Grain Boundary Structure Research Articles

Role of grain boundary structure on diffusion and dissolution during Ni/Al nanolaminate combustion

Ni/Al nanolaminates are reactive materials with customizable combustion characteristics. A common approach to synthesize the repeating Ni and Al nanolayers is physical vapor deposition, which often results in columnar grains with ⟨111⟩ texture and grain diameters on the order of a single layer thickness. Changes in grain size have been reported to affect combustion rates, yet the role of individual grain boundaries (GBs) on this process is unclear. Thus, this work investigates the role of the GB structure on atomic diffusion/dissolution and the resulting combustion reaction via molecular dynamics simulations. Nanolaminate combustion is simulated in bicrystal models containing columnar symmetric tilt GBs with ⟨111⟩ misorientation axis perpendicular to the Ni/Al interface. A range of GB misorientation angles is studied, and combustion in a Ni/Al nanolaminate without GBs is simulated for comparison. Combustion in bicrystal models reveals a rise in temperature with an exponential form prior to complete Al melting, while the model without GBs shows a linear temperature increase. Diffusion coefficients are measured for each bicrystal model, and separate Arrhenius fits are used to identify the first three combustion stages. Models containing higher energy GBs generally have higher diffusion coefficients and lower activation energies prior to complete melting of Al, while the GB structure shows little effect on dissolution after the Al layer melts. Thus, the GB structure plays a key role in Ni/Al nanolaminate ignition sensitivity but does not impact runaway combustion.

Interfacial atomic structure and electrical activity of nano-facetted CSL grain boundaries in high-performance multi-crystalline silicon

The interfacial structure of coincidence site lattice grain boundaries in multi-crystalline silicon plays a decisive role in their electrical behavior as revealed by high-resolution (scanning) transmission electron microscopy investigations. Considering only the global misorientation of the adjacent grains can lead to a false correlation between the structural and electrical properties of a grain boundary. The grain boundary habit plane as well as local deviations in the orientation and misorientation that induce additional structural defects need to be analyzed. Indeed, a Σ9 {221} grain boundary, presenting a perfect coincidence and periodicity at the atomic scale, appears electrically non-active. However, a grain boundary also identified as Σ9 {221} at the mesoscale is found to be composed of nano-sized triangular structures involving Σ3 {111} and Σ3 {112} facets at the nano-scale. This leads to the formation of grain boundary kinks and triple junctions that induce additional structural defects and turn the overall grain boundary interface electrically active. The possible origin of such a grain boundary dissociation, as well as its impact on the electrical activity, is discussed.

Relationship between carbon segregation and the carbide precipitation along grain boundary based on the structural unit model

Intragranular embrittlement is key to the weakening of properties of ultra-supercritical applied materials as HR3C heat-resistant steel in service and is considered to be related to the structure of grain boundary and the precipitation of carbides. In this work, the structural unit model was applied to study the structure of grain boundary and carbide formation with molecular dynamics simulation, and then, the precipitation tendency of carbides was analyzed through the structural similarity between grain boundary and carbide by using radial distribution function of the metal atom. At the same time, an electron backscatter diffraction study on serviced and treated HR3C heat-resistant steel was used for experimental exploration. Results show that the structural similarity between grain boundary and carbide depends on the grain boundary angle, wherein the structural similarity between grain boundary and carbide is higher for the grain boundary angle of 20–50°, while it is lower at the grain boundary angle close to 60° or smaller than 20°. The structure of Cr23C6 and Cr7C3 carbides is more similar to the local structure of grain boundary than that of Cr3C2 carbide. The grain boundary angle study on HR3C heat-resistant steel shows that the high content of 20–50° grain boundary indeed increases the precipitation of carbides, which is proposed to reduce the proportion of 20–50° grain boundaries by a proper procedure of cold deformation + heat treatment as 20% + 900 °C/6 h.

Interaction of radiation-induced defects with tungsten grain boundaries at across scales: a short review

As promising candidates for plasma-facing materials, tungsten-based materials suffer the irradiation of high-energy neutrons in addition to the hydrogen isotopes and helium irradiation and the high-thermal flux. Radiation-produced defects, e.g. self-interstitial atoms (SIAs) and vacancies (Vs), can induce the hardening and embrittlement of tungsten, meanwhile enhancing the retention of hydrogen isotopes and helium in tungsten. Reducing the grain size of materials to introduce a high density of defect sinks, e.g., grain boundaries (GBs) prevalent in nano-/ultrafine-crystalline materials, was demonstrated to be an effective approach for mitigating irradiation damage in tungsten. In this paper, we reviewed the theoretical advances in exploring radiation-resistance of nano-structured tungsten at across scales. It was concentrated on the results of molecular dynamics, molecular statics, and the object kinetic Monte Carlo simulations on the fundamental interaction of the radiation-created Vs and SIAs with the GB. These mechanisms include GB-promoted V/SIA migration and SIA-V recombination, interstitial-emission induced annihilation, coupling of the V migration close to the GB with the SIA motion within the GB, and interstitial reflection by the locally dense GB structure. We proposed the remaining scientific issues on the defect-GB interactions at across scales and their relation to experimental observations. We prospected the possible trends for simulating the radiation damage accumulation and healing processes in nano-structured tungsten in terms of the development of the across-scale computational techniques and efficiency of the GB-enhanced tolerance of tungsten to irradiation under complex in-service conditions.

In Situ Quantitative Tensile Testing of Antigorite in a Transmission Electron Microscope.

The determination of the mechanical properties of serpentinites is essential toward the understanding of the mechanics of faulting and subduction. Here we present the first in situ tensile tests on antigorite in a transmission electron microscope. A push‐to‐pull deformation device is used to perform quantitative tensile tests, during which force and displacement are measured, while the evolving microstructure is imaged with the microscope. The experiments have been performed at room temperature on beams prepared by focused ion beam. The specimens are not single crystals despite their small sizes. Orientation mapping indicated that several grains were well oriented for plastic slip. However, no dislocation activity has been observed even though the engineering tensile stress went up to 700 MPa. We show also that antigorite does not exhibit a purely elastic‐brittle behavior since, despite the presence of defects, the specimens accumulate permanent deformation and did not fail within the elastic regime. Instead, we observe that strain localizes at grain boundaries. All observations concur to show that under these experimental conditions, grain boundary sliding is the dominant deformation mechanism. This study sheds a new light on the mechanical properties of antigorite and calls for further studies on the structure and properties of grain boundaries in antigorite and more generally in phyllosilicates.

Hydrogen-modified interaction between lattice dislocations and grain boundaries by atomistic modelling

Dislocation plasticity in the vicinity of grain boundaries (GBs) plays a critical role in H-induced intergranular failure. Their interaction mechanisms under H environment, however, remain largely unexplored. Here, the underlying interaction of a screw dislocation with [11¯0] symmetric tilt GBs was studied by using molecular dynamics simulation, with special concerns on the role of solute H in it. Our results show several interaction mechanisms including dislocation dissociation, transmission, nucleation and reflection, depending on different glide planes and GB structures. The presence of H tends to transform these reactions into ones involving dislocation absorption due to H-hindered GB migration and H-enhanced localised plasticity. Furthermore, it is quantified that solute H leads to an increase in energy barrier for dislocation-grain-boundary interaction. After dislocation absorption, the GB segregated with H atoms is activated to a more disordered atomic structure, which can be correlated to the crack nucleation and hence the ultimate fracture. These findings advance a mechanistic understanding on H-induced plasticity-mediated intergranular failure.

The Coalescence Behavior of Two-Dimensional Materials Revealed by Multiscale In Situ Imaging during Chemical Vapor Deposition Growth.

Wafer-scale monocrystalline two-dimensional (2D) materials can theoretically be grown by seamless coalescence of individual domains into a large single crystal. Here we present a concise study of the coalescence behavior of crystalline 2D films using a combination of complementary in situ methods. Direct observation of overlayer growth from the atomic to the millimeter scale and under model- and industrially relevant growth conditions reveals the influence of the film-substrate interaction on the crystallinity of the 2D film. In the case of weakly interacting substrates, the coalescence behavior is dictated by the inherent growth kinetics of the 2D film. It is shown that the merging of coaligned domains leads to a distinct modification of the growth dynamics through the formation of fast-growing high-energy edges. The latter can be traced down to a reduced kink-creation energy at the interface between well-aligned domains. In the case of strongly interacting substrates, the lattice mismatch between film and substrate induces a pronounced moiré corrugation that determines the growth and coalescence behavior. It furthermore imposes additional criteria for seamless coalescence and determines the structure of grain boundaries. The experimental findings, obtained here for the case of graphene, are confirmed by theory-based growth simulations and can be generalized to other 2D materials that show 3- or 6-fold symmetry. Based on the gained understanding of the relation between film-substrate interaction, shape evolution, and coalescence behavior, conditions for seamless coalescence and, thus, for the optimization of large-scale production of monocrystalline 2D materials are established.

Grain-boundary structure and segregation in Nb3Sn coatings on Nb for high-performance superconducting radiofrequency cavity applications

We report on atomic-scale analyses of grain boundary (GB) structures and segregation in Nb3Sn coatings on Nb, prepared by the vapor-diffusion process, for superconducting radiofrequency (SRF) cavity applications, utilizing atom-probe tomography, high-resolution scanning transmission electron-microscopy and first-principles calculations. We demonstrate that the chemical composition of Nb3Sn GBs is correlated strongly with the diffusion of Sn and Nb at GBs during the coating process. In a sample coated with a relatively large Sn flux, we observe an interfacial width of Sn segregation at a GB of ∼3 nm, with a maximum concentration of ∼35 at.%. After post-annealing at 1100 °C for 3 h, the Sn segregated at GBs disappears and Nb segregation is observed subsequently at GBs, indicating that Nb diffused into the Nb3Sn GBs from the Nb substrate. It is also demonstrated that the amount of Sn segregation in a Nb3Sn coating can be controlled by: (i) Sn flux; and (ii) the temperatures of the Nb substrates and Sn source, which may affect the overall kinetics including GB diffusion of Sn and Nb. An investigation of the correlation between the chemical compositions of GBs and Nb3Sn SRF cavity performance reveals that the Nb3Sn SRF cavities with the best performance (high-quality factors at high accelerating electric-field gradients) do not exhibit Sn segregation at GBs. Our results suggest that the chemical compositions of GBs in Nb3Sn coatings for SRF cavities can be controlled by GB engineering and can be utilized to optimize fabrication of high-quality Nb3Sn coatings for SRF cavities.

Neural-network interatomic potential for grain boundary structures and their energetics in silicon

Artificial neural-network (ANN) interatomic potentials for simulating atomic structures and energetics of grain boundaries (GBs) in silicon were constructed and integrated into structural optimization and molecular dynamics (MD) algorithms. A training dataset including various atomic environments of symmetric tilt GBs was generated by performing density-functional-theory (DFT) calculations. The ANN potential after training was found to be capable of approximating the potential-energy surface at GBs even with dangling bonds and large atomic displacements at high temperatures, which cannot be well reproduced with empirical interatomic potentials. Additionally, reliability of the ANN potential for molecular simulations was evaluated. GB structures optimized or equilibrated by the ANN molecular simulations were also energetically lower for DFT calculations, without significant errors. The ANN potential is therefore expected to greatly reduce structural-optimization iterations and required time steps to acquire stable or equilibrium GB structures in MD simulations, enabling us to address even large-scale systems of general GBs in silicon, with high accuracy and low computational cost.

Базовые элементы структуры границ зерен наклона. Часть I. Ось разориентации [100

The computer simulation methods have been applied to calculate structure and energy of symmetric tilt grain boundaries with the misorientation axis [100]. The calculations have been carried out with the use of the structural-vacancy model previously developed by the authors. The misorientation angles of grain boundaries of common type have varied from 0º up to 90º while the increment has amounted 1º. The reverse density of coincidence sites in special grain boundaries has amounted Σ ≤ 53. The calculations have been carried out with the use of the Morse pair potential and the Cleri-Rosato many-body potential. It is shown that the dependence of the energy of grain boundaries on the misorientation angle when calculated with different potentials has a similar form, and their structure does not depend on the choice of potential and is in good agreement with high-resolution electron microscopic images. Only one special grain boundary is distinguished on the energy curve which is Σ5(013). It has been found that the structure grain boundaries may be represented by a limited number of atomic groups that have been named basic structural units. The structure of low-angle grain boundaries with the misorientation angle less than 8º is described by alternation of basic structural units of D types and ideal crystal, while the structure of low-angle grain boundaries with the misorientation angle ranging from 8º up to 13º is described by alternation of structural units of C and D types, from 14º up to 23º – B and C types. High-angle grain boundaries in the range of misorientation angles from 24º up to 37º contain only of basic structural units of B types, from 38º to 50º – A and B, more than 50º – only A.

Extremely flat surface full granular media with cap layer consisting of high-Ku CoPt grains and ferromagnetic oxide grain boundaries

Investigation of magnetic properties and nanostructure of stacked media with a CoPt-B2O3 granular recording layer (GRL) and granular cap layers (CL) with CoPt and a typical ferromagnetic oxide of rare earth (RE) oxide (RE oxide: Gd2O3, Nd2O3, Sm2O3, and CeO2) are reported. CoPt-RE oxide granular layer shows higher saturation magnetization and magnetic anisotropy than that of CoPtCrB continuous layer. In stacked media with CoPt-Gd2O3 granular CL (9 nm), nucleation field increases from 0.7 to 0.8 kOe and saturation field decreases from 21.5 to 12.5 kOe with comparable values to those of CoPtCrB continuous CL. This indicates that CoPt-RE oxide granular CL is effective to control intergranular exchange coupling. Reduction of coercivity distribution ratio (∆Hc/Hc) from 0.15 to 0.09 which is comparable to that of CoPtCrB continuous CL when CoPt-Gd2O3 granular CL thickness is changed from 4 to 9 nm is observed. This means CoPt-Gd2O3 granular CL is effective to reduce switching field distribution. Furthermore, significant reduction of surface roughness (Ra) for media with CoPt-Gd2O3 granular CL (9 nm) from 0.32 to 0.25 nm is observed. This reveals that CoPt-RE oxide granular CL has a great impact to induce stacked media with flat surface. Based on the cross-section observation by TEM, the media with CoPt-Gd2O3 granular CL shows continuous structure of grain boundaries from the GRL up to the CL and which results in one-on-one growth of magnetic grain with homogeneous columnar structure. Focusing on the surface morphology, media with CoPt-Gd2O3 granular CL shows smoother surface than that of media with CoPtCrB continuous CL which supports the Ra evaluation result. These results indicate that the employment of granular CoPt-RE oxide for CL is not only to reduce switching field distribution but also to flatten surface of the stacked media.

Phase field crystal simulation of effect of misorientation angle on low-angle asymmetric tilt grain boundary dislocation motion

Grain boundary affects the microstructure of metal material, and thus further its macroscopic properties. As is well known, under the action of applied stress, the grain boundary migrates. The structures and arrangements of grain boundary dislocations at different misorientation angles are very different, which affects the macrophysical and chemical properties of metal crystal. Therefore, it is of great theoretical and practical significance to study the dislocation structure and reaction mechanism of grain boundary under different misorientations for further studying the material properties.The phase field crystal method is used to simulate the low-angle asymmetric tilt grain boundary structure and dislocation motion on a nanoscale. From the perspective of the change of the position of the grain boundary dislocation motion under the applied stress and the change of the free energy of the crystal system, the influences of the misorientation angle on the low-angle asymmetric tilt grain boundary structure and the motion of the grain boundary dislocation are analyzed. The results show that the types of dislocation pairs of low-angle asymmetric tilt grain boundaries at different misorientation angles are the same. With the increase of misorientation angle, the grain boundary dislocation pairs increase, and n1n2 and n4n5 type dislocation pairs are more easily formed at the grain boundaries. Under the action of applied stress, the initial movement states of the grain boundary dislocation pairs at different misorientation angles are all climbing along the grain boundaries. As the system energy accumulates, the larger the misorientation angle is, the more the number of decomposed grain boundary dislocation pairs decomposed will be, and only in the dislocation pairs of n1n2 and n4n5 type there occurs decomposition reaction. There are four stages in the free energy curve of the low-angle asymmetric tilt grain boundary system at different misorientation angles, which correspond to the dislocation pairs climbing, dislocation pairs sliding and decomposition, dislocation pairs reaction to form single crystal, and the free energy rising process of the system. Further research shows that as the misorientation angle increases, the time for the single crystal system formed by the dislocation of grain boundary pairs to annihilate is required to be long.

Grain boundary properties of elemental metals

The structure and energy of grain boundaries (GBs) are essential for predicting the properties of polycrystalline materials. In this work, we use high-throughput density functional theory calculations workflow to construct the Grain Boundary Database (GBDB), the largest database of DFT-computed grain boundary properties to date. The database currently encompasses 327 GBs of 58 elemental metals, including 10 common twist or symmetric tilt GBs for body-centered cubic (bcc) and face-centered cubic (fcc) systems and the Σ7 [0001] twist GB for hexagonal close-packed (hcp) systems. In particular, we demonstrate a novel scaled-structural template approach for HT GB calculations, which reduces the computational cost of converging GB structures by a factor of ~ 3–6. The grain boundary energies and work of separation are rigorously validated against previous experimental and computational data. Using this large GB dataset, we develop an improved predictive model for the GB energy of different elements based on the cohesive energy and shear modulus. The open GBDB represents a significant step forward in the availability of first principles GB properties, which we believe would help guide the future design of polycrystalline materials.

Grain boundary structure and mobility in high-entropy alloys: A comparative molecular dynamics study on a Σ11 symmetrical tilt grain boundary in face-centered cubic CuNiCoFe

We employ atomistic computer simulations to study the structure and migration behavior of a Σ11 symmetrical tilt grain boundary in a 4-component model FCC high-entropy alloy (HEA) (CuNiCoFe). The results are compared to grain boundaries in elemental metals and a so-called ‘average-atom’ sample. We find that the repeating structural units characterizing the static grain boundary structure show the same repeating structural units for all samples, while the high temperature equilibrium grain boundary structure is most strongly influenced by presence of stacking faults. Under an applied synthetic driving force, this GB migrates by a mechanism assisted by partial dislocations in all materials. For this reason the grain boundary mobilities and stacking fault energies are directly related. Moreover, the HEA sample and the average-atom sample show almost identical mobilities suggesting that local chemical fluctuations play a minor role. Solute segregation to the GB in the HEA suppresses GB migration up to very high temperatures and might be the main cause for reduced grain growth in FCC HEAs.

Probing defect relaxation in ultra-fine grained Ta using micromechanical spectroscopy

The study of grain boundaries (GBs) in polycrystalline materials is a field of major interest, as many fundamental properties are influenced by their actual structure. One of the main challenges in investigating GBs is that electron microscopy techniques capable to resolve their atomistic arrangement require very small sample volumes of a few tens of nanometers and might therefore change the natural state of the GB via removal of occurring material constraint. To counteract this influence one could apply indirect measurements such as internal friction to probe for changes of the GB structure. However, with the drive towards smaller volumes most of these techniques are at their limit. The current work proposes a new approach based on mechanical spectroscopy of micron sized specimens applied in-situ in a scanning electron microscope. Applying this novel concept to ultra-fine grained Ta we investigate changes in the defect structure between the as-deformed condition and after a heat treatment. In fact, we detect a decrease in internal friction of about 50% upon annealing. This pronounced change occurs in conjunction with an increase in flow stress and hardness and is related to thermally induced GB relaxation.

Alloying Element Segregation and Grain Boundary Reconstruction, Atomistic Modeling

Grain boundary (GB) segregation is an important phenomenon that affects many physical properties, as well as microstructure of polycrystals. The segregation of solute atoms on GBs and its effect on GB structure in Al were investigated using two approaches: First principles total energy calculations and the finite temperature large-scale atomistic modeling within hybrid MD/MC approach comprising molecular dynamics and Monte Carlo simulations. We show that the character of chemical bonding is essential in the solute–GB interaction, and that formation of directed quasi-covalent bonds between Si and Zn solutes and neighboring Al atoms causes a significant reconstruction of the GB structure involving a GB shear-migration coupling. For the solutes that are acceptors of electrons in the Al matrix and have a bigger atomic size (such as Mg), the preferred position is determined by the presence of extra volume at the GB and/or reduced number of the nearest neighbors; in this case, the symmetric GB keeps its structure. By using MD/MC approach, we found that GBs undergo significant structural reconstruction during segregation, which can involve the formation of single- or double-layer segregations, GB splitting, and coupled shear-migration, depending on the details of interatomic interactions.

Migration Pinning and Roughening Transition of a Ni Grain Boundary

To date, much research has been conducted into the effect of migration pinning on the grain size in polycrystalline materials. However, effects of migration pinning on the grain-boundary structure and its transition have not been illuminated. Here, using transmission electron microscopy (TEM) we have explored the pinning effects for the grain boundary in a Ni bicrystal. During TEM specimen preparation, a hole was intentionally drilled in the middle of the grain boundary as a pinning point against grain-boundary migration. The specimen was heated to 600 °C. The grain boundary is driven to migrate by both the surface energy anisotropy and the total strain energy reduction. Grain-boundary facets with a plane orientation of {0 3 2}//{1 1 1} appear near the hole. The facets undergo a structural transition from atomically flat to rough with increasing distance from the hole. A pinning force exerted by the hole suppresses the migration of the grain boundary near the hole, indicating that the grain-boundary region away from the hole is subjected to a higher driving force. It certainly appears that the phenomenon originates from a change in driving force with the distance from the hole, being a signature of kinetic roughening.

Microstructure Evolution of AA1070 Aluminum Alloy Processed by Micro/Meso-Scale Equal Channel Angular Pressing

The Micro/meso-forming of commercially pure aluminum, AA1070, processed at room temperature by equal channel angular pressing (ECAP) with a die channel angle of 90° through 4 deformation passes has been conducted. Microstructure features, such as grain size, misorientation angle distributions and the developed texture during the four deformation passes of micro/meso-ECAP have been investigated by Electron backscattering diffraction (EBSD) technique. Then, hardness measurements over the cross-section of the processed samples were correlated with the EBSD analysis. EBSD scans revealed that extended shear bands are formed and represent the microstructural feature induced during micro/meso-forming. Whereas, a non-uniform grain structure consisting of intensive low-angle grain boundaries was obtained in the first pass, a uniform ultrafine-grained structure of high-angle grain boundaries (in the range of 1–2 μm) was achieved at the fourth pass. Consequently, a significant improvement in the hardness value to 65.3%, with respect to the starting material, was achieved due to the enhancement of the fine grain structure. The texture analysis exhibited that the high plastic shear strain associated with micro/meso-scale ECAP was capable to develop a weak texture in the flow plane compared to the starting texture.

Effect of grain boundary misorientation angle on diffusion behavior in molybdenum-tungsten systems

Grain boundary (GB) diffusion plays a key role in the long-term service processes of engineering materials at elevated temperatures. GB diffusion processes are strongly relying on the misorientation angle of the two adjacent grains. However, the orientation dependence of GB diffusion is not well understood. In this work, two types of tungsten coating were deposited onto a substrate consisting of a single molybdenum crystal to forming diffusion couples. The diffusion profiles of isolated GBs have been measured by electron backscatter diffraction and energy dispersive X-ray spectroscopy. The effect of the tungsten GB misorientation angle (θ) on the diffusivity of molybdenum was investigated. GB structures with different misorientation angles were resolved using high-resolution transmission electron microscopy. As θ increased from 5° to 18°, the spacing between the dislocations composing the GBs decreased from greater than 10 to 3 atomic spaces. Furthermore, when θ was greater than 18° a 0.53 nm wide ordered structure was formed, and this structural change increased the GB energy. Consequently, the GB diffusion coefficient increased by 3- and 7- orders of magnitude fold as compared to the volume diffusion coefficient for low- and high-angle GBs, respectively. The GB energies were approximately 1.1 and 2.1 J m−2 in low- and high-angle GBs, respectively. This is the first time that tungsten GB energies were determined through experimental method.

Effect of Local Terrace on Structure and Mechanics of Graphene Grain Boundary

In this study, the effects of local terrace of the substrate on the structure and mechanical properties of graphene grain boundaries (GBs) during CVD growth have been explored by phase field crystal (PFC) modeling and molecular dynamics (MD) simulations. It is found the GBs are significantly disturbed for weak surface disturbance with bulge height of only 3.4 A. The distance between GBs and bulge plays an important role in determining the morphologies of GBs, and the aperiodic and curved GBs can be observed which is attributed to several representative structures, like 5-6|6-7, 5-6-7 defects and GBs deflection. In general, there are four fracture modes for GBs with weak surface disturbance depending on the existence of 5-6-7 and 5-6|6-7 defects. While for strong surface disturbance with bulge height of 10 A, the interaction between topological defects at bulged graphene and GBs will locally offset nearby 5-7 dislocation pairs and there are two fracture modes depending on the structural integrality of GBs....