Spherical Pores Research Articles

Molecular Dynamics Study of the Development of the Dislocation Structure of an FCC Crystal Containing an Ensemble of Spherical Pores under an External Force Action

Radiation swelling is one of the most urgent problems in radiation materials science. Its successful solution requires an understanding of the features of the mechanisms of pore healing. In this regard, there are various experimental and theoretical works devoted to this topic. At present, due to the increase in the power of computing tools, computer simulation allows for more and more complex studies and is used, among other things, in the field of materials science. In this paper, we present the results of molecular dynamics simulations devoted to the study of the processes of healing of a group of spherical pores in an fcc crystal subjected to shear deformation. As the study showed, for pores located in close proximity to each other, the formation of a common dislocation loop is characteristic, which is formed as a result of the attraction of individual loops having sections with opposite signs of helical orientation. The formation and subsequent development of such a loop contributes to a decrease in the free volume localized in the crystal in the form of pores. In addition, structural transformations that occur when a shock wave is generated in the computational cell, which creates additional stresses, are considered separately. In this case, the formation of the loop described above and the subsequent collapse of individual pores are also observed. Taking into account that the simulation was carried out at temperatures insufficient to activate diffusion processes, and the temperature control procedure was used in model experiments with wave generation, we can conclude that the shock wave is the cause of pore collapse even in the absence of high temperatures, and one of the main mechanisms of this process is the development of the dislocation structure of the ensemble of pores. Keywords: crystal, model, deformation, pore, dislocation.

Молекулярно-динамическое исследование развития дислокационной структуры ГЦК-кристалла, содержащего ансамбль сферических пор, при внешнем силовом воздействии

Radiation swelling is one of the most urgent problems in radiation materials science. Its successful solution requires an understanding of the features of the mechanisms of pore healing. In this regard, there are various experimental and theoretical works devoted to this topic. At present, due to the increase in the power of computing tools, computer simulation allows for more and more complex studies and is used, among other things, in the field of materials science. In this paper, we present the results of molecular dynamics simulations devoted to the study of the processes of healing of a group of spherical pores in an fcc crystal subjected to shear deformation. As the study showed, for pores located in close proximity to each other, the formation of a common dislocation loop is characteristic, which is formed as a result of the attraction of individual loops having sections with opposite signs of helical orientation. The formation and subsequent development of such a loop contributes to a decrease in the free volume localized in the crystal in the form of pores. In addition, structural transformations that occur when a shock wave is generated in the computational cell, which creates additional stresses, are considered separately. In this case, the formation of the loop described above and the subsequent collapse of individual pores are also observed. Taking into account that the simulation was carried out at temperatures insufficient to activate diffusion processes, and the temperature control procedure was used in model experiments with wave generation, we can conclude that the shock wave is the cause of pore collapse even in the absence of high temperatures, and one of the main mechanisms of this process is the development of the dislocation structure of the ensemble of pores.

DETECTION OF STRUCTURAL CHANGES IN A BCC CRYSTAL-LE DYNAMICS UNDER LASER INFLUENCE BY THE METHOD OF MOLECULAR

The presented work presents the results of molecular dynamics modeling of changes in the surface layer of the computational cell under a short-term high-energy impact. Interest in this topic is because the processes occurring in the surface layer, which is in a liquid state, will subsequently have an impact during its crystallization, and, as a result, will affect various physical and geometric characteristics of the surface of the material as a whole. The model constructed and described in the work, in which the temperature of the computational cell is distributed in accordance with the solution of the linear problem of heat conduction, made it possible to reveal the discontinuity of the surface layer, which consists in the localization of excessfree volume in the form of a group of spherical pores. The sizes of these imperfections, as well as the duration of their existence, have differences when modeling different energy densities of laser radiation. Further research made it possible to reveal the conditions under which the pores remain stable throughout the entire simulation time, as well as to reveal the relationship between the crystallographic orientation of the “solid-liquid” interface and the sizes of the formed pores.

Role of microstructure reactivity and surface diffusion in explaining flash (ultra-rapid) sintering kinetics

The competition between sintering and coarsening is cited by numerous authors as one of the potential factors for explaining the ultra-rapid sintering kinetics of flash sintering. In particular, surface diffusion is a mechanism decreasing the driving force of sintering by changing the initial highly reactive microstructures (particle contact) into poorly reactive porous skeleton structures (spherical porosity). We show by finite element simulations that flash SPS experiments high specimen temperatures close to 2000 °C. These high temperatures are not sufficient to explain the ultra-rapid sintering kinetics if typical spherical pore theoretical moduli are employed. On the contrary, reactive experimentally determined moduli succeed in explaining the ultra-rapid sintering kinetics. Mesoscale simulations evidenced that the origin of such reactive experimental moduli is a porous skeleton geometry with a significant delay in surface diffusion and particle rearrangement. This highlights the important role of the surface diffusion negation (favoring higher stress intensification factor) in flash sintering.

Semi-solid wire-feed additive manufacturing of AlSi7Mg by direct induction heating

• Demonstration of an induction heating based additive manufacturing process. • The novel process enables high speed additive manufacturing with build rates of around 20 mm 3 /s (0.054 g/s). • Adaptation of induction power during the process allows multilayer specimen fabrication. • Defects in the novel process are analysed for the first time using µCT to pave the way for their future prevention. In this study, a novel process is presented in which direct induction heating with frequencies in the MHz range is used for the wire-feed additive manufacturing of the alloy AlSi7Mg. The high frequency of 1.5 MHz enables processing of 1.2 mm diameter wires without the need for indirect heating via a nozzle. The feasibility of the process is proven by the experimental identification of a proper process window regarding the influential parameters such as the distance between inductor and substrate, induction power and wire feed rate for the fabrication of single layers. Furthermore, a strategy for the successful fabrication of multi-layered cubes is developed. The microstructure of the cubes exhibits a characteristic variation along the build direction. Micro-computed tomography is used to reveal defects like lack of fusion and spherical pores in test cubes. The presented results are used to derive possible process improvements, which will allow the novel process to be used as a fast and powder-free alternative metal additive manufacturing route in future.

Regulation of the fabric wet adhesion property through structural design of desirable scale: A theoretical suggestion

It is a universal phenomenon existing in both manufacture and everyday life that fabrics tend to adhere under wet condition. The phenomenon is closely related to the interaction of the fabric surface structure and the liquid. Among the complex structures and numerous varieties of fabrics, the features of typical fabric structures are employed to establish a geometrical model (including structural configuration of spherical protrusion, cylinder, loop, and pore) in order to investigate wet adhesion property under the existence of massive liquid media. Through mechanical analysis on material-liquid bridge system (consisting of fabric material, liquid bridge adhering to the material, and the solid-liquid interface), a quantitative computational theoretical model is built with parameters including critical distance [Formula: see text], radius of the structure R*, apparent receding contact angle [Formula: see text], wetted sphere angle [Formula: see text], and so on, finding the rules and proposing a series of preferred ranges of regulating structural parameters to effectively achieve desirable fabric wet adhesion property in different circumstances. The research findings could be applied to the scientific prediction and on demand regulation of fabric wet adhesion property. They are of great significance with regard to improving the convenience of fabric wet processing, application properties under wet environments, apparel health and comfort for human body, and so on.

Pore morphology effect on elastic and fluid flow properties in Bakken formation using rock physics modeling

Unconventional geo-resources are critical due to their important contributions to energy production. In this energy transition and sustainability era, there is an increased focus on CO2-enhanced oil recovery (CO2-EOR) and geological CO2 storage (GCS) in unconventional hydrocarbon reservoirs, and the extraction of hot fluid for energy through enhanced geothermal systems. However, these energy solutions can only be achieved through efficient stimulation to develop a complex fracture network and pore structure in the host rocks to extract heat and hydrocarbon, or for CO2 storage. Using Bakken formation well data and rock physics models, this study aimed to identify the post-depositional effect of pore structure on seismic velocity, elastic moduli, and formation fluid; and further predict the best lithofacies interval for well landing, and the implications for fluid (gas, oil, and water) recovery in naturally- and often systematically-fractured geosystems. The KT and DEM models' predictions show distinct formation intervals exhibiting needle-like pores and having higher seismic velocities (Vp and Vs) and elastic moduli (K and µ), relative to other formation intervals that exhibit moldic pores. At the same fluid concentration, the needle-like pores (small aspect ratios) have a higher impact on elastic moduli, Vp, and Vs than on the moldic spherical pores with all other parameters held constant. Vp is affected more than Vs by the properties of the saturating fluid (gas, oil, or water) with Vp being greater in Bakken formation when it is water-saturated than when it is gas-saturated. Vs exhibit the reverse behavior, with Vs greater in the gas-saturated case than in the water-saturated case. Further, analyses suggest that the middle Bakken formation will have a higher susceptibility to fracturing and faulting, and hence will achieve greater fluid (oil and water) recovery. Our findings in this study provide insights that are relevant for fluid production and geo-storage in unconventional reservoirs.Article highlightsIntegrated well log data and rock physics models.Investigated the effect of changes in pore structure on elastic properties and fluid flow in shale.Increase in porosity causes a reduction in elastic moduli and seismic velocities.Vp is more affected by pore geometry than Vs depending on density and properties of saturating fluid.Lithofacies with needle−like pores are more susceptible to fracturing than lithofacies with intragranular pores.

Interpretation of micromorphic constitutive relations for porous materials at the microscale via harmonic decomposition

Micromorphic theories became an established tool to model size effects in materials like dispersion, localization phenomena or (apparently) size dependent properties. However, the formulation of adequate constitutive relations with its large number of constitutive relations and respective parameters hinders the usage of the full micromorphic theory, which has 18 constitutive parameters already in the isotropic linear elastic case. Although it is clear that these parameters are related to predicted size effects, the individual meaning of single parameters has been rather unclear.The present work tries to elucidate the interpretation of the constitutive relations and their parameters. For this purpose, a harmonic decomposition is applied to the governing equations of micromorphic theory. The harmonic modes are interpreted at the microscale using a homogenization method for a simple volume element with spherical pore. The resulting boundary-value problem at the microscale is solved analytically for the linear-elastic case using spherical harmonics resulting in closed-form expressions for all of the elastic 18 parameters. These values are used to predict the size effect in torsion of slender foam specimens. The predictions are compared with respective experimental results from literature.

Defect-based analysis of the laser powder bed fusion process using X-ray data

Due to high production costs and a limited reproducibility of quality, the high potential of laser powder bed fusion (LPBF) has not been fully exploited yet. In fact, internal defects can have a detrimental effect on the fatigue behaviour and cause final component failure. Therefore, process-induced defects must be localized and evaluated at a higher level of detail. The present study deals with the correlation amongst pores and LPBF process parameters in AlSi10Mg components. Computed tomography (CT) allows an extensive examination of internal defects. Within this work, a total number of 2,939,830 pores detected in 96 cylindrical samples were analysed using CT. The formation of pores can be adjusted by varying the modified volume energy density, for example, by using various laser scanning speeds. Furthermore, the effect of powder preparation scan strategies (pre-heating and pre-sintering) on the formation of different pore types as well as the general reproducibility was examined. For instance, the shielding gas flow, contaminated protective windows of the lasers as well as prior powder preparation influence the formation of pores. Using prior laser powder preparation reduced the total number of pores at high scanning speeds up to 45%. When the scanning speed is increased, the number of spherical pores decreases and large and irregularly shaped pores appear. Interestingly, only the pre-heating process resulted in a reduced formation of spherical pores at low scanning speeds (1000 mm/s).

Effective elastic properties and wave surfaces of rock materials containing multiple cavities and cracks (effective field approach)

The paper is devoted to simulation of the effective elastic properties of rock materials containing two substantially different types of defects simultaneously: random sets of ellipsoidal pores (cavities) and elliptical cracks. For solution of the homogenization problem and calculation of the effective elastic properties of such materials, the self-consistent effective field method is used. For composites with one type of heterogeneities, the method coincides with the Mori–Tanaka method. The method allows deriving analytical expressions for the effective elastic stiffness tensors of the materials containing pores and cracks of various scales. These tensors have correct symmetry with respect to tensor indices and provide physically reasonable values of the effective elastic constants in wide regions of porosity and crack density. The materials with pores and cracks of substantially different sizes and of close sizes are considered. The method predicts substantially different effective elastic constants of the materials with these microstructures. Comparison of the components of the effective elastic stiffness tensors for the materials with various values of porosity and crack density are presented. Wave surfaces of acoustical waves in the materials containing pores and cracks are constructed. It is shown that for materials with spherical pores and circular cracks, the shapes of these surfaces are close to ellipsoidal. The results of the paper can be used for determination of fracture level in damaged rock materials by acoustical methods.

Characterization of the deposit-foaming of pure aluminum and Al-Mg-0.7Si alloys using directed energy deposition based on their metallurgical characteristics and compressive behaviors

This study focuses on the manufacturing of porous materials using the directed energy deposition (DED) technique, which is a 3D printing technique. A foaming agent was mixed with pure aluminum and Al-Mg-0.7Si powder to produce deposited aluminum foam, and its foaming characteristics, metallic structure, and compressive properties were analyzed. The foaming agent formed pores inside the deposited materials through a chemical reaction, and it produced Al 3 Ti, an inter-metallic compound, through the reaction with aluminum powders. Kirkendall voids were generated by the dispersion of Al 3 Ti. Despite the generated inter-metallic compound, no distinct changes were observed in the micro-hardness of the foamed materials compared to that of the non-foamed materials. We also analyzed the effects of laser power on the porosity, number of pores, and deposited height. We fabricated porous materials with a maximum porosity of 35.26% at a laser power of 1100 W using pure aluminum, and fabricated porous materials with a maximum porosity of 18.56% using the Al-Mg-0.7Si alloys. During the deposition process, pure aluminum formed oxide particles more easily compared to the alloys. Consequently, the oxides stabilized the generation and growth of pores, leading to high porosity in pure aluminum. Furthermore, spherical pores were uniformly distributed in pure aluminum, whereas merged pores with irregular shapes were distributed in Al-Mg-0.7Si. This difference is attributed to the viscosity of different melting pools. In the compression test of the non-porous and porous samples, although Young’s modulus and compressive strength were reduced by the internal pores, the specific energy absorption of deposited-foamed samples was higher than that of non-porous materials. The internal pores were compressed and disappeared due to the compressive load, but for Kirkendall voids around Al 3 Ti, they were not completely closed, remaining as micro-pores.

Discovering the role of the defect morphology and microstructure on the deformation behavior of additive manufactured Ti–6Al–4V

The defects remain a significant issue for additive manufactured titanium alloys, which hinders their further market penetration. In this work, we proposed a new understanding of the inhomogeneous microstructure formation affected by defects with detailed EBSD characterization, and how the defects and inhomogeneous microstructures determined the strain accumulation behavior by in-situ SEM tensile testing. Smaller prior β grains and coarser α laths were identified around the defects. In-situ tensile behavior investigations revealed that irregularly-shaped and elongated defects with high stress intensity factor (ΔKI) could induce severe strain accumulation, showing that the defect morphology was the primary factor that could determine the strain accumulation behavior. In the meantime, the inhomogeneous microstructures around the defects could further contribute to the strain accumulation, with the coarser α laths more favorable for deformation and stronger microtexture for easier slip transmission. In addition, microcracks were found initiated from a spherical pore with small loading, and arrested due to the negligible strain accumulation. Further characterization showed that oxidation, which was found more probable in spherical pores, was the cause of this microcrack initiation.

Pore Space Connectivity in Different Rock-Physics Methods—Similarity and Differences

This study is focused on the analysis of pore space connectivity in reservoir rocks. This parameter is of vital importance for the oil and gas industry since it controls hydraulic permeability. Five methods of rock physics are used for this goal. Three of these methods (self-consistent version of generalized singular approximation, Berryman self-consistent method, and differential scheme) take into account the pore space connectivity implicitly. The other two methods, the f-model of the generalized singular approximation and a similar modification of the Berryman method suggested in this work, allow for quantifying the connectivity via a special parameter (f-parameter). In order to reveal a physical meaning of this parameter, two simple models of carbonate rock (porous-cracked limestone) are considered. The first model is a double porosity model containing spherical pores and cracks. The second model contains only spherical pores, and their connectivity is expressed via the f-parameter. The pores and cracks are filled with brine and gas. Application of the two groups of methods for modeling the effective elastic properties of the carbonate rock gives a possibility of relating the f-parameter to the characteristics of the cracks and pores. The f-parameter is shown to be controlled by the relative crack volume in the total pore space. An increase in crack porosity and crack density leads to an increase in the f-parameter. A good correlation of the f-parameter with crack density is demonstrated. It is shown that for the porosity range 2–20%, a relationship between the f-parameter and crack density ε, in general, has the form f=alog10(ε)2+blog10(ε)+c for ε≤εmin. For the crack density less than εmin the f-parameter can be approximated by a constant value fmin. The values of εmin and fmin and coefficients a, b, and c depend on the porosity of spherical pores, saturation type, and pair of methods used for finding the link. These results give f-models an advantage in searching zones of the enhanced permeability and quantifying the ability of these zones to filtrate fluids.

Electrochemical Characteristics of Pitch Coated Silicon/Graphite Anode Composites Using Surface Modification Process

The electrochemical characteristics of pitch coated graphite / silicon composites were investigated as anode material in lithium ion batteries. The anode materials were prepared with etched graphite using nickel chloride hexahydrate to create spherical pores on the graphite surface. The pore size was controlled depending on the calcination retention time and optimized for the insertion of silicon nanoparticles. Scanning electron microscopy, x-ray diffraction and thermogravimetric analysis were used to analyze the physical properties of pitch coated graphite / silicon composites. The electrochemical characteristics of the batteries were investigated by charge-discharge cycle, rate performance, cyclic voltammetry and electrochemical impedance spectroscopy tests in the eletrolyte of 1.0 M LiPF6 ( EC : DMC : DEC = 1 : 1 : 1 vol % ). Graphite/silicon/pitch electrode showed better electrochemical properties than the graphite electrode, and even nickel etched graphite was superior to graphite. Also, it was confirmed that both capacity and rate performance were significantly improved when the ratio of graphite, silicon, and pitch is 8 : 1 : 1 ( G8 Si1 P1 ). It is found that G8 Si1 P1 have the initial discharge capacity of 680 mAh / g, the capacity retention ratio of 90 % during 100 cycles and the retention rate capability of 91 % in 2 C / 0.1 C, 87 % in 5 C / 0.1 C.

The capillary and thermal performance of the porous silicon nitride ceramics with nearly spherical pore structure

AbstractThe capillary and thermal performance of porous Si3N4 ceramics with nearly spherical pore structure has been investigated by altering the addition and diameter of pore‐forming agent polymethyl methacrylate (PMMA) in this work. An exponential model is used to evaluate the liquid uptake capacity of porous Si3N4 ceramics. Porous Si3N4 ceramics fabricated by 5 μm PMMA with 40 wt.% addition possess the lowest capillary time constant and show the best capillary performance owing to the perfect balance between friction resistance and capillary force. The thermal conductivity of porous Si3N4 ceramics is significantly impacted by their porosity. Alexander model with an exponent of .96 is suitable for predicting the thermal conductivity of porous Si3N4 ceramics due to its R‐squared up to .99. Moreover, with the addition and diameter of PMMA decrease, the flexural strength of porous Si3N4 ceramics increases. These results support the application of porous Si3N4 ceramics in the field of mass and heat transfer.

Using a plasma FIB system to characterise the porosity through the oxide scale formed on 9Cr-1Mo steel exposed to CO2

Focused ion beam microscopy and scanning electron microscopy have been used to characterise the porosity of the oxide scale of an experimental 9Cr–1Mo steel sample exposed for 4580 h in a CO2-rich environment. The magnetite shows a high frequency of spherical pores (~ 1 µm3) with no interconnectivity. The Cr-rich spinel layer shows greater interconnectivity, but no single pore spans the total oxide scale. A mechanism for the formation of the different morphologies observed across the scale is proposed, linking porosity changes across the oxide scale to the carburisation and elemental segregation of Cr within the substrate.Graphical abstract

Formation and evolution mechanisms of micropores in powder metallurgy Ti alloys

Mechanical properties and thermal deformation capabilities of powder metallurgy (P/M) Ti alloys are extremely susceptible to pores in the sintered parts. However, deciphering the pores evolution mechanisms is exceedingly challenging and heretofore this problem has yet been unequivocally understood. To address it, 3D X-ray computed tomography (XCT) is herein employed to perform this investigation. It is found that three types of pores encompassing branch-like pores (type Ⅰ), flat pores (type Ⅱ), and snatchy spherical pores (type III) form in the green compacts after the cold isostatic pressing, which is fundamentally governed by the irregularity of powder profiles. During sintering, type I pores are first to be decomposed at the junctions where pores branch triggered by stress concentration and subsequently undergo splitting and spheroidizing, till converting to (near) spherical pores. Type II and III pores are basically showing analogical behavior excluding the branch break-up step. Moreover, pores orientation distribution is turned out to be temperature independent, nevertheless, pores connectivity and localized porosity are closely related to the sintering temperature. Besides, a spot of pores is inspected inside partial large-sized master alloy powder particles, i.e., at 1200 °C, which possibly originates from the Kirkendall’s effect between the alloying elements and the Ti matrix.

Evaluation of pore scattering in transparent ceramics: A simplified model for nanometric spherical pores

AbstractIt is well known that light scattering by nanometric pores is adverse to realize a high transparency in ceramics; however, a suitable model that gives this phenomenon a quantitative evaluation is still lacking. In this work, a simplified model based on Rayleigh's theory was developed in order to unravel the light scattering by nanometric pores, taking polycrystalline KNNB‐xSm ceramics with high transparency (>67% at 780 nm) as example. The validity of this model was further verified by its good agreement with experimental results. Based on detailed simulations, we came to the conclusion that the pore volume fraction p is a more important parameter on the transparency of KNNB‐xSm ceramics than pore size dpore. Our findings in this work might provide inspiring insight to obtain highly transparent KNN‐based ceramics and more.

THE ELASTICITY THEORY PROBLEM FOR A HOLLOW SPHERE WITH A DISPLACED CAVITY CENTER

In this paper, the elastic state of a sphere is studied, inside which there is a spherical cavity (spherical pore) at an arbitrary distance from the center of the sphere. Expressions for the displacement components, strain, and stress tensors depending on the geometrical parameters of the problem and the pressure values on the surfaces of the outer and inner spheres were obtained.

A micromechanical analysis of strain concentration tensor for elastoplastic medium containing aligned and misaligned pores

In this paper, making use of Eshelby tensor for the elastoplastic medium, will be presented an approach for the strain concentration tensor for materials that present spherical or spheroidal pores. The pores are embedded in a homogeneous isotropic matrix which presents an elastoplastic behavior, governed by the von Mises yield criterion and isotropic hardening. In this context, it will also be analyzed the behavior of a porous material through the influence of the extraction of the isotropic part of the anisotropic operator. The effective properties of the material in each incremental step are obtained through the micromechanical Mori-Tanaka's model. Through elastoplastic strain concentration tensor for aligned or misaligned pores, it was observed that the pore direction represents a greater influence on the elastoplastic behavior of the material studied than aspect ratio (length divided by pore diameter). This approach has the advantage of being easily incorporated into conventional mean-field homogenization (MFH) schemes for the case where inclusion stiffness is neglected and presents different geometries or directions in 3D space.