Boundary Network Research Articles

Effect of grain boundary engineering on corrosion behavior of nickel-based alloy 825 in sulfur environment

Grain boundary engineering (GBE) by appropriate deformation and heat treatment processes was applied to nickel-based alloy 825 due to its excellent corrosion resistance. The microstructure of nickel-based alloy was analyzed by electron backscatter diffraction (EBSD) and the effect of GBE on the corrosion resistance in high sulfur-containing environments was studied by electrochemical method. The results show that the ratio of low Σvalue coincidence site lattice (CSL) grain boundary is obviously improved from 47.1% to 65.5%. The special angle grain boundaries Σ3, Σ9, Σ27 and so on are randomly distributed on the network of the large angle grain boundaries, which destroy the connectivity of the original grain boundary network, and effectively block the corrosion cracking of the material along them. The corrosion resistance of nickel-based alloy 825 in high sulfur environments is enhanced verified by polarization curve and scanning electrochemical microscope, providing references for corrosion protection during the exploitation of high sulfur gas reservoirs.

Amorphous complexions alter the tensile failure of nanocrystalline Cu-Zr alloys

Grain boundary-based mechanisms are known to control the plastic deformation and failure of nanocrystalline metals, with manipulation of the boundary structure a promising path for tuning this response. In this study, the role of interfacial structural disorder on plasticity and failure of nanocrystalline Cu-Zr alloys is investigated with in situ scanning electron microscopy tensile deformation experiments. Two model materials are created, one with only the typical ordered grain boundaries and another with amorphous intergranular films interspersed into the boundary network, while the microstructures are otherwise identical. Hence, the importance of complexion type on plasticity and failure is isolated by only varying complexion structure. The tensile experiments show that failure of the samples containing amorphous films is significantly retarded, as evidenced by an increase in the cross-sectional area reduction, a decrease in the occurrence of shear-dominated failure, a decrease in strain localization, and fracture surfaces with more elongated dimple features. As a whole, this study provides direct evidence that structural disorder at the grain boundaries can be beneficial for improving the ductility of nanocrystalline metals.

IMPROVING THE METHODOLOGY FOR MONITORING THE STATE OF SPECIAL-PURPOSE GEODETIC NETWORK POINTS

The practical creation of a Unified Federal Geoinformation system (EFGIS) of the egrn in the Russian Federation is largely determined by the quality of the geodetic basis of the cadastre, information about which is necessary when performing cadastral works. Reference boundary networks (OMS) are an element of the geodetic basis and were created during various periods of development of the real estate cadastre. Over the past decades, their technical condition has not been evaluated, so it is necessary to monitor the MHI. Currently, there is no modern method for monitoring the state of points of reference boundary networks. This necessitates the development and testing of methods for monitoring the state of OMS points, as a special-purpose geodetic network. In addition, the absence or insufficiently complete list of regulatory requirements for the geodetic basis of the cadastre, causes the emergence of new, with the existing old registry errors that occur in the data of the EGRN.

Impact of selenium addition to the cadmium-zinc-telluride matrix for producing high energy resolution X-and gamma-ray detectors

Both material quality and detector performance have been steadily improving over the past few years for the leading room temperature radiation detector material cadmium-zinc-telluride (CdZnTe). However, although tremendous progress being made, CdZnTe still suffers from high concentrations of performance-limiting defects, such as Te inclusions, networks of sub-grain boundaries and compositional inhomogeneity due to the higher segregation coefficient of Zn. Adding as low as 2% (atomic) Se into CdZnTe matrix was found to successfully mitigate many performance-limiting defects and provide improved compositional homogeneity. Here we report record-high performance of Virtual Frisch Grid (VFG) detector fabricated from as-grown Cd0.9Zn0.1Te0.98Se0.02 ingot grown by the Traveling Heater Method (THM). Benefiting from superior material quality, we achieved superb energy resolution of 0.77% at 662 keV (as-measured without charge-loss correction algorithms) registered at room temperature. The absence of residual thermal stress in the detector was revealed from white beam X-ray topographic images, which was also confirmed by Infra-Red (IR) transmission imaging under cross polarizers. Furthermore, neither sub-grain boundaries nor their networks were observed from the X-ray topographic image. However, large concentrations of extrinsic impurities were revealed in as-grown materials, suggesting a high likelihood for further reduction in the energy resolution after improved purification of the starting material.

Grain-Size-Governed Shear Failure Mechanism of Polycrystalline Methane Hydrates

The shear failure mechanism of polycrystalline gas hydrates is critical for understanding marine geohazards related to gas hydrates under a changing climate and for safe gas recovery from gas hydrate reservoirs. Since current experimental techniques cannot resolve the mechanism on a spatial and temporal nanoscale, molecular simulations can assist with proposing and substantiating nanoscale failure mechanisms. Here, we report the shear failure of polycrystalline methane hydrates using direct molecular dynamics simulations. Based on these simulations, we suggest two modes of shear behavior, depending on the grain sizes, d, in the polycrystal: grain-size-strengthening behavior with a d1/3 grain size dependence for small grain sizes and grain-size-weakening behavior for large grain sizes. Through the crossover from strengthening to weakening behavior, the failure mode changes from shear failure with a failure plane parallel to the applied shear to tensile failure with a failure plane lying at an angle with the applied shear, spanning a network of grain boundaries. The existence of such a change in mechanism suggests that the Hall–Petch breakdown in methane hydrates is due to a change from grain boundary sliding to tensile opening being the most important failure mechanism when the grain size increases.

A modelling approach to describe the DC current-voltage behaviour of low-voltage zinc oxide varistors

Zinc oxide varistors are among the most important surge arresters. The current-voltage (I–V) behaviour of these devices can be approximated by a model of electrical networks of grain boundaries arranged in 3D space, where nonlinear resistors mimic the effective grain boundary behaviour of the varistor microstructure. A simplified version of the network model, namely parallel circuits of chains of serially connected resistors of different length (path model), is derived from a fully 3D model of the polycrystalline microstructure. A comparison of the path model with a complex resistor network based on the 3D model demonstrates that the network of highly nonlinear resistors has a very similar DC I–V characteristic as parallel paths of these resistors connected in series. The path model, whose parameters were adapted from microscopic I–V measurements, is able to reproduce the macroscopic I–V behaviour of real low-voltage zinc oxide varistors.

Fracture Energy Measurement of Prismatic Plane and \u03a32 Boundary in Cemented Carbide

The grain boundary network of WC in WC-Co is important, as cracks often travel intergranularly. This motivates the present work, where we experimentally measure the fracture energy of Σ2 twist grain boundaries between WC crystals using a double cantilever beam opened with a wedge under displacement control in a WC-10wt%Co sample. The fracture energy of this boundary type was compared with cleaving {10overline{1}0} prismatic planes in a WC single crystal. Fracture energies of 7.04 ± 0.36 Jm−2 and 3.57 ± 0.28 Jm−2 were measured for {10overline{1}0} plane and Σ2 twist boundaries, respectively.

Advances in CdZnTeSe for Radiation Detector Applications

Detection of X- and gamma-rays is essential to a wide range of applications from medical imaging to high energy physics, astronomy, and homeland security. Cadmium zinc telluride (CZT) is the most widely used material for room-temperature detector applications and has been fulfilling the requirements for growing detection demands over the last three decades. However, CZT still suffers from the presence of a high density of performance-limiting defects, such as sub-grain boundary networks and Te inclusions. Cadmium zinc telluride selenide (CZTS) is an emerging material with compelling properties that mitigate some of the long-standing issues seen in CZT. This new quaternary is free from sub-grain boundary networks and possesses very few Te inclusions. In addition, the material offers a high degree of compositional homogeneity. The advancement of CZTS has accelerated through investigations of the material properties and virtual Frisch-grid (VFG) detector performance. The excellent material quality with highly reduced performance-limiting defects elevates the importance of CZTS as a potential replacement to CZT at a substantially lower cost.

Optimization of selenium in CdZnTeSe quaternary compound for radiation detector applications

X- and gamma-ray detectors are increasingly becoming an essential tool for science and technology in various fields, including homeland security, nonproliferation, nuclear security, medical imaging, astrophysics, and high energy physics. Cd1−xZnxTe1−ySey (CZTS) is emerging as a next-generation compound semiconductor for such applications. CZTS was found to possess a very low concentration of Te inclusions and free from sub-grain boundary networks. Being a quaternary compound with varying alloy compositions, optimization of the composition was performed to determine the minimum amount of selenium required to produce CZTS with reduced defects. The optimized composition was found to be x = 0.10 and y = 0.02, i.e., Cd0.9Zn0.1Te0.98Se0.02, for excellent material properties as a radiation detector. The resulting material was free from sub-grain boundary networks and with a highly reduced concentration of Te inclusions. The bulk dark resistivity obtained was in the range of 1–3 × 1010 Ω cm with the highest achieved mobility-lifetime product of ∼6.6 × 10−3 cm2/V for the optimized CZTS composition. Impurity analyses were performed by the glow discharge mass spectroscopy technique, and the results showed relatively high impurity concentrations compared to commercial detector-grade CdZnTe. Thus, CZTS has room for further improvement with additional purification of the starting materials.

Measuring simulated hydrogen diffusion in symmetric tilt nickel grain boundaries and examining the relevance of the Borisov relationship for individual boundary diffusion

Grain boundaries are known to act as important diffusion pathways in metals. Here we examine how hydrogen diffusion in nickel grain boundaries is affected by different grain boundary types. We simulate 26 [100] symmetric tilt grain boundaries at several temperatures and develop a modified mean squared displacement method for isolating the grain boundary diffusivity from the bulk diffusivity. We analyze the diffusivity data for the different boundaries and find the grain boundary diffusivity to be higher than the bulk diffusivity up to a temperature where the two converge. We test a model developed by Borisov et al. (Phys. Met. Metallogr.17, 1964, 80-84) that relates grain boundary diffusion to grain boundary interfacial energy. Despite the fact that the Borisov relationship has never been used to examine solute interstitial diffusion before, it fits the simulated diffusion data well for temperatures where the grain boundary diffusivity is higher than the bulk diffusivity. Some parameters in the Borisov relationship, related to the geometry of the grain boundary and its activated states, have always taken on assumed values because they are hard to measure or acquire. The fit of the simulated data provides estimates for these values that are physically realistic and provides evidence that the use of the traditionally assumed values should be reconsidered. This work also demonstrates that while there is a need for further validation, the Borisov relationship can reasonably relate solute interstitial diffusivity of hydrogen to the interfacial energy of individual [100] symmetric tilt nickel grain boundaries, whereas in the past it has been used to relate self diffusion and interfacial energy of a grain boundary network as a whole.

Self-driven near-UV and visible light detection based on ITO/Gd-doped BiFeO3/Au heterostructure

Multiferroic BiFeO3 materials have driven great interest due to their potential in solar-spectrum energy harvesting, optoelectronic and photodetection devices. Here we report effects of electric-field poling on electronic hybridization and domain structure, and their correlations with photovoltaic responses in the ITO/(Bi0.93Gd0.07)FeO3 ceramic/Au heterostructure under 405 nm and 532 nm irradiations. Photovoltaic conversion, photoresponsivity (R) and specific detectivity (D*) are sensitive to ceramic thickness, photon energy, light intensity and electric-field poling. The photoresponsivity and detectivity in the 1 kV/cm poled photovoltaic cell under low-intensity 405 nm irradiation can respectively reach ∼4.5 × 10−2 A/W and 2.5 × 1011 Jones, which are larger than ∼2.8 × 10−2 A/W and 1.56 × 1011 Jones in the unpoled cell. This study demonstrates fast response times of ∼1 × 10−3 s and ∼2 × 10-2 s respectively under 405 nm and 532 nm irradiations. The improved photoresponse was driven jointly by the p-n junction, the field-modulated Schottky barriers and the network of grain boundaries and domain walls.

Enhanced deep carbon cycle marked by the upsurge of silica-undersaturated nephelinitic magmatism at the Proterozoic-Phanerozoic boundary

Abstract The temperature of the upper mantle was a principal factor controlling the style of plate tectonics and influencing magmatism and metamorphism on Earth over geological history. Recent studies emphasized that Earth’s tectonic style has transited into the modern plate tectonics since the late Neoproterozoic, which is characterized by a global network of plate boundaries with deep and cold oceanic plate subduction. However, the consequence of the establishment of modern plate tectonics to Earth’s mantle temperature and deep carbon cycle has not been fully understood. Here we apply statistical analysis on the geochemical data of continental igneous rocks and identify an increased magnitude of nephelinitic volcanism at the end of the Ediacaran. Nephelinitic rocks, a silica-undersaturated high-alkaline rock group, are mostly formed by low-degree melting of carbonated mantle sources. We link their widespread emergence with an enhanced mantle cooling event and a dramatically increased flux of crustal carbonates transporting to the mantle. The rapid cooling of the mantle was ascribed to the onset of modern-style plate tectonics with global-scale cold oceanic and continental subduction since the late Neoproterozoic. The declined upper-mantle temperature could not only favor the low-degree melting but also allow the subduction of carbonates into the deep mantle without decarbonation at shallow depth. Considering the high oxygen fugacity feature of the nephelinitic rocks and some other high-alkaline volcanism, the establishment of modern plate tectonics and thereafter enhanced mantle cooling and deep carbon cycle might contribute to the high-level atmospheric oxygen content during the Phanerozoic.

Correlative statistical microstructural assessment of precipitates and their distribution, with simultaneous electron backscatter diffraction and energy dispersive X-ray spectroscopy

Modern engineering alloys have bespoke microstructures, where features such as precipitates are used to control properties. In many Ni-based alloys, carbo-nitride precipitates are introduced to strengthen and improve performance. These precipitates can be distributed throughout the microstructure and niobium rich carbides are often found at grain boundaries. In this work, we used combined energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) to characterise the population of these precipitates. Processing of the EDS signal is used to label the Mo/Nb-rich precipitates, and their size and location are measured from maps using a circular Hough transform. This label map is combined with the grain boundary network (from EBSD analysis). Statistical analysis, using ANOVA testing, reveals differences in chemistry between carbides found in Ni-rich matrix grain interiors, on random high angle boundaries and on special boundaries (Σ3 and Σ9). These results are compared between wrought and power metallurgy product forms. These distributions are discussed in the context of their performance within demanding environments, such as reactor core internals.

In situ scanning tunneling microscopy study of 2-mercaptobenzimidazole local inhibition effects on copper corrosion at grain boundary surface terminations

New insight on local inhibition effects of 2-mercaptobenzimidazole (MBI) on early stage intergranular corrosion of copper in hydrochloric acid solution is reported from in situ analysis at the nanometer scale and comparison with 2-mercaptobenzothiazole (MBT) effects in the same pre-adsorption and corrosion testing conditions. Macroscopic cyclic voltammetry analysis, including grains and grain boundary (GB) network, showed a passivation-like behavior in the Cu(I) oxidation range, specific to MBI since not observed with MBT and assigned to the anodic formation of a surface film of Cu(I)-MBI reaction products protecting against dissolution. Electrochemical scanning tunneling microscopy analysis revealed net intergranular dissolution, mitigated by the imperfect protection provided by the anodically formed MBI layer. It also showed local accumulation of reaction products in the GB surface regions, blocking preferential dissolution. For random GBs, blocking by local accumulation of reaction products was dominant, in agreement with the expected higher reactivity of these GBs generating more Cu(I) ions under anodic polarization and thus less efficiently protected by the anodically formed MBI layer. For Coincidence Site Lattice (CSL) boundaries, mitigated net dissolution was more frequently observed. Coherent twins showed equally efficient inhibition in the GB surface region than on adjacent grains. MBI inhibition was less efficient than MBT inhibition with more Cu(I) reaction products generated on the grains to form a surface film and their preferential local accumulation more frequently observed in the GB surface regions.

Effects of aging temperature and grain boundary character on carbide precipitation in a highly twinned nickel-based superalloy

ABSTRACT The precipitation of carbides at grain boundaries in nickel-based superalloys and austenitic stainless steels can significantly affect the grain boundary related properties of the bulk material. This study evaluated the growth and dissolution of carbides in grain boundary engineering (GBE) treated nickel-based superalloy (Inconel 690) during aging treatment using scanning electron microscopy and transmission electron microscopy. The grain boundary network was stable during aging below 1000°C. At lower aging temperatures of 715 and 800°C, a small number of tiny carbide precipitates were observed on Σ3c grain boundaries. In contrast, plate-like carbides precipitated near both sides of Σ3i grain boundary and on one side of Σ9 grain boundary, and large carbide precipitates formed on Σ27 and random grain boundaries. At higher aging temperatures of 900 and 1000 °C, large carbide precipitates with a more consistent morphology precipitated at Σ9, Σ27, and random grain boundaries. Carbide dissolution was common during aging at these higher temperatures, which promoted the growth and redistribution of other carbides. These experimental findings provide valuable insights into the effect of aging temperature and grain boundary character on carbide precipitation behaviours.

Intergranular stress corrosion cracking in simulated BWR water of 316L stainless steels manufactured with different procedures

Stress corrosion cracking (SCC) of 316 L stainless steels manufactured with different procedures were investigated in simulated BWR environment. The effects of grain boundary network on SCC are discussed. Twin boundaries show stronger resistance to SCC than other types of boundaries. Grain size has significant influence on SCC though residual strain has more effect. Smaller grains result in more SCC initiation sites, while larger grains result in that the crack propagates locally deeper due to less restrictions from less triple points in the grain boundary network. Theoretical calculation is implemented to estimate the grain size effect on crack propagation.

Grains boundary networking interconnection design for robust flexible perovskite solar cell

Due to perovskite crystal brittleness, the flexibility of perovskite photovoltaic devices arises as one major challenge to be addressed before any practical application of high efficiency perovskite solar cells. We report a new method that monomer ETPTA were introduced into perovskite intermediate phases to address the intrinsic fragility of these materials with mechanically reinforcing grains boundary network. The formation of macromolecular intermediate phases with long chain polymers leads to polymer/perovskite composite cross-linker. The cross-linker network function as a dynamic elastic protective layer over the perovskite grain boundary and yielding excellent mechanical stability against tensile strain.

Improved corrosion behavior of a novel Fe85Cr4Mo8V2C1 tool steel processed by laser powder bed fusion

Laser powder bed fusion (LPBF) of tool steels offers a great potential for tailoring material properties besides technical advantages like great freedom of design or function integration. In this study, a novel high-strength Fe85Cr4Mo8V2C1 tool steel processed by LPBF and by casting was examined in a comparative study. By means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) for both conditions a microstructure composed of martensite, austenite and complex carbides was detected. Very high cooling rates of LPBF yield a significant microstructural refinement across the building direction with much more homogenous distribution of the principal phases compared with the cast counterparts. The corrosion behavior was investigated in NaCl solutions by means of potentiodynamic polarization measurements as well as surface analysis with SEM and X-ray photo electron spectroscopy (XPS) after immersion testing. Particulary in more aggressive solutions where active dissolution dominates, i.e. in 3.5% NaCl (pH 7) and in 0.035% NaCl (pH 3), the LPBF samples exhibit a retarded corrosion activity according to more positive corrosion potentials and significantly lower corrosion current densities as well as reduced anodic current densities. A wave-like corrosion pattern was detected reflecting melt track-related microstructural features, i.e. local grain coarsening and preferred crystal orientation. In milder solution, 0.035% NaCl (pH 7), Fe85Cr4Mo8V2C1 exhibits for both processing conditions relatively low free corrosion rates. For LPBF samples a complex, fine grain boundary network supports the formation of thicker but slightly more permeable layers composed mainly of iron oxides which retard the corrosion activity especially at higher anodic polarization.

Evolution of microscopic strains, stresses, and dislocation density during in-situ tensile loading of additively manufactured AlSi10Mg alloy

The AlSi10Mg alloy produced by laser powder bed fusion (LPBF) possesses a novel microstructure and higher mechanical properties compared with its casting counterpart. So far, the crystallographic orientation-dependent lattice strains, average phase stresses, and dislocation density during the tensile loading of the LPBF AlSi10Mg are not well understood. This fact is impeding further optimization of microstructure and mechanical properties. High energy synchrotron X-ray diffraction providing deep penetration capability and phase-specific measurements of various bulk properties of crystal materials is applied to investigate the LPBF AlSi10Mg under loading. The crystallographic orientation-dependent lattice strains and elastoplastic properties of the Al matrix are assessed. The average phase stresses are calculated to quantify load partitioning between the Al and Si phases. The nano-sized Si particles that bear high-stress are efficient strengthening particles. The maximum value of the average phase stress of Si reaches up to ~2 GPa. Based on the modified Williamson-Hall and the modified Warren-Averbach methods, the dislocation density and its evolution during the plastic deformation are determined. A multistage strain hardening behavior is detected in the Al matrix, which is associated with the interactions between the dislocations and the cell boundary network.

Effect of nitrogen on grain boundary character distribution in 316 stainless steel

Influence of nitrogen content and percentage of prior rolling deformation on evolution of grain boundary character distribution in 316L(N) stainless steel, during grain boundary engineering type thermo-mechanical processing, have been studied in the present work. 10% rolling deformation followed by annealing produces highest fraction of low energy Σ3 boundaries and high fraction of low energy grain boundary network in high nitrogen SS, leading to improved corrosion resistance.