Bulk Behavior Research Articles

Boundary logarithmic corrections to the dynamical correlation functions of one-dimensional spin- 12 chains

The asymptotic dynamical correlation functions in one-dimensional spin chains are described by power-laws. The corresponding exponents characterize different bulk and boundary critical behavior. We present novel results for the logarithmic contribution to the boundary correlations of an isotropic Heisenberg chain. The exponent of the logarithm, $\lambda=1$, is derived using a renormalization group technique. We confirm our analytical results by comparing with numerical quantum Monte Carlo data.

A thermodynamics-based multi-physics constitutive model for variably saturated fractured sorptive rocks

A multi-physics constitutive mathematical model is developed using non-equilibrium thermodynamics and continuum mechanics to investigate the temporal changes in the poromechanical behaviour of variably saturated sorptive shale rocks during gas production. The model specifically integrates the effects of matrix gas desorption and fracture two-fluid flow on bulk behaviour. The working of the model is studied through the numerical example of gas production from a continuum quarter wellbore geometry solved by the finite element method . The numerical results support the field observations that gas productivity remains unaffected during early production operations whereas the water recovery is strongly dependent on capillary pressure redistribution in the fracture network induced by the applied drawdown. Although the drawdown is limited by field operations, a higher drawdown will not necessarily result in more water production unless the capillary forces are altered in the water-wet micro-fracture network. The sensitivity analysis showed important parameters which if not incorporated into the constitutive model fail to accurately capture the bulk behaviour such as the presence of water in the fracture network, fracture permeability , and fractal dimension (representing the pore structure). It was shown how neglecting two-fluid flow and multi-physics effects in the fracture network can have significant impacts on the accuracy of gas production data. The research findings provide valuable insights to understand how poromechanics coupled with capillarity and desorption plays a role in causing bulk volumetric changes during gas production from shale formations .

Study of polycarbonate–polystyrene interfaces using scanning transmission electron microscopy spectrum imaging (STEM‐SI)

AbstractPolymer blends are important for both commercial utility and scientific understanding. The degree of interfacial mixing in polymer blends is important since it influences the blends' mechanical properties. Understanding bulk properties in multiphase polymeric materials requires knowledge of the interfacial properties of the materials. The characterization of the interface, in terms of its width and composition profile, provides insight about the bulk behaviour of the material. Chemical microscopy through electron energy‐loss spectroscopy (EELS) in a transmission electron microscope is gaining popularity to characterize narrow polymer–polymer interfaces. In this work, we show how scanning transmission electron microscopy spectrum imaging, a spatially resolved energy‐loss spectroscopy, can be employed to calculate the interfacial width in a pair of immiscible polymers, taking a polycarbonate–polystyrene (PC‐PS) bilayer as an example. By mapping peaks unique to each of the blend constituents at several points across the interface, we show how the interfacial profile concentrations can be determined. With this method we calculated the interfacial width in the PC‐PS bilayer sample to be approximately 32 nm, even utilizing low resolution spectrometers, which are more widely available. Using the technique described with higher resolution EELS instruments having a better signal‐to‐noise ratio, a higher spatial resolution can be achieved. Using EELS chemical fingerprints of polymers that have been developed earlier, the technique presented here has the potential for effective visualization and morphological measurements of phase‐differentiated polymer blends. This paper is an attempt to enable a new user to characterize polymer–polymer interfaces using chemical microscopy. © 2022 Society of Industrial Chemistry.

Reverse scaling of a bonded-sphere DEM model: Formulation and application to lignocellulosic biomass microstructures

We explore scaling laws for adapting a bonded-sphere discrete element method (BS-DEM) model developed for woody structural mechanics at the millimeter scale to model the mechanics of realistic lignocellulosic biomass microstructures. Two scaling approaches, i.e., the reverse coarse graining (RCG) and equivalent bulk behavior (EBB), are proposed based on the classical mechanics principles and assessed in the single-particle compression and rectangular cuboid block tension tests. The EBB approach is recommended for BS-DEM models with general 3D geometries and is applied to simulate the microindentation test on a realistic microscale pinewood specimen. Simulations are performed to elucidate the impact of specimen thickness and loading position on the specimen’s force–deformation behavior. The range of Young’s modulus obtained from the calibrated BS-DEM simulations can match the experimental measurements. This is the first-of-its-kind study that has explored scale-bridging modeling approaches in biomass micromechanics and has proposed solutions based on BS-DEM models in the microscale. • First-of-its-kind reverse scaling laws for bonded-sphere discrete element model. • Reverse coarse graining applicable for maintaining equivalent single-bond behavior. • Equivalent bulk behavior recommended for BS-DEM model with general 3D geometries. • Impact of specimen thickness and loading position on material response analyzed. • Young’s moduli from simulations using EBB can match the experimental measurements.

Dipolar Dispersion Forces in Water-Methanol Mixtures: Enhancement of Water Interactions upon Dilution Drives Self-Association.

Mixtures of short-chain alcohols and water produce anomalous thermodynamic and structural quantities, including molecular segregation into water-rich and alcohol-rich components. Herein, we used molecular dynamics simulations with polarizable models to investigate interactions that could drive the self-association of water molecules in mixtures with methanol (MeOH). As water was diluted with MeOH, significant changes in the distribution of molecules and solvation properties occurred, where water exhibited a clear preference for self-association. When common structural quantities were analyzed, it was found that there was a clear reduction in water-water hydrogen bonding and tetrahedral order (both in terms of typical bulk behavior), contrary to the observed water self-association. However, when dipolar dispersion forces between all molecules as a function of system composition were analyzed, it was found that water-water dipolar interactions became significantly stronger with dilution (6-fold stronger interaction in 75% MeOH compared to 0% MeOH). This was only observed for water, where MeOH-MeOH interactions became weaker as the systems were more dilute in MeOH. These forces result from specific dipole orientations, likely occurring to adopt lower energy configurations (i.e., head-to-tail or antiparallel). For water, this may result from lost other interactions (e.g., hydrogen bonding), leading to more rotational freedom between the dipole moments. These intriguing changes in dipolar interactions, which directly result from structural changes, can therefore explain, in part, the driving force for water self-association in MeOH-water mixtures.

Scour around underwater unexploded ordnances (UXOs): An experimental and computational investigation

Underwater unexploded ordnances (UXOs) pose a significant threat to human health and safety as well as the environment. The understanding of the scour around UXOs is essential for remediation actions. Many previous studies simplified UXOs as cylinders. We report an experimental and computational study on the hydrodynamics and morphodynamics around a real UXO and the comparison with a cylinder. With detailed flume data and simulation results, we show the UXO shape has a great impact on the hydrodynamics and the consequent scour and burial process. The shape effects also convolute with the flow’s angle of attack, making it even more important to consider the shape of UXOs. Hence, two angles of attack, 45° and 90° were investigated. The bulk behavior of scour around UXOs is akin to scour around any slender object placed on a sediment bed. However, the detailed scour process is significantly different. Our results show that flow contraction near the two ends of UXOs, recirculation behind the object, vortex shedding downstream, and jet-like flow under the object all play dominant roles during different stages of scour. In addition, we generalized the scour process around UXOs and other objects by applying a scaling analysis based on first principles of physics. A fitted saturation growth-rate equation relating scour volume and time collapses data reasonably well.

Relating the kinetics of grain-boundary complexion transitions and abnormal grain growth: A Monte Carlo time-temperature-transformation approach

Grain boundaries undergo thermally-activated, first-order transitions that result in discontinuous changes of interfacial properties. Importantly, grain boundary transitions lead to changes in bulk material properties (e.g., embrittlement) and/or behavior (e.g., abnormal grain growth). Numerous studies have been completed on the equilibrium states of grain boundaries and their transitions (i.e., complexion transitions), but there have been far fewer investigations of complexion transition kinetics; complexion transitions occur on the atomic-scale and are therefore challenging to detect experimentally. In this work, a 3D Potts grain growth model with stochastic complexion transitions was employed to investigate complexion transition kinetics. A Johnson-Mehl-Avrami-Kolmogorov (i.e., JMAK) approach was used to extract nucleation and growth rates (i.e., transformation rates), while point process analyses and correlation functions were used to infer complex interrelated nucleation and growth events. Time-temperature-transformation (TTT) diagrams, in particular grain-boundary complexion, transformed grain, and abnormal grain TTT diagrams, were constructed to summarize the progress of complexion-related transformations. Such diagrams relate complexion-induced grain growth to the underlying complexion transitions and, in the case of abnormal grain growth (AGG), permit one to assess the role of AGG as a temperature-dependent, time-displaced indicator of complexion transitions. Overall, this work details a theoretical framework that can be used to better understand complexion transition kinetics as well as to develop tools for the design of bulk microstructures.

Efficient calculation of the free energy for protein partitioning using restraining potentials

An approach for the efficient simulation of phase-separated lipid bilayers, for use in the calculation of equilibrium free energies of partitioning between lipid domains, is proposed. The methodology exploits restraint potentials and rectangular aspect ratios that enforce lipid phase separation, allowing for the simulation of smaller systems that approximately reproduce bulk behavior. The utility of this approach is demonstrated through the calculation of potentials of mean force for the translation of a transmembrane protein between lipid domains. The impact of the imposed restraints on lipid tail ordering and lipid packing are explored, providing insight into how restraints can best be employed to compute accurate free-energy surfaces. This approach should be useful in the accurate calculation of equilibrium partition coefficients for transmembrane protein partitioning in heterogeneous membranes, providing insight into the thermodynamic driving forces that control this fundamental biophysical phenomenon.

Nanostructured CuFeSe2 Eskebornite: An efficient thermoelectric material with ultra-low thermal conductivity

Bulk and nanoparticles of CuFeSe2 eskebornite have been synthesized using vacuum-sealed tube melting and facile hydrothermal method, respectively. The phase purity of bulk and nano-CuFeSe2 is confirmed through powder X-ray diffraction. Field emission-SEM micrographs of nano-CuFeSe2 depict the randomly distributed plate-like agglomerated particles. Further, TEM studies reconfirm the plate-like CuFeSe2 nanoparticles with an average particle size of ∼ 102 nm. The electronic states of Cu1+, Fe3+, and Se2- in CuFeSe2 are confirmed using XPS analysis. VSM measurements depict the weak ferromagnetic behavior in both bulk and nano-CuFeSe2 samples. The electrical conductivity of bulk CuFeSe2 is ∼ 0.6x104 S/m, and it reduces to 0.07x104 S/m at 300 K for nano-CuFeSe2, whereas the Seebeck coefficient increases from ∼ 12 µV/K to ∼ 48.4 µV/K at 300 K. Intriguingly, an ultra-low thermal conductivity of ∼ 0.73 W/mK at 300 K has been achieved for nano-CuFeSe2, which leads to the maximum figure of merit, zT of ∼ 0.02 at 645 K.

Varied Magnetic Phases in a van der Waals Easy-Plane Antiferromagnet Revealed by Nitrogen-Vacancy Center Microscopy.

Interest in van der Waals materials often stems from a desire to miniaturize existing technologies by exploiting their intrinsic layered structures to create near-atomically thin components that do not suffer from surface defects. One appealing property is an easily switchable yet robust magnetic order, which is only sparsely demonstrated in the case of in-plane anisotropy. In this work, we use widefield nitrogen-vacancy (NV) center magnetic imaging to measure the properties of individual flakes of CuCrP2S6, a multiferroic van der Waals magnet known to exhibit weak easy-plane anisotropy in the bulk. We chart the crossover between the in-plane ferromagnetism in thin flakes down to the trilayer and the bulk behavior dominated by a low-field spin-flop transition. Further, by exploiting the directional dependence of NV center magnetometry, we are able to observe an instance of a predominantly out-of-plane ferromagetic phase near zero field, in contrast with our expectation and previous experiments on the bulk material. We attribute this to the presence of surface anisotropies caused by the sample preparation process or exposure to the ambient environment, which is expected to have more general implications for a broader class of weakly anisotropic van der Waals magnets.

Low-field NMR investigation on interaction of ZnO nanoparticles with reservoir fluids and sandstone rocks for enhanced oil recovery

The use of nanoparticles (NPs) can considerably benefit enhanced oil recovery (EOR) by changing the wettability of the rock, improving the mobility of the oil drop, and decreasing the interfacial tension (IFT) between oil and water. Prior to the application of nanoparticles in oil fields, it is essential to conduct measurements at the laboratory scale. However, the estimation of reservoir wettability is difficult in most laboratory experiments. Practicably, ZnO NPs were used to modify the rock surface wettability, lower the IFT at the oil/water interface, and reduce the interaction of chemical adsorption, such as (surfactant) onto reservoir rock surface to solve various challenges in oil production and EOR operations. Upon confining both ZnO-based nanofluid and the crude oil into sandstone, deviations from the corresponding pure bulk dynamical behaviors were observed with low-field nuclear magnetic resonance (LF-NMR) relaxometry. The expected deviations from the pure bulk behaviors were attributed to the well-known confinement effect. The wettability test results before and after surface variations of formation water (FW) with the addition of three different NP concentrations (0.05, 0.075, and 0.1) wt% ZnO reflected significant changes to its wettability. Among the treatments of Berea sandstone cores with ZnO NPs, the percentage of clay-bound H2O/free fluid index was maximum in 1.0 pore volume (PV) NP treatment. The ratio of NMR relaxations, which determines the affinity of fluids toward solids, by the 1.0 PV NP treatment is reported to have the most potential with higher affinity for FW and less affinity for crude oil toward the pore walls. Hence, LF-NMR allows monitoring of nanofluid and crude oil characteristics in the pores of rock samples and may potentially be applied in further EOR studies.

Well-Ordered Nanonetwork Metamaterials from Block Copolymer Templated Syntheses.

Through the morphological evolution to give highly optimized complex architectures at different length scales, fine-tuned textures for specific functions in living organisms can be achieved in nature such as a bone core with very complicated porous architecture to attain a significant structural efficiency attributed to delicately structured ligaments and density gradients. As inspired by nature, materials with periodic network structures (i.e., well-defined porous textures) in the nanoscale are appealing and promising for innovative properties. Biomimicking from nature, organic and/or inorganic nanonetworks can be synthetically fabricated, giving broadness and effectiveness when tuning the desired properties. Metamaterials are materials whose effective properties do not result from the bulk behavior of the constituent materials but rather mainly from their deliberate structuring. The performances of fabricating metamaterials will depend on the control of size, shape, order, and orientation of the forming textures. One of the appealing textures for the deliberate structuring is network architecture. Network materials possess self-supporting frameworks, open-cell character, high porosity, and large specific surface area, giving specific functions and complexity for diverse applications. As demonstrated by recent studies, exceptional mechanical performances such as negative thermal expansion, negative Poisson's ratio, and twisting under uniaxial forces can be achieved by the effect of the deliberate structuring with nanonetwork textures. In contrast to a top-down approach, a bottom-up approach is cost-effective, and also it can overcome the size limitation to reach nanoscale fabrication. It can be foreseen that network metamaterials with a feature size of tens of nanometers (referred as nanonetwork metamaterials) may provide new comprehension of the structure and property relationships for various materials. The self-assembly of block copolymers (BCPs) is one of the most used methods to build up well-ordered nanostructured phases from a bottom-up approach with precise control of size, shape, and orientation in the thin films for realistic applications. In this account, we summarize recent advancements in the fabrication of nanohybrids and nanoporous materials with well-ordered nanonetwork textures even with controlled helicity by combining block copolymer self-assembly and templated syntheses for mechanical and optical applications with superior properties beyond nature as metamaterials as well as chiral metamaterials with new properties for chiroptic applications such as chiral plasmonics, beam splitter, and negative refraction. The description of the fundamental facets of a nonconventional structure-property relationship with the characters of metamaterials and the state-of-the-art methodologies to fabricate nanonetworks using block copolymer self-assembly will stimulate research activities for the development of nanonetwork metamaterials with exceptional individual and multifunctional properties for futuristic devices.

C/C–SiC composites derived from single‐source poly(silylene–acetylene) precursors

AbstractCarbon‐fiber‐reinforced carbon–silicon carbide (C/C–SiC) composites were prepared by impregnating carbon fibers with ethynylphenyl‐terminated poly(silylene–acetylene) (EPTSA) as a single‐source precursor with subsequent hot pressing and pyrolysis. The structural evolution, crystallization behavior, and graphitization of bulk C–SiC ceramics, as well as their mechanical properties and ablation behavior, were investigated. The EPTSA precursor starts to transform into inorganic SiC ceramic materials at 800°C, which is characterized by an amorphous structure with weight loss, shrinkage, and densification between 800 and 1000°C. The formation of SiC crystals inhibited the growth of the graphitic structure between 1000 and 1200°C. As the temperature was raised, both graphite and SiC crystals continued to grow, and the crystalline forms became more complete. The carbon‐fiber cloth (T300CF)‐reinforced C–SiC composite (T300CF/C–SiC) prepared using polymer infiltration and pyrolysis (PIP) exhibited excellent mechanical properties. After five PIP cycles, the flexural strength, flexural modulus, and interlaminar shear strength of the T300CF/C–SiC composite reached 169 MPa, 32.5 GPa, and 9.38 MPa, respectively. In addition, the chopped‐carbon‐fiber‐reinforced C–SiC composite fabricated using the PIP process demonstrated good oxyacetylene‐torch ablation properties.

Understanding Morphology, Bulk Properties, and Binding of Silk Particles for 3D Printing

Silk fibroin has emerged as a leading biomaterial for biomedical applications. 3D printing has been successfully used for printing with silk fibroin, albeit in the form of a bioink, in direct-write 3D printers. However, in the form of bioinks, stability and mechanical attributes of silk are lost. An innovative alternative to producing 3D printed solid silk constructs is silk milled into powder for printing in a binder jetting printer. In this work, we focus on characteristics of silk powder to determine suitability for use in 3D printing. Two different silk powders are compared with hydroxyapatite powder, a known biomaterial for biomedical constructs. We have investigated powder size and shape by Camsizer X2 and Scanning Electron Microscope and bulk behaviour, dynamic flow behaviour, and shear behaviour by FT4 powder rheometer. Preliminary printing tests were conducted in an in-house custom-built printer designed for silk powder. It was found that silk powder has low flowability and stability. Therefore, to print solely out of silk powder, a 3D printer design will need sophisticated techniques to produce flow to ensure even distribution and consistent thickness of powder layers during the printing process. It was also found that high concentrations of formic acid (>75 to 99 wt.%) can fuse particles and therefore be used as a binder ink for 3D printing. The printer design challenges for silk powder are discussed.

Influence of cryogenic impacting and succeeding annealing on electrochemical corrosion behavior of coarse-grain Cu bulk in sodium chloride solution

PurposeThis paper aims to study the electrochemical corrosion performance of ultrafine-grained (UFG) Cu bulk in 0.5 M NaCl solution.Design/methodology/approachUFG Cu bulk were prepared by impacting at −196°C and following heat treatment. The electrochemical corrosion behaviors of coarse-grained (CG), impacted and subsequently annealed at 190°C Cu bulks were studied.FindingsAll the bulks displayed typical active-passive-transpassive behaviors (dual passive films without stable passive regions). The resistance to corrosion of impacted Cu bulk was notably superior to that of CG Cu bulk, and subsequently annealing further improved its corrosion resistance.Social implicationsExcept for mechanical properties, corrosion performance has been considered to be one of the most important aspects in bulk UFG metallic materials research for the prospective engineering applications.Originality/valueCryogenic impacting could effectively reduce grain size of CG Cu bulk to UFG scale and induce high density dislocation. Subsequent annealing resulted in a further decrease of grain size even to nanoscale, as well as nanometer twins. The grain refinement, high density dislocation and annealing twins effectively enhance the passivation capability, resulting in an increase in the corrosion resistance.

Local Ice-like Structure at the Liquid Water Surface.

Experiments and computer simulations have established that liquid water's surfaces can deviate in important ways from familiar bulk behavior. Even in the simplest case of an air-water interface, distinctive layering, orientational biases, and hydrogen bond arrangements have been reported, but an overarching picture of their origins and relationships has been incomplete. Here we show that a broad set of such observations can be understood through an analogy with the basal face of crystalline ice. Using simulations, we demonstrate a number of structural similarities between water and ice surfaces, suggesting the presence of domains at the air-water interface with ice-like features that persist over 2-3 molecular diameters. Most prominent is a shared characteristic layering of molecular density and orientation perpendicular to the interface. Lateral correlations of hydrogen bond network geometry point to structural similarities in the parallel direction as well. Our results bolster and significantly extend previous conceptions of ice-like structure at the liquid's boundary and suggest that the much-discussed quasi-liquid layer on ice evolves subtly above the melting point into a quasi-ice layer at the surface of liquid water.

Crystallization Study of Pb Additive Se-Te-Ge Nanostructured Alloys using Non-isothermal Differential Scanning Calorimetry

Nanotechnology started a new era in research and continuous escalations due to its potential applications. In modern optoelectronics, Pb additive nano-chalcogenides are becoming promising materials. For a memory and switching material, the thermal stability and glass-forming ability are vital, and the glass should be thermally stable from a technological point of view. This research article investigates the thermal behaviour of bulk (Se80Te20)94-xGe6Pbx (x = 0, 2, 4 and 6) samples synthesized using the melt quench procedure non-isothermal differential scanning calorimetry at four heating rates from 5 to 20°Cmin-1. Further, the overall crystallization study, which includes thermal stability, ease of glass formation, fragility, glass relaxation and glass-crystallization transformation kinetics of investigated alloys using different empirical formalisms, is reported and discussed. Compositional and heating rate dependence of recorded characteristic temperatures and other deduced parameters of reheated alloys are also discussed. The correlation between deduced parameters is also established to define the utility of these materials in practical applications.

Influence of divalent ion concentration on the adhesion behaviour of sulfonate self-assembled monolayers (SAM)

Surfactant-assisted oil recovery is usually employed for the production of residual oil after primary and secondary recovery techniques have been exhausted. The loss of injected surfactants via adsorption on to the porous media surfaces impede the efficiency of the procedure and greatly impact process economics. To address this issue, the choice of surfactant and aqueous fluid composition are varied. In particular, the divalent ion concentration is a property that potentially influences surfactant adsorption significantly. It is this aspect which has been addressed in this study. In this study, atomic force microscope (AFM). tips were functionalised with a self-assembled monolayer (SAM) of the alkanethiol, sodium 11-mercaptoundecanesulfonate (otherwise known as MUS) with the –SO3-headgroups oriented outwards towards the aqueous solutions, this is a model for the well-studied surfactant, sodium dodecyl sulfate, SDS. The adhesion forces between these “surfactant-functionalised” tips and Bandera Brown sandstone surfaces were measured in brine solutions of varying CaCl2 concentrations. Observations deviate from previous studies where adsorption of surfactant headgroups in the bulk solution is enhanced via Ca2+ bridging with the surface. Rather, measured adhesion forces decreased with increase in CaCl2 concentration. The results are reversible and also occurs in a reducing environment when the iron in the Bandera brown is reduced for iron (III) to iron (II). The observed behaviour is interpreted in terms of Ca2+ preferentially bridging between neighbouring headgroups on the tip due to the dense packing of molecules non-representative of respective bulk behaviours and acting as a barrier to adhesive contact. Thus, this supposed micellization at the tip aided by the formation of Ca2+ salt bridges between alternate headgroups more closely mimics a surfactant micelle, rather than dispersed individual monomers. The findings of this study provide insight into nanoscale anionic surfactant interactions in general and within sandstone reservoirs in particular where surfactant adsorption is driven largely by monomers rather than micelles. Investigating surfactant adsorption using microscopic techniques such as AFM allow for the determination of interactions at mineral-fluid interfaces and constitute an effective screening methodology for ascertaining enhanced oil recovery process feasibility.

Coupled effects of particle overall regularity and sliding friction on the shear behavior of uniformly graded dense sands

Particle morphology has been regarded as an important factor affecting shear behaviors of sands, and covers three important aspects, i.e. global form (overall shape), local roundness (large-scale smoothness), and surface texture (roughness) in terms of different observation scales. Shape features of different aspects can be independent of each other but might have coupled effects on the bulk behavior of sands, which has been not explored thoroughly yet. This paper presents a systematic investigation of the coupled effects of the particle overall regularity (OR) and sliding friction on the shear behavior of dense sands using three-dimensional (3D) discrete element method (DEM). The representative volume elements consisting of ideal spheres and irregular clumps of different mass proportions are prepared to conduct drained triaxial compression simulations. A well-defined shape descriptor named OR is adopted to quantify particle shape differences of numerical samples at both form and roundness aspects, and the particle sliding friction coefficient varies from 0.001 to 1 to consider the surface roughness effect equivalently in DEM. The stress-strain relationships as well as peak and critical friction angles of these assemblies are examined systematically. Moreover, contact network and anisotropic fabric characteristics within different granular assemblies are analyzed to explore the microscopic origins of the multi-scale shape-dependent shear strength. This study helps to improve the current understanding with respect to the influence of the particle shape on the shear behavior of sands from different shape aspects.

Ab initio investigation of the screw dislocation-hydrogen interaction in bcc tungsten and iron

The interaction of hydrogen with 1/2<111> screw dislocations is investigated in body-centered cubic tungsten and iron using ab initio calculations. Different core reconstructions are evidenced, depending on the number of hydrogen atoms introduced inside the dislocation core. Corresponding interaction energies are highly attractive in both metals, with a significant contribution of zero point energy associated with H vibrations, particularly in Fe. The pinning by hydrogen of the dislocation in its reconstructed core remains efficient for a local atomic fraction of hydrogen as low as 1/5. Other octahedral interstitial sites close to the reconstructed core are also attractive, contrary to the perfect bcc crystal where these sites are unstable and where hydrogen lies in the tetrahedral sites. Hydrogen recovers its bulk behavior only beyond the eighteenth neighbor octahedral sites. A simple pair interaction model is parameterized on ab initio calculations and used with a mean-field approximation to predict the concentration profiles of hydrogen segregated in and around the dislocation core. This segregation model predicts that dislocation core-sites remain completely decorated by hydrogen atoms up to at least 750 K and 200 K in W and Fe, respectively.