), similar (X2) and larger (X3) than iron atoms, respectively, corresponding to phosphorus (P), antimony (Sb) and tin (Sn). With this, we were able to evidence that segregation is dominated by atomic size and local hydrostatic stress. For low angle GBs, where the elastic field is produced by dislocation walls, X1 atoms segregate preferentially at the limit between compressed and dilated areas. Contrariwise, the positions of X2 atoms at GBs reflect the presence of tensile and compressive areal regions, corresponding to extremum values of the σXX and σYY components of the strain tensor. Regarding high angle GBs Σ5 (310) (θ = 36.95°) and Σ29 (730), it was found that all three types of solute atoms form Fe9X clusters within B structural units (SUs), albeit being deformed in the case of larger atoms (X2 and X3). In the specific case of Σ29 (730) where the GB structure can be described by a sequence of |BC.BC| SUs, it was also envisioned that the C SU can absorb up to four X1 atoms vs. one X2 or X3 atom only. Moreover, a depleted zone was observed in the vicinity of high angle GBs for X2 or X3 atoms. The significance of this research is the development of a QA methodology capable of ascertaining the atomic position of solute atoms for a wide range of GBs, as a mean to highlight the impact of the solute atoms’ size on their locations at and near GBs.

Study on hydrogen segregation at individual grain boundaries in pure nickel by scanning Kelvin probe force microscopy

The hydrogen segregation at different individual grain boundaries (GBs) in pure nickel has been visually revealed by the contrast of the Volta potential measured by scanning Kelvin probe force microscopy (SKPFM) before and after hydrogen charging. By combining the Electron Back Scattered Diffraction (EBSD) analyses, both several typical low-Σ special GBs and the general GBs with different misorientation have been studied. The results show that the segregation degree of hydrogen atoms at GBs strongly depends on the GB structure. The hydrogen trapping capacity of the low-Σ special GBs is sequenced. For the general GBs with different misorientation, the evolution of the hydrogen segregation degree with the GB misorientation coincides with the prediction of the GB structure model.

Machine learning to predict aluminum segregation to magnesium grain boundaries

Magnesium alloys are good candidates for a number of applications due to their high strength-to-weight ratio, but other properties like corrosion resistance, formability, and creep are still a concern. In magnesium-aluminum alloys, Mg17Al12 phase precipitates at the grain boundaries (GBs) can have important implications on the mechanical and corrosion behavior. In order to better understand the effects, atomistic segregation of aluminum to GBs must be evaluated first. This study uses atomistic simulations to quantify aluminum segregation energetics for training a machine learning model. Aluminum atoms were iteratively placed at various atomic sites near 30 different 〈0001〉 symmetric tilt grain boundaries (STGBs) in magnesium. The results show how aluminum segregation is affected by GB structure and the local atomic environment. The ability to compute grain boundary physical properties of interest using machine learning techniques can have broad implications for the area of grain boundary science and engineering.

A moiré theory for probing grain boundary structure in graphene

Multiscale microscopy spanning the atomistic, moiré, and meso scales has enabled engineering the equilibrium structure of graphene. However, temporal restrictions on in-operando imaging techniques make the moiré scale the finest accessible spatial resolution, thereby limiting our understanding of atomistic mechanisms of non-equilibrium processes in graphene. In order to include atomic scale features with in-operando microscopy, we develop a moiré metrology theory that infers the atomic scale structure from the moiré scale, creating a bridge to in-operando microscopy. The theory is based on atomic scale models that govern the atomistic structure and are promoted to the moiré scale by simulation. We introduce this through a relevant application: nuclei coalescence of graphene during chemical vapor deposition. We develop two mechanistic atomic scale models that govern the propagation and structure of grain boundaries, illuminating how edge dislocations, disconnections, and grain boundaries form from the attachment of individual dimers. The atomistic models are brought to the moiré scale through bond convolution simulations and the resultant moiré metrology theory is tested on results from in-operando scanning tunneling microscopy. By showing that we can identify atomic scale defects from moiré patterns, we highlight how moiré metrology can enable decision making during growth from in-operando observation of graphene structure, paving the way for the design of graphene atomistic structure under scalable synthesis conditions.

The Role of Grain Boundaries in Charge Carrier Dynamics in Polycrystalline Metal Halide Perovskites

AbstractBoundaries of perovskite grains forming polycrystalline films are the most sensitive place for the formation of the defect states and accumulation of impurities. The intra‐bandgap states located at the boundaries forming traps for charge carriers are commonly discussed as causing nonradiative carrier recombination and reducing carrier mobility. Grain boundaries (GB) may also disturb carrier motion by creating scattering centres and barriers, however, their role was less frequently considered. On the other hand, grain boundaries may also facilitate the splitting of exciton states and the generation of free charge carriers. Consequently, the grain boundaries are responsible or affect a wide range of perovskite properties and processes strongly influencing the performance of perovskite‐based optoelectronic devices. Nevertheless, abundant literature data related to the properties of grain boundaries and their influence are highly controversial. In this minireview, we discuss the most importnat literature data related to the structure and properties of grain boundaries and their role in device performance.

The Effect of Grain Boundaries on Plastic Deformation of Olivine

AbstractThe plastic deformation of olivine has been studied for decades. However, the precise role of grain boundaries during deformation in, for example, the dislocation‐accommodated grain‐boundary sliding regime, remains poorly understood. Specifically, we lack knowledge regarding the manner in which grain boundaries interact with other defects, such as dislocations, during deformation. To investigate the interaction of dislocations and grain boundaries, we analyzed the structure and distribution of grain boundaries in a polycrystalline aggregate of Fo50 deformed in torsion (Hansen, Zimmerman, Dillman, & Kohlstedt, 2012, https://doi.org/10.1016/j.epsl.2012.04.016). We characterized the microstructure of the aggregate using electron‐backscatter diffraction and transmission electron microscopy in three perpendicular directions. An increase in plastic strain is associated with the development of a strong crystallographic preferred orientation and a grain‐boundary plane distribution that evolves from a uniform distribution to one dominated by (010)‐type planes. We use the m' factor, to evaluate the potential for transmission of dislocations across grain boundaries based on the relative orientations of slip systems between neighboring grains. With progressive deformation, our analysis indicates an increase in abundance of apparently slip‐transparent boundaries until moderate strains ( = 4) are reached. Based on these observations, we propose that specific types of grain boundaries are created by dislocation activity and that the input of dislocations into grain boundaries facilitates grain‐boundary sliding. Our results provide insight into the microphysics of olivine deformation and highlight the importance of the coupled study of inter‐ and intragranular mechanisms during rock deformation.

Multiscale analyses of the interaction between dislocation and Σ9 symmetric tilt grain boundaries in Fe–Si bicrystals by nanoindentation technique

Dislocation–grain boundary (GB) interaction plays a crucial role in the strengthening mechanism of polycrystalline metals. In this study, we evaluated the interaction between dislocation and two types of Σ9 symmetric tilt GBs using combined experimental and computational nanoindentation analyses. The experimental and simulation results are obtained at different spatial scales and are complementary. Experimental nanoindentation, in which the indent position was 1.0 μm offset from the GBs, showed that a considerable plastic zone was formed in the adjacent grain in the case of Σ9{114}, whereas this zone was absent in the case of Σ9{221}. Atomistic analysis provided information on the activated Burgers vector caused by a large resolved shear stress and the change in the GB structure due to dislocation absorption. Moreover, from experimental and computational nanoindentation analyses of the case in which the indent position was exactly on the GB, it was found that the load required for dislocation nucleation from the GB in Σ9{221} was slightly higher than that in the case of Σ9{114}. Dislocation trajectories observed in the on-GB experiments were similar to those in the on-GB simulations, where an activated Burgers vector was detected. The origin of the effects of GBs on dislocation transmission and nucleation was discussed based on the mechanical responses and dislocation behaviors obtained in experiments, activated slip directions in atomistic simulations, and theoretical models for dislocation–GB interaction.

On the structure of (111) twist grain boundaries in diamond: atomistic simulations with Tersoff-type interatomic potentials

In nanocrystalline diamond thin films, the high grain boundary density and the associated grain boundary properties may dominate the overall material behavior. In the present work, we systematically investigate (111) twist grain boundaries in diamond carbon by using atomistic simulations with Tersoff-type potential. Our work reveals the relation between atomic scale grain boundary structure and bicrystallography, bond deformation, point defect population. A comparative study with diamond silicon highlights general trends in the grain boundary energy as a function of the misorientation. We predict a transition between glide-plane grain boundary position and shuffle-plane grain boundary with increasing misorientation in diamond cubic materials with a (111) twist grain boundary.

Cumulative defect structures for experimentally attainable low thermal conductivity in thermoelectric (Bi,Sb)2Te3 alloys

Manipulation of thermal transport properties by defect engineering is an important but yet unresolved issue in thermoelectrics, since rational design strategies of multiple defect structures for minimizing lattice thermal conductivity are lacking. The key is to comprehend complex interaction between different frequency-dependent phonon scattering processes by multiple defect structures. Herein, individual contributions in lattice thermal conductivity reduction from each defect structure—point defects (0-dimensional, 0D), dislocations (1D), grain boundaries (2D), and nanoparticles (3D)—are characterized based on experimental results of commercial (Bi,Sb)2Te3-based alloys fitted to Debye-Callaway model. Then, the cumulative contributions by multiple defect structures are investigated interactively by estimating total phonon relaxation time. Therefore, the interactive role of multiple defect structures in reducing lattice thermal conductivity is elucidated. To extend the approach to other thermoelectric materials, PbTe-based alloys with multiple defect structures of point defects and grain boundaries were modeled as well. This approach enables to provide a thorough design of defect structures for realizing the experimentally attainable low lattice thermal conductivity in thermoelectric materials.

Reduced Viscosity of Mg2GeO4 with Minor MgGeO3 between 1000 and 1150 °C Suggests Solid-State Lubrication at the Lithosphere–Asthenosphere Boundary

Tectonic plates are thought to move above the asthenosphere due to the presence of accumulated melts or volatiles that result in a low-viscosity layer, known as lithosphere–asthenosphere boundary (LAB). Here, we report experiments suggesting that the plates may slide through a solid-state mechanism. Ultrafine-grained aggregates of Mg2GeO4 and minor MgGeO3 were synthetized using spark plasma sintering (SPS) and deformed using a 1-atm deformation rig between 950 °C and 1250 °C. For 1000 < T < 1150 °C, the derivative of the stress–strain relation of the material drops down to zero once a critical stress as low as 30–100 MPa is reached. This viscosity reduction is followed by hardening. The deformation curves are consistent with what is commonly observed in steels during the shear-induced transformation from austenite to martensite, the final material being significantly harder. This is referred to as TRansformation-Induced Plasticity (TRIP), widely observed in metal alloys (TRIP alloys). It should be noted that such enhanced plasticity is not necessarily due to a phase transition, but could consist of any kind of transformation, including structural transformations. We suspect a stress-induced grain-boundary destabilization. This could be associated to the transient existence of a metastable phase forming in the vicinity of grain boundaries between 1000 and 1150 °C. However, no such phase can be observed in the recovered samples. Whatever its nature, the rheological transition seems to occur as a result of a competition between diffusional processes (i.e., thermally activated) and displacive processes (i.e., stress-induced and diffusionless). Consequently, the material would be harder at 1200 °C than at 1100 °C thanks to diffusion that would strengthen thermodynamically stable phases or grain-boundary structures. This alternative scenario for the LAB would not require volatiles. Instead, tectonic plates may slide on a layer in which the peridotite is constantly adjusting via a grain-boundary transformation.

Hydration-driven stabilization and volume collapse of grain boundaries in Mg2SiO4 forsterite predicted by first-principles simulations

Abstract Grain boundaries in mantle minerals are of critical importance to geophysical and geochemical processes of the Earth’s interior. One of the fundamental issues is to understand how the water (H2O) component influences the properties of grain boundaries in silicate materials. Here, we report the results of the structure and stability of several tilt grain boundaries in Mg2SiO4 forsterite over the pressure range 0 to 15 GPa using density functional theory-based first-principles simulations. The results suggest greater energetic stability and hydration-driven volume collapse (negative excess volume) at zero pressure for the majority of hydrous grain boundaries relative to the anhydrous (dry) ones. All the hydrous grain boundaries become increasingly favorable at elevated pressures as the calculated hydration enthalpy systematically decreases with increasing pressure. The hydrous components at the interfacial regions are predominantly in the hydroxyl form and, to a lesser extent, in the molecular H2O form. Their calculated ratio ranges from 1.6 to 8.7 among the different grain boundary configurations. Our structural analysis also reveals that the hydroxyls are bound to either both Mg and Si or to Mg only. In comparison, the molecular species are bound only to Mg sites. Besides direct oxygen-hydrogen bonding, intermolecular hydrogen bonding becomes important with compression. On the basis of our results, we suggest that local atomic rearrangements caused by dissociative adsorption of water facilitate efficient compaction of the boundary interfaces, which, in turn, results in greater relative stability of hydrous grain boundaries. This means that water prefers to be incorporated within the grain boundaries over the bulk of silicate materials.

Atomistic study of the structure and deformation behavior of symmetric tilt grain boundaries in [formula omitted]-zirconium

We have investigated the structure, energetics and mechanical deformation of symmetric tilt grain boundaries (STGB) in hexagonal close-packed Zr (α-Zr). Molecular dynamics (MD) simulations are performed to determine the equilibrium structures of STGBs with [0001], [11¯00] and [112¯0] tilt axes with respect to the misorientation angle. We characterized the atomic structure and energy of STGBs, determined within the coincidence site lattice model, for the tilt axes considered. Subsequently, we performed simulations of uniaxial tensile loading to evaluate their mechanical response under mechanical deformation. Our results provide a comprehensive characterization of STGBs in α-Zr in a wide variety of conditions, and therefore substantially contributing to the understanding of the mechanical properties of α-Zr-based materials.

Evolutionary inverse design of defects at graphene 2D lateral interfaces

Grain boundaries (GBs) in two-dimensional (2D) materials often have a profound impact on various material properties from mechanical to optical to electronic, yet predicting all possible GB formations is a challenge. Here, we introduce a workflow based on an evolutionary algorithm for exploring possible GBs formed at a lateral 2D interface. In a departure from conventional genetic algorithm based structure optimization methods, we perform genetic operations in the near interface region that allow us to be computationally efficient. We benchmark our method using graphene, which is a well-studied 2D material with a wide range of point defects. An empirical potential was used as the surrogate of the evolutionary search. More than 11.5 × 106 structures in total were evaluated for 128 GB orientations, and for each orientation, the ten best structures are recorded. A subset of low energy GBs predicted by empirical potential based search was relaxed by first-principles calculations and used to validate the energetic rank order. With the validated formation energy, we rank-ordered the best 128 GB structures and performed a detailed statistical analysis of primitive rings to find the correlation between the ring distribution and the formation energy. We found that for low energy GBs (below 0.5eV/Å), Stone–Wales defects will dominate, while structures with a higher energy (0.5–1.1eV/Å) show an increasing population of heptagons and nine-membered rings to form seven-nine defect pairs. For structures with energy higher than 1.1eV/Å, the percentage of octagons and nine-membered rings increases, which indicates that these two types of rings are not energetically favorable. Our proposed methodology is broadly applicable to explore defective low dimensional materials and represents a powerful tool that enables a systematic search of GBs of lateral interfaces for 2D materials.

Machine Learning to Predict Aluminum Segregation to Magnesium Grain Boundaries

Magnesium alloys are good candidates for a number of applications due to their high strength-to-weight ratio, but other properties like corrosion resistance, formability, and creep are still a concern. In magnesium-aluminum alloys, M17Al12 phase precipitates at the grain boundaries (GBs) can have important implications on the mechanical and corrosion behavior. In order to better understand the effects, atomistic segregation of aluminum to GBs must be evaluated first. This study uses atomistic simulations to quantify aluminum segregation energetics for training a machine learning model. Aluminum atoms were iteratively placed at various atomic sites near 30 different 〈0001〉 symmetric tilt grain boundaries (STGBs) in magnesium. The results show how aluminum segregation is affected by GB structure and the local atomic environment. The ability to compute grain boundary physical properties of interest using machine learning techniques can have broad implications for the area of grain boundary science and engineering.

Arrangement of polyhedral units for [0001]-symmetrical tilt grain boundaries in zinc oxide

Polycrystalline ZnO is used in a wide variety of electrical applications, and its properties are largely influenced by crystalline defects, such as grain boundaries (GBs). Therefore, in this study, the atomic structures of [0001]-symmetrical tilt GBs in ZnO have been characterized by scanning transmission electron microscopy, and the misorientation dependence of the atomic structures around GBs is thoroughly discussed based on the polyhedral-unit model. In addition to theoretical calculations, the polyhedral-unit arrangements for arbitrary tilt angles are described and predicted by a number-theory-based approach. The predicted structural-unit arrangements agreed well with those experimentally observed, indicating that geometrical restrictions determine the ZnO GB structures. Owing to the crystallographic relationship between the structural-unit Burgers vector and the GB-plane normal, the structural-unit arrangement was transformed at approximately 30∘, which is associated with a rigid-body translation from one grain to another.

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.