Grain Boundaries Research Articles

Atomistic Probing and Identification of Point Defects and Grain Boundaries of 1H and 1T' Molybdenum Telluride Monolayer.

The substantial structural defects frequently observed in fabricated transition-metal dichalcogenide (TMD) samples inevitably affect the device performance. The molybdenum telluride (MoTe2) monolayer can easily generate phase transitions between the 1H and 1T' phases due to a small energy barrier. However, distinguishing and identifying various defects during experiments is challenging. In this study, we comprehensively explore point defects and grain boundaries in MoTe2 using first-principles calculations. We simulate the corresponding scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) images to characterize different types of defects. The same type of point defects in the 1T' phase exhibits lower formation energies than those in the 1H phase. The grain boundaries of the 1T' phase form more easily, with corresponding formation energies ranging from 0.07 to 0.14 eV/Å. The partial densities of states indicate that the electronic properties of the 60°, 60°-glide, and 120° grain boundaries (GBs) in the 1T' phase are similar, while various types of defect rings in the 1H phase differ significantly. Our theoretical results effectively reduce the primary cost of characterizing defects and provide essential guidance for experimental references and identifications.

The spallation characteristics of polycrystalline aluminum with helium bubbles under a wide range of shock stresses

The spallation behavior of polycrystalline Al with helium (He) bubbles (poly Al–He) under unsupported shock loadings at a wide range of impact velocities was investigated via molecular dynamics simulations. We found that the microstructural features during shock compression and release processes for the poly Al–He are highly analogous to those observed in polycrystalline Al (poly Al), indicating that the bubbles studied here do not have a significant influence on the mechanical deformation before tension. During the tension process, the expansion-merging of He bubbles dominates damage accumulation and leads to the ultimate fracture of the metal, the same as that in a single crystal with He bubbles. The presence of grain boundaries (GBs) does not exhibit an apparent effect on the evolution of He bubbles, resulting in comparable expansion rates for the bubbles in different locations (i.e., near GBs or at grain interiors). Additionally, the nucleation of voids occurs subsequent to bubble expansion due to the much higher critical stress. Voids are preferentially nucleated on GBs when the material is solid and at liquid parts when the material is partially molten, demonstrating that GBs and melting can strongly facilitate void nucleation. However, He bubbles significantly impede void nucleation and growth, resulting in a much smaller quantity and volume of voids formed in the poly Al–He, compared to the poly Al. Furthermore, the critical stress for void nucleation and the spall strength of the metal matrix are reduced by He bubbles.

Understanding the influence of hydrogen on BCC iron grain boundaries using the kinetic activation relaxation technique (k-ART)

Abstract Hydrogen embrittlement (HE) poses a significant challenge to the mechanical integrity of iron and its alloys. This study explores the influence of hydrogen atoms on two distinct grain boundaries (GBs), $\Sigma37$ and $\Sigma3$, in body-centered-cubic (BCC) iron. Using the kinetic activation relaxation technique (k-ART), an off-lattice kinetic Monte Carlo approach with an EAM-based potential, extensive catalogs of activated events for atoms in both H-free and H-saturated GBs were generated.
Studying the diffusion of H, we find that, for these systems, while GB is energetically favorable for H, this element diffuses more slowly at the GBs than in the bulk. The results further indicate that the $\Sigma 3$ GB exhibits higher stability in its pure form compared to the $\Sigma 37$ GB, with notable differences in energy barriers and diffusion behaviors. Moreover, with detailed information about the evolution landscape around the GB, we find that the saturation of a GB with hydrogen both stabilizes the GB by shifting barriers associated with Fe diffusion to higher energies and reducing the number of diffusion events. For the $\Sigma 37$ GB, the presence of hydrogen causes elastic deformation, affecting the diffusion of Fe atoms both at the GB and in adjacent positions. This results in new diffusion pathways but with higher diffusion barriers, unlike for the $\Sigma 3$ GB. These results indicate that the presence of hydrogen rigidifies the direct GB interface layers while allowing more atoms to be active for the $\Sigma 37$ GB. This provides a microscopic basis to support the existence of competing mechanisms compatible with either plasticity (such as hydrogen enhanced localized plasticity --- HELP) or energy-dominated (hydrogen enhanced decohesion mechanism ---HEDE) embrittlement, with the relative importance of these mechanisms determined by the local geometry of the GBs.

Failure Mechanism Analysis of Thermal Barrier Coatings Under a Service Simulation Environment

In this paper, the ceramic coating of thermal barrier coatings (TBCs) was prepared on the surface of the tube specimens by Electron Beam Physical Vapor Deposition (EB-PVD) process. Subsequently, a service simulation was conducted using a simulation device to analyze the failure behavior of the TBCs. The effects of high-temperature sintering and CaO-MgO-Al2O3-SiO2 (CMAS) corrosion on the microstructural evolution, phase structural changes, and insulation performance of the thermal barrier coatings were investigated. The results indicated that with increasing high-temperature sintering time, the “feather” structures at the boundaries of the columnar grains evolve into the “tentacle” structure that facilitates the fusion of adjacent columnar grains, resulting in increased grain diameter and wider gaps. No transformation from t’-ZrO2 to the monoclinic phase m-ZrO2 occurred during the high-temperature sintering process. Over time, CMAS wets the coating surface and infiltrates the interior of the coating, causing corrosion to the Yttria-stabilised zirconia (YSZ) and accelerating sintering. A new phase, ZrSiO4, was formed after corrosion without inducing the transition.

Study of the boundary migration of recrystallized grains under inhomogeneous deformation field by crystal plasticity – phase field simulation

ABSTRACT In this study, a grain boundary (GB) migration model based on crystal plasticity (CP) and phase field (PF) was constructed for recrystallization of magnesium alloys. The model introduced the stored energy field obtained by CP into the PF simulations, aiming to analyze the effect of the inhomogeneous stored energy generated under the plastic deformation on the GB migration. It was found that the curvature plays a more significant role in the inhomogeneous stored energy field than in the uniform deformation field. Although grains with hard orientation have a lower deformation storage energy, they are still partially swallowed due to mutual influence between GBs near the triple junction. The high energies in the original GBs increase the migration rate at the triple junctions locally for a short period of time, but it is not enough to make a significant difference over extended periods at mesoscopic scales. The simulation results in this study are helpful in explaining the previous experimental observations.

Single-Atom Suture.

In atomically thin two-dimensional (2D) materials, grain boundaries (GBs) are ubiquitous, displaying a profound effect on the electronic structure of the host lattice. The random configuration of atoms within GBs introduces an arbitrary and unpredictable local electronic environment, which may hazard electron transport. Herein, by utilizing the Pt single-atom chains with an ultimate one-dimensional (1D) feature (width of a single atom and length up to tens of nanometers), we realized the suture of the electron pathway at GBs of diversified transition metal dichalcogenides (TMDCs). Theoretical calculations reveal that the construction of Pt single-atom sutures (SAS) prompts the emergence of electronic states proximal to the Fermi level, effectively modulating the transformation of the electronic structure from semiconductivity to metallicity. This transformation underscores the pivotal role of Pt SAS in reconfiguring the electron pathway. Benefiting from this, the Pt SAS-MoS2 emerges as an excellent catalyst, exhibiting an overpotential of 41 mV at 10 mA cm-2 and a Tafel slope of 54 mV dec-1 in hydrogen evolution reaction. Our results offer an understanding of the electron conduction pathway contributed by ultraordered atomic arrangement and the innovative mechanisms for future potential catalysts with an optimized architecture.

Precipitation-controlled grain boundary engineering in a cast & wrought Ni-based superalloy

Grain boundary engineering (GBE) describes the various approaches to control the fraction and composition of special boundaries in designing engineering materials. For example, complex thermo-mechanical processing routes are harnessed to increase the fraction of Σ3 twin boundaries, bringing about improvements to strength, toughness, and corrosion properties in austenitic stainless steels. However, high-temperature gas turbine engine components designed for the most demanding applications must be manufactured from Ni-based superalloys. To provide the required high-temperature strength, their microstructures are highly complex and consist of an austenitic γ-matrix, γ' precipitates, and grain boundary (GB) carbides, and/or borides. GBE approaches for Ni-based superalloys have been reported to reduce in-service GB cracking via targeted engineering of the GB microstructure. However, the same complex microstructures severely limit the processability of the most advanced Ni-based superalloys required to develop the next generation gas turbine engines.To tackle this challenge, we propose precipitation-controlled GBE for Ni-based superalloys with limited formability, via formation of GB serrations and precipitation upon slow cooling. We showcase our approach by achieving controlled GB precipitation of GB-γ' and M6C carbides in the cast & wrought Ni-based superalloy René 41 via a simple heat treatment. Ductility improvements are predicted via crystal plasticity modeling due to increased slip transmission compatibility. Instead of generating additional Σ3 twin boundaries only, these interfaces are directly incorporated into the GB network. It is shown that precipitation-controlled GBE is enabled by GB-γ' nucleating heterogeneously on M6C whereby competitive coarsening of GB-γ' provides the driving force. The fundamental mechanisms of our GBE approach are discussed and summarized in a qualitative microstructural model.

Point defects at grain boundaries can create structural instabilities and persistent deep traps in metal halide perovskites.

Metal halide perovskites (MHPs) have attracted strong interest for a variety of applications due to their low cost and excellent performance, attributed largely to favorable defect properties. MHPs exhibit complex dynamics of charges and ions that are coupled in unusual ways. Focusing on a combination of two common MHP defects, i.e., a grain boundary (GB) and a Pb interstitial, we developed a machine learning model of the interaction potential, and studied the structural and electronic dynamics on a nanosecond timescale. We demonstrate that point defects at MHP GBs can create new chemical species, such as Pb-Pb-Pb trimers, that are less likely to occur with point defects in bulk. The formed species create structural instabilities in the GB and prevent it from healing towards the pristine structure. Pb-Pb-Pb trimers produce deep trap states that can persist for hundreds of picoseconds, having a strong negative influence on the charge carrier mobility and lifetime. Such stable chemical defects at MHP GBs can only be broken by chemical means, e.g., the introduction of excess halide, highlighting the importance of proper defect passivation strategies. Long-lived GB structures with both deep and shallow trap states are found, rationalizing the contradictory statements in the literature regarding the influence of MHP GBs on performance.

Vector substrate design for grain boundary engineering: boosting oxygen evolution reaction performance in LaNiO3.

The realization and subsequent control of emerging structural and electronic phases in solid materials has significantly enhanced their functionalities, thereby benefiting both fundamental research and practical applications. The grain boundary (GB), as a transitional region within the crystal lattice, exhibits atomic shifts and distinct energy profiles. These unique characteristics offer a promising avenue for the discovery of advanced active catalytic phases for carbon, oxygen, hydrogen, and nitrogen evolution/reduction reactions. However, the challenge lies in isolating and controlling the quantity of grain boundaries in conventional catalysts, which hinders the identification of their functional attributes. In this study, we successfully engineered the (001)/(110), (001)/(111), and (110)/(111) GBs in LaNiO3 (LNO) using a vector substrate design approach. Subsequent evaluation of these GBs in the oxygen evolution reaction (OER) revealed that LNO (110)/(111) exhibited the fastest surface reconstruction into Ni oxyhydroxide and the most superior OER performance, achieving 2.36 mA cm-2 at η = 400 mV. This outstanding performance is attributed to its strongest Ni-O covalency and the proximity of the O 2p-band center to the Fermi level. This research aims to address the challenges associated with isolating and controlling GBs for optimized OER performance, while also providing comprehensive insights into the relationship between GBs and surface reconstruction behaviors.

Impact of grain boundaries on the electronic properties and Schottky barrier height in MoS2@Au heterojunctions.

Using density functional theory (DFT) calculations we thoroughly explored the influence of grain boundaries (GBs) in monolayer MoS2 composed of S-polar (S5|7), Mo-polar (Mo5|7), and (4|8) edge dislocation, as well as an edge dislocation-double S vacancy complex (S4|6), and a dislocation-double S interstitial complex (S6|8), respectively, on the electronic properties of MoS2 and the Schottky barrier height (SBH) in MoS2@Au heterojunctions. Our findings demonstrate that GBs formed by edge dislocations can significantly reduce the SBH in defect-free MoS2, with the strongest effect for zigzag (4|8) GBs (-20% reduction) and the weakest for armchair (5|7) GBs (-10% reduction). In addition, a larger tilt angle in the GBs leads to a more pronounced decrease in the SBH, suggesting that the modulation of SBH in the MoS2@Au heterostructure and analogous systems can be accomplished by GB engineering. Our findings also suggest that planar defects with high mobility in MoS2 may contribute to the memory switching effect observed in MoS2-based memtransistors and the reduction caused by the presence of planar defects can partially contribute to the discrepancy observed between experimental measurements and theoretical SBH predictions at the MoS2@Au heterojunction.

DFT Simulations of the Self-healing Behavior of a W〈110〉/W〈112〉 Grain Boundary in the Presence of Coexisting Point Defects

Light impurity atoms (LIAs), such as hydrogen and helium, tend to aggregate at pre-existing intrinsic point defects. This aggregation leads to detrimental effects, particularly in environments such as those foreseen in nuclear fusion reactors. There, such impurities would be ubiquitous, resulting in unacceptable material behavior that would unqualify the material as a Plasma Facing Material (PFM). One option to delay the degradation in performance is the use of nanostructured tungsten (NW), showing a large density of grain boundaries (GBs). Although we have already addressed the behavior of a single LIA in a GB, in this work we present the combined synergistic effects of the simultaneous presence of multiple LIAs, vacancies and Self-Interstitial Atoms (SIA) at semicoherent W/W interfaces using ab initio methods. Our results reveal a complex and interesting process in the competition between LIAs and SIAs. When the number of SIAs is low, He appears to hinder their recombination with vacancies, therefore casting doubts on the self-healing provided by NW. However, in the presence of larger numbers of SIAs, their mutual repulsion leads to the opposite behavior. Thus, a thorough thermodynamic assessment in which the evolution of the system may be tracked emerges as the crucial subsequent step in these investigations.

Triplet Reflection at Boundaries of Grains and Its Influence on Optical and Scintillation Characteristics of Organic Composite Detectors

Triplet Reflection at Boundaries of Grains and Its Influence on Optical and Scintillation Characteristics of Organic Composite Detectors

Role of twin thickness gradient and grain size gradient in work hardening of gradient nanotwinned metals

Abstract Gradient nanotwinned (GNT) structures exhibit superior strength and work hardening compared to homogeneous nanotwinned (HNT) structures. However, the relative effectiveness of twin thickness gradient (TTG) versus grain size gradient (GSG) on enhancing material properties remains unclear. In this study, the tensile behavior of various homogeneous, TTG, GSG, and dual gradient (DG) nanotwinned (NT) specimens was investigated using molecular dynamics simulations (MDs). The mechanisms of plastic deformation and the additional dislocation densities in different gradient structures were compared. The results indicate that both TTG and GSG structures facilitate dislocation slip within grains by stabilizing grain boundaries (GBs), dispersing shear bands, and promoting grain rotation. For the components in the DG structure, the extra dislocation density generated by the DG structure is approximately the average of the densities produced by the TTG and GSG structures individually. For the gradient structure as a whole, an appropriately designed DG can yield a higher extra dislocation density than a single gradient. This study contributes to the further enhancement of work hardening in NT metals through microstructural design.

Computational Characterization of the Structure, Energy, Strengths, and Fracture Resistances of Symmetric Tilt Grain Boundaries in Ice.

Using an interatomic potential that can capture the tetrahedral configuration of water molecules (H2O) in ice without the need to explicitly track the motion of the O and H atoms, coarse-grained (CG) atomistic simulations are performed here to characterize the structures, energy, cohesive strengths, and fracture resistance of the grain boundaries (GBs) in polycrystalline ice resulting from water freezing. Taking the symmetric tilt grain boundaries (STGBs) with a tilting axis of ⟨0001⟩ as an example, several main findings from our simulations are (i) the GB energy, EGB, exhibits a strong dependence on the GB misorientation angle, θ. The classical Read-Shockley model only predicts the EGB - θ relation reasonably well when θ < 20° or θ > 45° but fails when 20° < θ < 45°; (ii) two "valleys" appear in the EGB-θ landscape. One occurs at θ = 22° for Σ14(2̅31̅0) GB, and the other is at θ = 32° for Σ26(3̅41̅0) GB. These two GBs might be the most common in polycrystalline ice; (iii) all the STGB structures under consideration here are found to be a collection of edge dislocations with a Burgers vector of b = 1/3⟨112̅0⟩. The core structure of this edge dislocation is composed of a pentagon and a heptagon atomic ring. The separation and orientation of the structure units (SUs) at the GB exhibit a strong dependence on θ; (iv) the length of an atomic bond within the SUs, rather than EGB and θ which are often used in the literature, is identified as one controlling parameter that dictates the intrinsic GB cohesive strength; (v) characterization of the fracture resistance of the GB containing an initial crack is beyond the reach of nanoscale atomistic simulations but is feasible in concurrent atomistic-continuum (CAC) simulations that can simultaneously retain the atomic GB structure together with the long-range stress field within one model. The above findings provide researchers with a stepping stone to understand the complex microstructure of polycrystalline ice and its response to external forces from the bottom up. Such knowledge may be consolidated into constitutive rules and then transferred into the higher length scale models, such as cohesive zone finite element models (CZFEMs), for predicting how polycrystalline ice fractures at laboratory and even geophysical length scales.

Enriched Electrophilic Oxygen Species on Ru Optimize Acidic Water Oxidation.

Ruthenium oxide (RuO2) is considered one of the most promising catalysts for replacing iridium oxide (IrO2) in the acidic oxygen evolution reaction (OER). Nevertheless, the performance of RuO2 remains unacceptable due to the dissolution of Ru and the lack of *OH in acidic environments. This paper reports a grain boundary (GB)-rich porous RuO2 electrocatalyst for the efficient and stable acidic OER. The involvement of GB regulates the valence state of Ru and weakens the interaction between Ru and O, effectively facilitating *OH adsorption and *OOH formation. Notably, achieved a record-high catalytic activity (145mV at 10mAcm-2) with a low Tafel slope (40.9mVdec-1) and a remarkable mass activity of 332mAmg-1 Ru at 1.5V versus reversible hydrogen electrode is achieved. Additionally, the porous RuO2 exhibits superb stability with an ultra-low degradation rate of 26µVh-1 over a 50-day durability test. This study opens a viable pathway for the development of efficient and robust Ru-based acidic OER electrocatalysts.

Mechanisms Behind Graphitization Modification in Polycrystalline Diamond by Nanosecond Pulsed Laser.

The ultraprecision machining of diamond presents certain difficulties due to its extreme hardness. However, the graphitization modification can enhance its machinability. This work presents an investigation into the characteristics of the graphitization modification in polycrystalline diamond induced by a nanosecond pulsed laser. In this paper, the morphology of microgrooves under laser modification was observed, material deposition and graphitization in different regions were researched, and the regularities of microgrooves at different laser powers were obtained. A molecular dynamics (MD) simulation was carried out to reveal the mechanism behind graphitization modification; when the pulse laser acts on the diamond surface and the temperature rises to the critical temperature of graphitization, the graphite crystal nuclei form and grow, resulting in the graphitization modification. It was confirmed that the existence of grain boundaries (GBs) contributed to the graphitization of polycrystalline diamond during laser modification. It was predicted that a lower laser power could cause a higher proportion of graphitization. The results of ablation thresholds and the effect of the defocusing position on the graphitization of diamond showed that for a fixed laser power, the highest graphitization ratio could be obtained when the defocusing quantity was optimized. Finally, the results of precision grinding experiments verified the feasibility of using laser graphitization pretreatment to improve the efficiency and quality of precision grinding.

The effect of local geometry and relative energy on grain boundary area changes during grain growth in SrTiO3

AbstractThis study uses high‐energy X‐ray diffraction microscopy of SrTiO3 to identify correlations between grain boundary (GB) area changes and the motion direction of neighboring GBs to investigate interfacial energy minimization mechanisms during grain growth. The local GB area changes were measured near triple lines (TLs) to isolate the effects of neighboring GBs. These area changes were then correlated to the migration direction and curvature of the neighboring GBs present at the TL, providing an alternative metric associated with lateral expansion for describing GB migration. Additionally, this study extracted GB dihedral angles, which reflect the relative GB energy, to test whether low energy GBs replace high energy GBs (i.e., GB replacement mechanism) and, thus, can be used to predict a GB's migration direction. The majority of GBs did not exhibit local area changes reflective of the GB replacement mechanism, and the dihedral angles were not reliable indicators of GB motion. However, the expansion and shrinkage of GBs moving away from their center of curvature was more often consistent with the grain boundary replacement mechanism. These results suggest that growth for certain GB configurations is governed by relative energy differences while others are governed by curvature.

Study on the mechanical properties of gradient grains based on molecular dynamics

ABSTRACT This study used molecular dynamics simulations to investigate the plastic deformation mechanisms of gradient nano-grained (GNG) copper. The results show that GNG structures have higher yield stress compared to uniform nanocrystalline structures, with the g = 0.30 model exhibiting a significant increase in flow stress. GNG structures can effectively distribute atomic stress, transferring it from coarse grains to fine grains and reducing local stress concentration. Coarse grains undergo severe shear deformation, while grain boundary (GB) activities transition from migration in coarse grains to rotation in fine grains. GNG structures enhance the ability to coordinate deformation through GB motion, suppress dislocation formation, and facilitate interactions between dislocations and GBs, thereby improving strength and toughness.

Regulating Strain and Suppressing Defect States Through Polymer Ionization and Chemical Chelation for Efficient Inverted Flexible Perovskite Solar Cells.

Flexible perovskite solar cells (FPSCs) have great promise for applications in wearable technology and space photovoltaics. However, the unpredictable crystallization of perovskite on flexible substrates results in significantly lower efficiency and mechanical durability than industry standards. A strategy is investigated employing the polymer electrolyte poly(allylamine hydrochloride) (PAH) to regulate crystallization and passivate defect states in perovskite films on flexible substrates. The weakly acidic precursor allows PAH to undergo partial ionization, leading to the protonation of some ─NH3 + groups into ─NH3 +. Simulations and experimental results indicate that multifunctional PAH forms strong chemical interactions with precursors of perovskite materials including Formamidine Hydroiodide (FAI) and Lead Iodide (PbI2), facilitating homogenous nucleation and growth of crystals of perovskite films. High-resolution transmission electron microscopy (HR-TEM) reveals that PAH strongly anchors to grain boundaries (GBs), consistent with findings from photo-induced force microscopy-based infrared spectroscopy (PiFM-IR). PAH improves the uniformity distribution of Young's modulus between the grains and GBs, facilitating stress relief in the perovskite films. Therefore, the champion efficiency of PAH-modified FPSCs reaches 24.19% and exhibits strong bending durability, retaining 87.9% of their initial efficiency after 2500 bending cycles (r = 5 mm), demonstrating their practical application potential in outdoor wearable electronic products.

Deformation Behavior and Microstructural Evolution Coordinated Regulation by Compression Deformation for Metastable Ti Alloy.

In this paper, in order to investigate the harmonious relationship between the compression deformation behavior of metastable β titanium alloy and the microstructure evolution, the β solution-treated Ti-10V-2Fe-3Al (Ti-1023) alloy was compressed at room temperature and its deformation behavior was analyzed. Optical microscopy (OM) and field emission electron microscopy (FESEM) were used to study the microstructure evolution of alloys at different strain rates. The results show that the stress-induced martensite transformation (SIMT) is more easily activated by low strain rate compression deformation, which is conducive to improving its comprehensive mechanical properties. With the decrease in strain rate, the α″ martensite content increases significantly, the average grain size decreases substantially, and the Low Angle Grain Boundary (LAGB) volume fraction decreases correspondingly. In addition, after compression at different strain rates, the misorientation angle (MA) of the β matrix is mainly concentrated in the LAGBs. The change is small with the decrease in strain rate, but the α″ martensite orientation difference angle shows some peaks, which are ~60°, ~85°, and ~95°, respectively. Simultaneously, the strain rate has an important effect on the content and type of martensitic twins. Finally, the fracture morphology analysis shows that with the increase in strain rate, the fracture mode changes from ductile fracture to brittle fracture. The fracture surface presents a significantly elongated cavity along the direction of maximum shear stress.