Types Of Grain Boundaries Research Articles

Nonsymmetrical Segregation of Solutes in Periodic Misfit Dislocations Separated Tilt Grain Boundaries.

Interfacial segregation is ubiquitous in mulit-component polycrystalline materials and plays a decisive role in material properties. So far, the discovered solute segregation patterns at special high-symmetry interfaces are usually located at the boundary lines or are distributed symmetrically at the boundaries. Here, in a model Mg-Nd-Mn alloy, we confirm that elastic strain minimization facilitated nonsymmetrical segregation of solutes in four types of linear tilt grain boundaries (TGBs) to generate ordered interfacial superstructures. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy observations indicate that the solutes selectively segregate at substitutional sites at the linear TGBs separated by periodic misfit dislocations to form such two-dimensional planar structures. These findings are totally different from the classical McLean-type segregation which has assumed the monolayer or submonolayer coverage of a grain boundary and refresh understanding on strain-driven interface segregation behaviors.

Mapping Grains, Boundaries, and Defects in 2D Covalent Organic Framework Thin Films.

To improve their synthesis and ultimately realize the technical promise of two-dimensional covalent organic frameworks (2D COFs), it is imperative that a robust understanding of their structure be developed. However, high-resolution transmission electron microscopy (HR-TEM) imaging of such beam-sensitive materials is an outstanding characterization challenge. Here, we overcome this challenge by leveraging low electron flux imaging conditions and high-speed direct electron counting detectors to acquire high-resolution images of 2D COF films. We developed a Fourier mapping technique to rapidly extract nanoscale structural information from these TEM images. This postprocessing script analyzes the evolution of 2D Fourier transforms across a TEM image, which yields information about polycrystalline domain orientations and enables quantification of average domain sizes. Moreover, this approach provides information about several types of defects present in a film, such as overlapping grains and various types of grain boundaries. We also find that the pre-eminent origin of defects in COF-5 films, a prototypical boronate ester-linked COF, arises as a consequence of broken B-O bonds formed during polymerization. These results suggest that the nanoscale features observed are a direct consequence of chemical phenomena. Taken together, this mapping approach provides information about the fundamental microstructure and crystallographic underpinnings of 2D COF films, which will guide the development of future 2D polymerization strategies and help realize the goal of using 2D COFs in a host of thin-film device architectures.

On the relationship between modification of Bi2O3 by Sb and type of grain boundaries in ZnO-based varistors

The possibility of eliminating of electrically inactive areas from varistor body with maintaining varistor electrical parameters can be achieved by modifying Bi2O3 in such a way that a Bi-rich layer which surrounds the grains of ZnO is as thin as possible and by minimizing of the amount of additives so as not to form electrically inactive areas in varistor. It has been proven that modification of Bi2O3 with small quantities of other MeO results in significant changes in the varistor structure, thereby gaining the uniform distribution of the intergranular phase. The work contains the results on which Sb2O3 were added in an amount of 15 mol%. After mixing with Bi2O3 they were sintered and shivered. Modified Bi2O3 was added to the varistor bulk in quantities of 1.0, 0.3, 0.2 and 0.1 mol% to study the varistor properties. The best results were obtained for the varistor doped with 0.3 mol% Bi2O3 modified by Sb, which has the best compatibility with the mathematically model calculated by statistical methods describing 100% of conductive grain boundaries. The obtained results of the varistors microstructure modification led additionally to reduce amounts of additives for about 30% and thus to lower at about 10% of production costs.

Shear-coupled grain boundary migration in bicrystal Ni with metallic dopant segregation

The shear-coupled grain boundary (GB) migration in bicrystal Ni with metallic dopant segregation was investigated by the molecular dynamics simulations. Different from the approximately linear relation of the GB migration of pure bicrystal Ni with the nominal shear strain, the curve of doped bicrystal Ni can be divided into three stages. The threshold strain, saturated strain, and saturated GB migration displacement can be used to characterize them. They are considerably affected by the Cr concentration in GB, temperature, and dopant type. The higher the dopant concentration is or the lower the temperature is, the greater the resistance to GB migration is. Cu dopant induces the greatest resistance, Cr and Fe dopants have great effect on the GB migration, but Co has almost no influence. All these hindering effects can be explained from the variation of the number of pinning points induced by the dopant atoms in GB.

Local Inhibition by 2-mercaptobenzothiazole of Early Stage Intergranular Corrosion of Copper

Corrosion inhibition by 2-mercaptobenzothiazole (MBT) at the surface termination of various types of grain boundaries (GBs) was studied at the nanometer scale on microcrystalline copper in HCl acid solution using in situ electrochemical scanning tunneling microscopy (ECSTM). Macroscopic electrochemical analysis by cyclic voltammetry showed highly effective inhibition of Cu(I) active dissolution blocked by MBT pre-adsorption in a potential range of 0.15–0.2 V. ECSTM analysis of the initial stages of intergranular corrosion confirmed the mitigation of net intergranular dissolution by the pre-adsorbed MBT surface layer but also revealed the local accumulation of reaction products in the GB regions. For Coincidence Site Lattice boundaries other than coherent twins, intergranular dissolution, mitigated by the pre-adsorbed MBT layer, and protection by intergranular formation of a film of reaction products were observed. For random GBs, protection by reaction products was dominant, in agreement with their more reactive intrinsic character, generating more Cu(I) ions under anodic polarization and thus promoting the formation of a protective film of reaction products. Coherent twins did not show preferential intergranular reactivity compared to adjacent grains, indicating equally strong efficiency than on grains. These results bring new insight on how inhibition operates locally at various types of GBs.

The role of binding energies for phosphorus and sulphur at grain boundaries in copper

In recent years it has been found that the segregation energy for phosphorus atoms to grain boundaries (GB) can vary strongly with atomic position in copper. This should be contrasted to the unique value for the segregation energy that has been used for property predictions of creep strength and ductility, which are crucial properties for copper canisters for final disposal of spent nuclear fuel. In the present paper, segregation energies for P and S atoms at different atomic sites for three symmetric tilt GB types: Σ3, Σ5 and Σ11 have been computed using density functional theory (DFT). We found that the segregation energies decrease with distance from the GB plane. However, for the GBs studied, the largest energies are not found at the GB planes, but instead at the first neighboring sites relative to the GB planes. The segregation energies are typically much larger for Σ5 than for Σ3 and Σ11. Subsequently, an effective segregation energy has been defined and it is used to predict the occupancy of P and S atoms at the GBs. Based on these data, the release of free energy during cavity formation has been computed. The release of free energy is lower for copper with phosphorus (Cu-OFP) than for phosphorus free copper (Cu-OF) indicating that cavity generation occurs more readily in the latter material. The data shows with good margin that creep cavitation in Cu-OFP does not lead to brittle creep rupture and that the creep ductility is quite satisfactory.

Dislocation–grain boundary interaction-based discrete dislocation dynamics modeling and its application to bicrystals with different misorientations

Grain boundaries (GBs) have a significant influence on the mechanical properties of metallic materials. It has been a great challenge to describe dislocation interactions with various GBs. In the present article, a generalized dislocation–GB interaction model was constructed and then implemented in the three-dimensional multiscale discrete dislocation dynamics (DDD) framework. In the model, two dislocation–GB interaction mechanisms, i.e., dislocation absorption at GBs and dislocation emission from GBs, were considered. In order to make the dislocation–GB interaction model suitable for various GB types, a ‘coarse-graining’ approach was applied to deal with the process of dislocation absorption and emission. As the validations and applications of the proposed dislocation–GB interaction model, nanopillars containing a non-sigma large-angle GB and subjected to uniaxial compression were studied. The simulated results show that the bi-crystalline nanopillars possess a higher yield strength and flow stress, smaller stress-drop size than single-crystalline counterparts, which is consistent with earlier experimental observations in the literature. Afterward, the DDD simulation was employed to reveal the effect of GB misorientation on the mechanical responses of bicrystals with a large-angle-symmetric-tilt GB. Simulations indicate that the mechanical responses of bicrystals are affected by the GB structures and complex dislocation–dislocation and dislocation–GB interactions. In contrast, the dislocation absorption and emission events, as well as the evolution of resolved shear stress and dislocation density, do not depend on the GB misorientation angles or the GB strength (or the GB energy).

The Mobile and Pinned Grain Boundaries in 2D Monoclinic Rhenium Disulfide.

In bulk crystals, the kinetics of dislocations is usually hindered by the twining boundaries (TB) or grain boundaries (GB), rendering the well‐known grain boundary strengthening effects. Nevertheless, here it is found that in 2D rhenium disulfide (ReS2), twinning is much easier than dislocation slip. Consequently, the highly mobile TBs or GBs are inversely pinned by the relatively immobile dislocations. Due to the strong in‐plane covalent bonding, the GBs in high‐symmetry 2D materials such as graphene which consists of defects are immobile at room temperature. In contrast, in monoclinic 2D ReS2 several types of GBs (including TBs) can be readily generated and driven by mechanical loading. A complete library of the GBs in 2D ReS2 is established by the (in situ) atomic‐scale transmission electron microscopy (TEM) characterizations and density functional theory (DFT) calculations. The twinning (shear) stresses for 2D ReS2 are estimated as low as 4–30 MPa, one or two orders of magnitude lower than the traditional bulk materials. Full elucidation on the GB structures and especially the intriguing GB kinetics in such anisotropic 2D materials are of fundamental importance to understand the structure–property relationships and develop strain‐tunable applications for 2D materials in future.

A generalised hot cracking criterion for nickel-based superalloys additively manufactured by electron beam melting

Abstract The influences of microstructure and solidification parameters on the hot cracking behaviour of an additively manufactured Ni-based superalloy via selective electron beam melting (SEBM) have been investigated. Severe intergranular hot cracking is found to occur at grain boundaries (GBs) in the SEBM-built superalloy. A generalised cracking criterion that considers various process parameters along with the GB inclination is developed on the basis of a prevailing hot cracking criterion for columnar-grained microstructure. A parametric evaluation of influencing factors is quantitatively conducted to predict the optimal conditions that could prevent hot cracking while maintaining the desirable fine microstructure. With the aid of numerical simulations, it is determined that while micro-segregation occurs at all types of GBs, its occurrence at the divergent ones tend to promote cracking. Using the results of this work, hot cracking in additively manufactured Ni-based superalloy single crystals can be potentially mitigated.

Effects of grain boundary structures on primary radiation damage and radiation-induced segregation in austenitic stainless steel

Grain boundary (GB) engineering is crucial in the austenitic stainless steel (ASS) design for nuclear energy applications. In this work, the influence of different GB structures on radiation defect recombination and radiation-induced segregation (RIS) at different temperatures were investigated using molecular dynamics simulation. Four typical GBs in ASSs were selected as model structures. Results showed that GBs remained stable at various temperatures and they all exhibited better self-healing performance than single crystals in terms of radiation defects. However, except Σ3(112) GB, other three GBs cannot inhibit the radiation induced segregation, while promoting the radiation defect recombination. Calculation results showed that the higher Σ value of GBs can lead to a greater lattice mismatch near GBs, which not only results in stronger sink strength for radiation induced defects, but also provides more sites for solute atoms and causes greater segregations eventually. Owing to the intrinsic low Σ and large inclination angle characteristic, Σ3(112) GB achieves an excellent balance between the defect-absorption and RIS. This phenomenon provides a feasible route for the future GB design in ultra-high radiation tolerant materials.

Recrystallization-based grain boundary engineering of 316L stainless steel produced via selective laser melting

Grain boundary engineering (GBE) is a thermomechanical processing strategy to enhance the physical and mechanical properties of polycrystalline metals by purposely incorporating special types of grain boundaries—such as twin boundaries (TB)—in the microstructure. Because of the multiple strain-annealing cycles involved, conventional GBE is not directly applicable to near-net-shape parts, such as those produced via additive manufacturing (AM) technology. In this study, we explore a different GBE processing route that leverages TB multiplication during recrystallization of austenitic 316L stainless steel produced via selective laser melting (SLM). We find that recrystallization requires a minimum level of mechanical deformation, which scales with the laser scanning speed employed during SLM. We ascribe this relationship to the cell size and the amount of solute segregating at cell boundaries during rapid solidification, which are inversely and directly proportional to the laser scanning speed, respectively. The coarser the cell structure and the more uniform the chemical composition, the easier the nucleation and growth of recrystallized grains. Our results provide the groundwork for devising AM-compatible GBE strategies to produce high-performance parts with complex geometry.

Quantifying Electrocatalytic Reduction of CO2 on Twin Boundaries

Summary A protocol relying on the use of silver nanostructures with well-defined dimensions and morphologies (e.g., nanocubes and nanowires) has been developed to differentiate and quantitatively determine the atom-specific activities of different surface atoms toward catalyzing electrochemical CO2 reduction reaction (CO2RR). The atom-specific activity of the twin-boundary edges (TBEs) is comparable to (or slightly higher than) that of the single-crystal edges (SCEs) formed from the crossing of two {100} surfaces, and it is on the order of ∼16 attoampere/atom (aA/atom) toward electrochemical CO2RR. This value is more than two orders higher than the atom-specific activity of the {100} surface atoms, i.e., ∼0.1 aA/atom. The high catalytic activity of the TBEs, which represent a class of commonly existing surface defects in stable metal nanostructures with face-centered cubic lattices, is consistent with the DFT calculations, in which the bridge-type binding configuration of COOH∗ at the TBEs stabilizes this intermediate to promote overall CO2RR.

Effect of hydrogen on deformation behavior of Alloy 725 revealed by in-situ bi-crystalline micropillar compression test

Nickel-based Alloy 725 bi-crystalline micropillars with different types of grain boundaries (GBs) were compressed in hydrogen-free and in-situ hydrogen-charged conditions to investigate the hydrogen effect on the deformation behavior of the selected GBs. In the presence of hydrogen, the compressive stresses on the micropillars increase regardless of the GB type. It was proposed that this hydrogen-induced hardening behavior is the synergistic effect of hydrogen-enhanced dislocation multiplication and interactions, the pinning effect of hydrogen on dislocation motion, and hydrogen-enhanced lattice friction. Transmission electron backscatter diffraction (t-EBSD) results demonstrate that both low-angle GBs and high-angle GBs can effectively suppress dislocation transmission through the GBs, resulting in dislocations pile up along the GBs in the hydrogen-charged condition. In contrast, this behavior was not observed in the micropillars with twin boundaries.

Defects and grain boundary effects in MoS2: A molecular dynamics study

Mechanical properties of low-temperature large area chemical vapor deposited (CVD) transition metal dichalcogenides such as MoS2 are a function of crystallinity, which tends to deteriorate with the presence of grain boundaries (GBs) and defects. In this study, we report mechanical properties of polycrystalline as well as single crystal MoS2 containing defects and dopant atoms. To investigate mechanical properties we adopted computational approach using classical molecular dynamics (MD) simulation. Our calculated mechanical properties such as tensile strength, Young's modulus of single-crystal MoS2 are in good agreement with the existing literature and alter with the appearance of GBs and defects. Polycrystalline MoS2 samples exhibit GB strengthening i.e., Hall-Petch effects. A detailed investigation of a specific type of GB tilted sample also shows GBs insensitive fracture behavior. A small amount of sulfur vacancy and oxygen doping (<2%) exhibit ductility in the sample at the expense of failure strength. We also notice local plastic deformation which yields ductility in the sample. Our present study shows the detailed mechanism behind the plastic deformation behavior of single as well as polycrystalline sample.

Atomistic Modeling of Grain Boundary Migration in Nickel

Herein, the molecular dynamics (MD) simulations of pure Ni crystallites are performed to show the influence of the grain boundary (GB) geometry on the values of the activation energy of GB migration. The considered systems are bicrystal with Σ5[010] tilt plane boundary, spherical grain with initial curvature radius 5 nm, and polycrystalline 30 nm × 30 nm × 30 nm block. The motion of three types of GBs (flat, spherical, and polycrystalline) at constant temperatures and no applied forces is studied. The obtained values of activation energy are 0.45, 0.11, and 0.57 eV for flat, spherical, and polycrystalline types of GBs, respectively. These values are smaller than those that are reported in experimental works, which is a common issue for atomistic simulations of GB migration. Possible sources of such disagreement and ways to overcome it are discussed. The particular part of this work is devoted to the development of the automated analysis of polycrystalline structure. This analysis provides detailed information on grain size distribution and its evolution in time.

Study on crack growth and fatigue life in TC6 titanium alloy by integrated phase proportion and grain size modeling

In this paper, a theoretical model is developed to reveal the crack growth and fatigue life in polycrystalline alloys with dual-phase microstructure. The present quantitative analysis proves that the grain size and phase proportion can efficaciously affect the crack growth and fatigue life. The dislocations emitted from the crack tip and penetrated three types of grain boundaries (GBs) under cyclic shear stress. According the different boundaries, we obtained the calculation of three kinds of critical penetration stress (τcr) and dislocation stress (τ1). Moreover, it can be worked out the crack growth and fatigue life. As the results suggest that the fatigue crack growth reduces with decreases of grain size and increases of minor phases proportion (Xβ). Comparison of fatigue life prediction, we derived the fatigue fracture characteristics. In a word, the results offer awareness to improve the mechanical properties in dual-phase titanium alloy by optimizing the microstructures.

Extraordinary Response of H-Charged and H-Free Coherent Grain Boundaries in Nickel to Multiaxial Loading

The cohesive strength of Σ 3, Σ 5, and Σ 11 grain boundaries (GBs) in clean and hydrogen-segregated fcc nickel was systematically studied as a function of the superimposed transverse biaxial stresses using ab initio methods. The obtained results for H-free GBs revealed a quite different response of the coherent twinning boundary Σ 3 to the applied transverse stresses in comparison to the other GB types. While the cohesive strength of Σ 5 and Σ 11 GBs increased with increasing level of tensile transverse stresses, the strength of Σ 3 GB remained constant for any applied levels of transverse stresses. In the case of GBs with segregated hydrogen, the cohesive strength of Σ 3 was distinctly reduced for all levels of transverse stresses, while the strength reduction of Σ 5 and Σ 11 GBs was significant only for a nearly isotropic (hydrostatic) triaxial loading. This extraordinary response explains a high susceptibility of Σ 3 GBs to crack initiation, as recently reported in an experimental study. Moreover, a highly triaxial stress at the fronts of microcracks initiated at Σ 3 boundaries caused a strength reduction of adjacent high-energy grain boundaries which thus became preferential sites for further crack propagation.

Anomalous 3D nanoscale photoconduction in hybrid perovskite semiconductors revealed by tomographic atomic force microscopy

While grain boundaries (GBs) in conventional inorganic semiconductors are frequently considered as detrimental for photogenerated carrier transport, their exact role remains obscure for the emerging hybrid perovskite semiconductors. A primary challenge for GB-property investigations is that experimentally they need to be performed at the top surface, which is not only insensitive to depth-dependent inhomogeneities but also could be susceptible to topographic artifacts. Accordingly, we have developed a unique approach based on tomographic atomic force microscopy, achieving a fully-3D, photogenerated carrier transport map at the nanoscale in hybrid perovskites. This reveals GBs serving as highly interconnected conducting channels for carrier transport. We have further discovered the coexistence of two GB types in hybrid perovskites, one exhibiting enhanced carrier mobilities, while the other is insipid. Our approach reveals otherwise inaccessible buried features and previously unresolved conduction pathways, crucial for optimizing hybrid perovskites for various optoelectronic applications including solar cells and photodetectors.

Effects of severe-strain-induced defects on the mechanical response of two kinds of high-angle grain boundaries

ABSTRACT Grain boundaries of metallic materials subjected to severe plastic deformation exhibit significantly enhanced diffusivity and excess energy compared with their relaxed poly- or bi-crystalline counterparts even when the macroscopic degrees of freedom are the same in both types of grain boundaries. Boundaries of excess energy are/can be relaxed by annealing. As a first step in accounting for this experimentally observed high-energy state of general high-angle grain boundaries subjected to severe plastic deformation, a concept of localised basic shear units and the presence of localised extra free volume in these units situated in different locations in the grain boundaries, which was originally proposed to explain steady-state structural superplastic flow, is made use of. Using MD simulation, the mechanical response of these modified grain boundaries is compared with that of their relaxed state. The results are also compared with a case of a homogeneous distribution of extra free volume within the grain boundary. The localised shear units containing extra free volume introduced in the grain boundaries are found to alter their physical and mechanical features strongly, which, in turn, drastically affect, consistent with experimental results, the mechanical response of the heavily deformed material.

The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

A model of discontinuous precipitation of γ′ phase at grain boundaries (GBs) in polycrystalline Ni-based superalloys during continuous cooling from supersolvus temperature was developed. The model calculates the size and γ′ phase fraction in GB precipitates as a function of cooling rate and GB mobility. The model is based on the classical mechanism of diffusion-controlled precipitation. It considers fast diffusion of γ′-forming elements along the GB and the motion of the GBs under driving force induced by alloy decomposition. The model predicts either discrete GB gamma-prime particles and GB serration or the growth of fan-type structures depending on the cooling rate and GB mobility. We conclude from the model predictions that the GB mobility is the key factor controlling the type and morphology of GB precipitates. High GB mobility results in formation of fan-type structures, while the lower GB mobility leads to GB serration. The predicted dependence of the size of GB precipitates on the cooling rate is in good agreement with the experimental observations.