Grain Boundaries Research Articles

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.

Effect of dynamic recrystallization on microstructure and properties of multilayer Al foils friction stir welding

In this study, friction stir welding (FSW) was used to weld multi-layer Al foil and Al plate, and the defect-free welded joint was successfully obtained. In the stirring zone (SZ), due to the extrusion of the shoulder and the rotation of the stirring pin, the original layered structure of SZ is destroyed, and the original grains are destroyed and dynamically recrystalized to form fine equiaxed grains. With the decrease of welding speed, the heat input increases, the dynamic recrystallization degree (DRX) of SZ increases, the proportion of low angle grain boundaries (LAGBs) increases from 20.5% to 45.0%, and the grain size of SZ decreases from 1.53 to 0.93 μm, which affects the performance of the joint. The hardness of each welding parameter SZ is higher than that of the base material. With the increasing degree of DRX, the hardness of SZ continues to rise, and the electrical conductivity decreases from 57.5% international annealing copper standard (IACS) to 52.5%-56.2% IACS. The tensile strength of the joint reaches its maximum first and then decreases with the increase of DRX.

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.

Effects of grain boundary phases on recoil loops by in situ observation of magnetization behavior in nanocrystalline PrNd–Fe–B magnets

The grain boundary phase (GBP) has a significant influence on the magnetization behavior in nanocrystalline PrNd–Fe–B magnets. The current study demonstrates that reversible/irreversible magnetization behavior and the phenomenon of open recoil loops are related to both the nature of GBPs and the magnetization state by in situ observation. The optimization of GBPs nature (increase the volume fraction and improve the composition of GBPs) leads to the suppression of reversible magnetization behavior and the phenomenon of open recoil loops at low fields. Since the asymmetric magnetic domain structure appears only at low cycle fields, the openness phenomenon originates from the weak pinning grain boundaries (GBs). In addition, optimization of the GBPs also enhances pinning strength and uniformity, which contributes to the domain walls being pinned in the GBs at higher external fields. At this moment, the domain wall is dominated by irreversible magnetization behavior, and the openness phenomenon disappears. This proves that the coercivity mechanism is transformed from inhomogeneously weak pinning to homogeneously strong pinning with the optimization of GBPs. Consequently, the coercivity and squareness factor are significantly enhanced. This study sheds light on the understanding of the effects of GBP's nature on recoil loops and coercivity mechanism, and it also provides significant guidance for the development of advanced permanent magnets.

Structural-phase state and properties of 56GM/(W + WC(Ni)) composite alloy obtained by wire electron beam additive manufacturing

The authors investigated the microstructure and mechanical characteristics of 56GM steel-based composite produced by wire electron-beam additive manufacturing with the addition of W + WC(Ni) powders during printing. The analysis demonstrates that 56GM/(W + WC(Ni)) composite alloy is characterised by a gradient structure consisting of 56GM base layer, 56GM – 56GM/(W + WC(Ni)) intermediate layer and 56GM/(W + WC(Ni)) composite layer. The base layer of 56GM steel is characterized by a multidirectional acicular structure, which corresponds to the ferrite-martensite state. In 56GM – 56GM/(W + WC(Ni)) intermediate layer the acicular structure becomes less pronounced. In 56GM/(W + WC(Ni)) composite layer an equiaxed grain structure is formed, with an average grain size of 8.59 μm, along the boundaries of which cracks are observed. WC particles are located mainly along the boundaries of small grains and in small quantities inside the grains themselves. It was found that 56GM/(W + WC(Ni)) composite is mainly composed of α-Fe (~80.6 vol. %), Ni (~6 vol. %), WC carbide phase (~10.3 vol. %) and γ-Fe (3 vol. %). The structure and properties of initial 56GM steel change both in the area of direct addition of alloying powder and in the underlying layers due to diffusion processes and infiltration of W + WC(Ni). Microhardness values increase from ~3.5 GPa to ~6.5 GPa with distance from the substrate to the composite layer. In uniaxial tensile tests, the ultimate tensile strength and yield strength values reached 1100 – 1200 MPa and 835 MPa in the intermediate layer, respectively.

Effect of Ag solute atom and N vacancy on CrAlN Σ5 (012) [100] grain boundary properties by first-principles study

ABSTRACT The solute atoms and point defects at the nanocrystalline material grain boundary (GB) have a significant impact on their macroscopic properties. This study utilised first-principles calculations to investigate the impact of the single Ag solute atom and N vacancy defect on the properties of CrAlN Σ5 (012) [100] GB. The results indicate that Ag atoms preferentially segregate in the gap between two grains, increases the probability of N vacancies formed at GB. The volume effect of Ag atom causes GB expansion and reduces the tensile strength. N vacancy decrease tensile strength by causing GB contraction and increasing regions with low charge density. Ag atom and N vacancy increase the mobility of GB. During the sliding process of a clean GB, it initially transitions to a Kingery type GB and eventually to an unstable Duffy-Tasker type GB. The peak value of the sliding energy barrier for the Ag atom segregation GB is attributed to the separation of grains during the sliding process. Finally, the bonding mechanism of atoms at the GB interface was analysed using density of states (DOS) analysis. The research results provide a fundamental understanding of using GB segregation to change the mechanical properties of materials.

Ion Migration at Metal Halide Perovskite Grain Boundaries Elucidated with a Machine Learning Force Field.

Metal halide perovskites are promising optoelectronic materials with excellent defect tolerance in carrier recombination, believed to arise largely from their unique soft lattices. However, weak lattice interactions also promote ion migration, leading to serious stability issues. Grain boundaries (GBs) have been experimentally identified as the primary migration channels, but the relevant mechanism remains elusive. Using molecular dynamics with a machine learning force field, we directly model ion migration at a common CsPbBr3 GB. We demonstrate that the as-built GB model, containing 6400 atoms, experiences structural reconstruction over several nanoseconds, and only Br atoms diffuse after that. A fraction of Br atoms near the GB either migrate toward the GB center or along the GB through different migration channels. Increasing the temperature not only accelerates the ion migration via the Arrhenius activation but also allows more Br atoms to migrate. The activation energies are much lower at the GB than in the bulk due to large-scale structural distortions and favorable non-stoichiometric local environments available at GBs. Making the local GB composition more stoichiometric by doping or annealing can suppress the ion migration. The reported results provide valuable atomistic insights into the GB properties and ion migration in metal halide perovskites.

Study on the Microstructure and Corrosion Behavior of Dissimilar Aluminum Alloy Welded Joints Formed Using Laser Welding.

This study investigates the evolution mechanisms and electrochemical corrosion behavior of laser-welded joints (WJs) between 6063 and 6082 dissimilar aluminum alloys under varying welding powers. The analysis focused on the microstructure of the weld metal zone (WMZ), its grain boundary (GB) features, and its electrochemical corrosion properties. Data from the experiments indicate that a higher laser power (LP) leads to an increase in grain size within the WMZ. At an LP of 1750 W, the weld surface exhibits the poorest corrosion resistance, while other parameters show a relatively better resistance. Additionally, electron backscatter diffraction tests indicate that the high-angle GB fraction on the 6063-T6 side of the heat-affected zone exhibits a substantially reduced measurement compared to the 6082-T6 side. The corrosion form in the WMZ is intergranular, with energy-dispersive spectroscopy (EDS) scans revealing that the poor corrosion resistance is primarily due to the presence of a large amount of Mg2Si phase.

Ultra-large springback bending strain and its atomistic mechanism in Ni nanowires

Springback is one of the most interesting mechanical phenomena for bent metals after stress/strain release. Most previous studies investigate the springback behaviors of metals only with low bending angles; rarely do studies investigate these metals under large bending angles. Here, we investigated the springback behaviors of twin-structured Ni nanowires (NWs) at various bending angles through in-situ experiments and molecular dynamics (MD) simulations. We discovered that the springback angle and deformation mode of twin-structured Ni NWs can be divided into three stages, which have rarely been reported. The results revealed that, as the maximum bending angle increases, the plasticity of the NWs transfers from partial dislocations parallel to the twin boundaries (TBs) to extended and full dislocations on multiple slip systems, and then to grain boundary (GB) generation. Unlike previous studies which suggested that deformation-induced dislocations and GBs are irreversible, our results show that these defects are reversible upon unloading. The different recoverable abilities of these deformation-induced defects lead to a bending-angle-dependent springback phenomenon.

Influence of planar defects on the mechanical behaviors of spherical metallic nanoparticles

Abstract In present study, we adopt molecular dynamics simulations to investigate the influences of typical planar defects, including twin boundaries (TBs), stacking faults (SFs) and grain boundaries (GBs), on the mechanical properties of fcc copper nanoparticles. Groups of nanoparticle samples, including defect-free single crystal and those with specific defects, are examined for elastic modulus, yield strength, and deformation mechanisms. Detailed results reveal that the elastic behavior of nanoparticles can be well described by a modified theoretical model regardless the type of defects. While the planar defects have negligible influence on the elastic modulus, they significantly enhance the yield strength of nanoparticles. Notably, nanoparticles containing fivefold TBs exhibit the highest yield stress, i.e. ∼17.0 GPa, even surpassing that of the defect-free counterparts, i.e. ∼10.0 GP. Analysis of atomic deformation unravels that the distinct yielding behaviors are attributed to the activation of different slip systems and the nucleation of dislocations at specific preferential sites. These findings highlight the potential of fabricating planar defects to tailor the mechanical properties of metallic nanoparticles for targeted applications in nanotechnology and materials science.

Precipitation behavior of G-phase and its effect on mechanical properties of 30Cr2Ni4MoV steel

Precipitation behavior of G-phase and its effect on mechanical properties of 30Cr2Ni4MoV steel during thermal aging at 350–600 °C for 0–10,000 h are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atom probe tomography (APT), hardness and impact tests. Aging at 500 °C, the martensitic matrix of 30Cr2Ni4MoV steel basically keeps stable. With the lath width almost remaining unchanged, carbides coarsen to a slight degree, but G-phase will precipitate at grain boundary (GB). The G-phase is Ni16Mn6Si7 and its “nose” temperature is around 500–550 °C. Due to relatively high enrichment level of forming elements including Ni, Mn and Si at GB, the G-phase can nucleate at triple junction or adjacent to M23C6 at GB. Subsequently, the G-phase grows along GB until contacts with M23C6, and then coarsens along the width direction, which results in alternating distribution morphology of G-phase and M23C6 at GB. The Vickers hardness results show that the hardness remains stable due to the relatively stable microstructure after aging. After aging for 10,000 h, the hardness undergoes little decrease due to the slight coarsening of M23C6 and G-phase. Interestingly, the impact energy drops obviously after the precipitation of G-phase. The reason is that the G-phase at GB promotes the initiation and propagation of cracks during the impact process, leading to intergranular fracture (IGF).

Mesoscopic Modelling of Li Deposition in Sulfide-Based Solid Electrolyte Using Two-Dimensional Artificial Polycrystal Structure

Abstract Understanding Li nucleation and growth mechanism during charging in solid electrolytes (SEs) is essential in development of all-solid-state batteries with metallic Li, because the Li dendrite could cause internal short-circuit by penetration of SE. However, it is still under debate how the factors including degradation of SE layer, the stacking pressure, and the microstructure affect the Li nucleation and growth in SE. In this study, the coupled current-deposition-stress models using the two-dimensional artificial SE structures have been developed by combination of finite element method and Monte Carlo simulations. The model assumed that Li flux on the SE grain induces an eigen strain in the deposited Li region, and Li grows into the SE layer by breaking grain boundary (GB). Degradation of SE was modelled as the decrease of fracture strength of GB using a coefficient. The effects of these microstructure and operation factors on GB fracture and Li deposition have been evaluated and discussed.

Simulating hydrogen-controlled crack growth kinetics in Al-alloys using a coupled chemo-mechanical phase-field damage model

Environmentally Assisted Cracking (EAC) of 7xxx series aluminium alloys involves interactions between multiple physical phenomena, which ultimately influence the in-service life of critical components. In this work, we present a new model to study EAC in 7xxx series alloys, which is implemented in the multiphysics simulation framework, DAMASK. The chemo-mechanical model couples crack tip hydrogen generation, resulting from surface oxidation, and transport, with crystal-plasticity-governed intergranular crack propagation, through the microstructural trapping of hydrogen at dislocations, grain boundaries (GB), and crack tip stress fields. Large-scale simulations with realistic grain structures have been performed to provide novel insight into the dominant rate-controlling processes associated with intergranular EAC in 7xxx series aluminium alloys. The model was able to reproduce experimentally measured crack velocities under different loading conditions. Parametric studies indicate that, in addition to the GB network morphology, the crack growth rate was controlled by hydrogen generation at the crack tip with long-range diffusion having negligible influence. Additionally, the total hydrogen generated through crack tip oxidation appears to be more significant than the peak generation rate.

Tensile behavior and microstructure evolution of Mo-14 %Re alloy at room temperature and elevated temperatures

The present work investigates the tensile deformation behavior and fracture mechanism of Mo-14 %Re alloy (MR14) at both room and high temperatures. The results show that the tensile strength of MR14 alloy is 613 MPa at room temperature and 260 MPa at 1000 °C. Moreover, the MR14 exhibits remarkable ductility, with elongation surpassing 60 % both at ambient temperature and elevated temperatures. As the temperature rises, dynamic recrystallization and the rotation of subgrains lead to the evolution of texture during fracture, which has not been reported hitherto. A high proportion of low-angle grain boundaries (LAGBs) facilitates the dislocation transfer, thereby preventing the nucleation of cracks caused by stress concentration. The strong texture, abundant substructure, movable edge dislocation and mixed dislocation on the bundle structures are the main toughening factors. Additionally, during the high-temperature tensile deformation process, the MR14 alloy maintains a single-phase structure without precipitation, demonstrating excellent thermal stability. These research findings hold profound significance for broadening the application scope of the MoRe alloy.

Effect of grain boundaries on thermal transport in bi-layer graphene nano-ribbons

Defects and grain boundaries (GBs) in graphene often form during its growth and have been extensively characterized experimentally. Moreover, in graphene with two or more layers, distinct defect profiles have been identified in different layers. Although these defects and GBs are known to reduce the thermal transport in monolayer graphene, their impact on the overall thermal transport in graphene with two or more layers remains obscure, especially when unique defect profiles exist in different layers. In this study, we employ molecular dynamics simulations to investigate the role of GBs in one of the bi-layer graphene nanoribbons (GNRs), which results in a moiré-like pattern on one side of the GB. We discovered that while the GBs in one of the bi-layer GNR sheets reduce the overall in-plane thermal conductivity, κ, the reduction is mitigated by the pristine layer due to the interlayer van der Waals interaction. By closely examining different phonon modes in individual layers, we elucidate the κ reduction mechanisms in each layer. Our findings offer valuable insights into thermal engineering in graphene-based heterostructures as well as into exotic graphene bi-layer structures, such as those with moiré patterns.

Evolution of microstructure and properties of nanocrystalline NiCo alloy under high-energy laser shock

In this study, nanocrystalline NiCo alloy was prepared through electrodeposition and subjected to high-energy laser shock (HELS) treatment to investigate the influence of laser shock on the microstructures and properties. An analysis of phase composition, microstructural evolution, texture evolution, and hardness was conducted. The results indicated that under HELS, there was no change in phase composition, while there was a slight reduction in grain size, as well as in the quantities of Low-angle grain boundaries (LAGBs) and twin boundaries (TBs). The texture evolution revealed that HELS led to a more uniform overall texture due to grain boundary (GB) mediate deformation mechanisms, along with the generation of a significant amount of 9R phases, causing a shift in texture orientation from (110) to (100). The internal structure of nanocrystalline NiCo showed an accumulation of numerous dislocations post-laser shock, with randomly oriented dislocations interacting to form Lomer-Cottrell Locks (L-C Locks) and 9R phases. Additionally, interactions with large stacking faults (SFs) also resulted in their dissociation into multiple 9R phases. The hardness enhancement was mainly attributed to the accumulation of dislocations within the grain and the generation of a significant quantity of 9R phases.