Formation Of Inhomogeneities Research Articles

Experimental Study on the Anisotropy of Layered Rock Mass under Triaxial Conditions

Weak and hard inhomogeneous rock formations are typically encountered during tunnel excavations. The physical and mechanical properties and geological conditions of these rock formations vary significantly; thus, it is crucial to investigate the mechanical characteristics of deep bedded composite rock formations. Three-dimensional (3D) scanning and 3D printing were used to prepare composite rock specimens to simulate natural rock laminae. Triaxial compression tests were conducted to determine the influence of the bedding angle, rock composition, and confining pressure on the mechanical properties of the composite rock specimens. The anisotropic strength characteristics and the damage patterns of the composite rock specimens were analyzed under different confining pressures, and the failure mechanism during triaxial loading was revealed. The results show that the damage of the composite rock specimens with a bedding structure depends on the bedding dip angle and the rock formation. The stress-strain curves and peak strengths of the composite rock specimens have anisotropic characteristics corresponding to their failure modes. As the bedding dip angle increases, the peak strength of the three groups of specimens first decreases and then increases under different confining pressure levels. The compressive strength has a nonlinear relationship with the confining pressure, and the difference between the compressive strengths of specimens with different inclination angles decreases as the confining pressure increases. The Hoek–Brown strength criterion is a good predictor of the nonlinear increase in peak strength of the composite rock specimens under different confining pressures. The specimen with a β = 60°dip angle shows the most significant increase in the strength difference with increasing confining pressure. The results can be used as a reference for testing and analyzing the anisotropic mechanical properties of bedded rock masses.

A Melt Pool Temperature Model in Laser Powder Bed Fabricated CM247LC Ni Superalloy to Rationalize Crack Formation and Microstructural Inhomogeneities

This study of the laser powder bed fusion (LPBF) of γ′-strengthened Ni superalloy CM247LC focuses on the development of a melt pool temperature model to predict crack density within the alloy. This study also analyzes spatter and elemental evaporation, which might cause defects and inhomogeneities, at different melt pool temperatures. The melt pool temperature model provides more accurate predictions than the widely used energy density model. Spatter particles were collected and characterized to study their sizes and chemical compositions, compared with the virgin powder, recycled powder, and as-built samples, to probe the impact of their entrapment into the melt pool. This study also investigated Al evaporation, revealing that its extent does not correlate with the laser energy density and is believed to be rather limited by comparing the chemistry of the virgin powder and the build. Last, the impact of LPBF process parameters on the formation of these inhomogeneities, and accordingly crack formation, was studied using finite element analysis by estimating the maximum melt pool temperature and correlating it with the formation of the microstructural inhomogeneities. The morphology of the various cracking modes was associated with the process parameters.

Conditions for the Violation of Concentrational Homogeneity of Fe\u2013Ni Invar Alloys

Conditions for the formation of microconcentration inhomogeneities in Fe–Ni alloys, which can decrease their invar characteristics, are determined. The nickel separation can be reached in the course of short-term annealing as a result of deformation-induced nickel segregation, nickel redistribution between martensite and austenite and between ferrite and austenite as well.

Мікроструктура сплаву ВТ8 після випробувань на малоциклову втому

Specimens of titanium alloy VT8, which is used for the manufacture of gas turbine engine elements, were investigated in the initial state and after fracture toughness testing by methods of transmission electron microscopy and diffraction analysis. The features of the microstructure, structure morphology, the nature of phase distribution and structural components were established. Defects in the crystal structure, the formations of dislocation inhomogeneities and local concentrators of internal stresses were identified using JEM-200CX transmission electron microscope. The scalar dislocation density is determined by the secant method. The study of VT8 titanium alloy samples before and after destruction, which is used for the manufacture of GTE elements, using the methods of transmission electron microscopy and diffraction analysis was made. Microstructural investigations for a detailed analysis of the structure features, morphology and phase formations distribution, as well as their components establishment, the nature of crystal lattice defects, the formation of dislocation inhomogeneities and local concentrators of internal stresses were performed on a JEM-200CX transmission electron microscope. The scalar dislocation density was measured by the secant method. It is shown that the studied samples of VT8 titanium alloy are characterized by a two-phase (α + β) microstructure in the form of large -phase plates, 0.15 ... 0.76 μm in size, interspersed with an insignificant amount of thin-plate β-phase, with a size of 0.04 ... 0.21 μm. Based on scalar dislocation densities, the level of local internal stresses in the places of dislocation accumulations, which are sources of crack formation, was analytically estimated. Dispersed particles of secondary phases characterized by different sizes and different structure morphologies were identified. The calculated dislocation densities and an estimate of the average distance over which they move in the process of deformation are used as the basis for creating a statistical map of localized deformation level indicators in the alloy structural components and on the fracture surface. It is shown that as a result of fracture after testing for low-cycle fatigue, the dislocation density increases, the level of local internal stresses increases, and the formation of a cellular structure in the α- and β-phases and deformation grain-boundary defects occurs. Keywords: VT8 alloy, dislocation structure, microstructure, transmission electron microscopy, local internal stresses.

Seeded growth of ultrathin gold nanoshells using polymer additives and microwave radiation

Nanoshells made of a silica core and a gold shell possess an optical response that is sensitive to nanometer-scale variations in shell thickness. The exponential red shift of the plasmon resonance with decreasing shell thickness makes ultrathin nanoshells (less than 10 nm) particularly interesting for broad and tuneable ranges of optical properties. Nanoshells are generally synthesised by coating gold onto seed-covered silica particles, producing continuous shells with a lower limit of 15 nm, due to an inhomogeneous droplet formation on the silica surface during the seed regrowth. In this paper, we investigate the effects of three variations of the synthesis protocol to favour ultrathin nanoshells: seed density, polymer additives and microwave treatment. We first maximised gold seed density around the silica core, but surprisingly its effect is limited. However, we found that the addition of polyvinylpyrrolidone during the shell synthesis leads to higher homogeneity and a thinner shell and that a post-synthetic thermal treatment using microwaves can further smooth the particle surface. This study brings new insights into the synthesis of metallic nanoshells, pushing the limits of ultrathin shell synthesis.

Investigation of interface microstructure and mechanical properties of rotatory friction welded dissimilar aluminum-steel joints

The objective of this work is to analyse the effect of variations in IMC thickness and microstructure at interface on the mechanical properties, for the RFW joints of Al6061 and mild steel. Rotary friction welding is a solid-state joining process which is advantageous towards joining of dissimilar materials. The rotary friction welding of Al6061 and mild steel is performed at combinations of two rotational speeds (1200 rpm and 1400 rpm) and three feed rates (10 mm/min, 20 mm/min and 30 mm/min). The microstructural characterization, tensile and hardness testing of the joints is performed. A concave joint interface is noticed due to inhomogeneous plastic deformation along the radial direction from center to workpiece periphery. Similarly, inhomogeneous intermetallic compounds formation along the radial direction at the joint interface is observed, with higher intermetallic compound thickness near the workpiece periphery. The grain refinement on Al6061 side of the interface is attributed to the continuous dynamic recrystallization. Dynamic recovery is noticed at the mild steel side. The crack initiates at the IMC rich region near the periphery and follows the IMCs to grow radially inwards. Along the radial direction from periphery to the center, the reduction in IMC thickness and increase in DRX region thickness resists the growth of the crack. Brittle failure is observed in the IMC rich regions near the periphery and ductile failure is noticed near the center. An increase in IMC thickness reduces the joint strength and wider DRX region improves the joint strength. This combination led to the maximum tensile strength for the joints prepared with intermediate feed rate of 20 mm/min. A maximum joint tensile strength of 136 MPa is achieved, which corresponds to 65 % joint efficiency.

Radiation hardening and optical properties of materials based on SiO2

Preliminary studies have shown that the optical absorption spectra of radiation-colored glasses correspond to the spectral behavior of the scattering losses of an optically inhomogeneous medium. The reasons for the same optical changes in glasses of different compositions are the radiation-induced electric charge separation in the structurally nano-inhomogeneous glass volume, polarization and formation of nanometer optical inhomogeneities. The authors of this work prove that the radiation changes in the mechanical and optical properties of silicate glasses are of the same nature. The performed estimates indicate that the electric charge separation in the glasses occurs up to absorbed doses of about 1 MGy. The local electric charge separation due to the appearance of Coulomb forces leads to radiation hardening of the glasses. The estimated Coulomb hardening of the quartz glasses was ~ 107 Pa. The theoretical results were experimentally confirmed by measuring the mechanical properties of the glasses under high intensity proton irradiation as well as by testing the mechanical strength of a composite material based on quartz glass. Under proton irradiation with a dose rate of 5×103 Gy/s (energy of 8 MeV) up to threshold doses of ~ (1 – 5) ×106 Gy in the KU-1 quartz glasses, the decrement of acoustic vibrations decreased due to Coulomb hardening. After gamma irradiation with 1.34×105 Gy, the tensile strength of the composite material based on quartz glass increased by up to 20 MPa. This value is in the range of estimates of Coulomb hardening of quartz glasses. It is also shown that ionizing radiation does not affect the elastic modulus of materials based on SiO2.

Production Method under Surveillance: Laboratory Pilot-Scale Simulation of CH4–CO2 Exchange in a Natural Gas Hydrate Reservoir

The “guest exchange” of methane (CH4) by carbon dioxide (CO2) in naturally occurring gas hydrates is seen as a possibility to concurrently produce CH4 and sequester CO2. Presently, process evaluation is based on CH4–CO2 exchange yields of small- or medium-scale laboratory experiments, mostly neglecting mass and heat transfer processes. This work investigates process efficiencies in two large-scale experiments (210 L sample volume) using fully water-saturated, natural reservoir conditions and a gas hydrate saturation of 50%. After injecting 50 kg of heated CO2 discontinuously (E1) and continuously (E2) and a subsequent soaking period, the reservoir was depressurized discontinuously. It was monitored using electrical resistivity, temperature and pressure sensors, and fluid flow and gas composition measurements. Phase and component inventories were analyzed based on mass and volume balances. The total CH4 production during CO2 injection was only 5% of the initial CH4 inventory. Prior to CO2 breakthrough, the produced CH4 roughly equaled dissolved CH4 in the produced pore water, which balanced the volume of the injected CO2. After CO2 breakthrough, CH4 ratios in the released CO2 quickly dropped to 2.0–0.5 vol %. The total CO2 retention was the highest just before the CO2 breakthrough and higher in E1 where discontinuous injection improved the distribution of injected CO2 and subsequent mixed hydrate formation. The processes were improved by the succession of CO2 injection by controlled degassing at stability limits below that of the pure CH4 hydrate, particularly in experiment E2. Here, a more heterogeneous distribution of liquid CO2 and larger availability of free water led to smaller initial degassing of liquid CO2. This allowed for quick re-formation of mixed gas hydrates and CH4 ratios of 50% in the produced gases. The experiments demonstrate the importance of fluid migration patterns, heat transport, sample inhomogeneity, and secondary gas hydrate formation in water-saturated sediments.

Visual Determination of Transformer Oil Quality Parameters

During operation of the transformer, the oil in it undergoes profound changes usually referred to as aging. The electrical insulating properties of it deteriorate, and sediments accumulate on the active part, which makes it difficult to remove heat. Inhomogeneous solid formations can induce not only the voltage breakdown, but also impair the parameters such as viscosity and other char-acteristics of the oil, which are important for the reliable operation of the power transformers. Aromatic groups initially is present in the transformer oil composition during the operation of the oil, and because of the aging processes, the concentration of the unsaturated cyclic com-pounds increases. The aim of this work is to develop a new method for the determination of the aromatic compounds and colloidal particles. This goal is achieved using a visual inspection of the transformer oil samples in the visible range. The most significant result of the paper is the establishment of a correlation between the acid number of the transformer oil (KOH), the tan-gent of the dielectric loss angle and the type of oil image under the visible light illumination. The significance of the obtained results lies in the fact that the presented method based on the analy-sis of the scattered and transmitted radiation, makes it possible to determine the presence of the aromatic compounds and colloidal particles in the transformer oil, as well as to define their con-centration and size. This method simplifies the analysis of the transformer oil quality, and re-duces the cost of the research, which is an important factor for the electric power industry.

Formation of a phase separated structure in the Zr–Cu–Fe–Al alloys by thermo-mechanical processing

The article analyzes the effect of the addition of iron in the Zr–Cu–Al system metallic glassy alloys on the phase separation process during thermo-mechanical treatment. The addition of Fe causes formation of nanostructured regions of chemical inhomogeneity after cold rolling and annealing below the glass transition temperature. The resulted effect on the mechanical properties of the investigated alloys is established. Changes in the structure during thermo-mechanical treatment are studied in detail by transmission electron microscopy. The increase in the microhardness of the alloys during the formation of inhomogeneities in the amorphous matrix is explained by the difficulty of propagation of the shear bands formed during deformation.

Crossover of the relative heat transport contributions of plume ejecting and impacting zones in turbulent Rayleigh-Bénard convection (a) (a)Contribution to the Focus Issue Turbulent Thermal Convection edited by Mahendra Verma and Jörg Schumacher.

Turbulent thermal convection is characterized by the formation of large-scale structures and strong spatial inhomogeneity. This work addresses the relative heat transport contributions of the large-scale plume ejecting vs. plume impacting zones in turbulent Rayleigh-Bénard convection. Based on direct numerical simulations of the two dimensional (2-D) problem, we show the existence of a crossover in the wall heat transport from initially impacting dominated to ultimately ejecting dominated at . This is consistent with the trends observed in 3-D convection at lower Ra, and we therefore expect a similar crossover to also occur there. We identify the development of a turbulent mixing zone, connected to thermal plume emission, as the primary mechanism for the takeover. The mixing zone gradually extends vertically and horizontally, therefore becoming more and more dominant for the overall heat transfer.

Formation and Characterization of Concentration Inhomogeneities in Melt-Grown Crystals

Formation and Characterization of Concentration Inhomogeneities in Melt-Grown Crystals

Nanoscale InN clusters and compositional inhomogeneities in InGaN epitaxial layers quantified by tip-enhanced Raman scattering

We investigate the compositional homogeneity of InGaN thin films with a high In content grown by migration-enhanced plasma-assisted metal-organic chemical vapor deposition. Micro-Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) are used to analyze the local InGaN composition on the micro- and nanoscale. Based on conventional micro-Raman mapping, the InGaN composition for all samples appears uniform but shows indications for intrinsic phase separations. TERS, a nanoscopic technique with a high spatial resolution far below the diffraction limit, verifies the formation of nanoscale compositional inhomogeneities. The dimensions of these compositional fluctuations observed in TERS are confirmed by scattering-type scanning near-field infrared nanoscopy (s-SNIN). In contrast to s-SNIN, we show that TERS furthermore enables the quantification of the In content in the different compositional regions and even allows the identification of InN nanoclusters near the surface of the epitaxial films.

Characterization of complex die-pressed Al2O3 green compact using liquid immersion, X-ray tomography, and numerical simulations

This study examined the compaction behavior of a green ceramic component with a complex shape formed by die pressing at 50 MPa using spray-dried alumina. Compared to a simple cylindrical sample, the sample with a complex shape revealed a higher degree of microstructural inhomogeneity and crack formation. Granule deformation and pore distribution at different sample locations were observed by optical microscopy after infiltrating liquid into the voids of a green compact. The refractive index of the immersion liquid should be different slightly from that of alumina for better observations. X-ray micro-computed tomography was also used to visualize the pore distribution and crack shape. Numerical simulations based on the Drucker-Prager/Cap model were performed to distinguish the stress and displacement distribution within the compact. The significant stress gradient at the crack initiation point could explain crack formation, whereas the application of a higher pressure resulted in a further increase in stress gradient.

Impact of neodymium doping on the dielectric and electrical properties of CCTO synthesized by a facile sol–gel technique

In this work, Ca1–xNdxCu3Ti4O12 (x = 0, 0.3 mM) ceramics were prepared by a facile sol–gel technique. The structure and crystalline nature of the samples were analyzed by X-ray diffraction characterization technique. FESEM analysis envisaged the inhomogeneous formation of ceramic material and confirmed the grain size was greatly affected by the dopant. The EDS analysis confirmed the existence of Ca, Cu, Nd, Ti, and O which illustrates the purity and stoichiometric ratios of the products. A colossal dielectric constant (ɛr = 5.5 × 105) was achieved for Neodymium-doped CaCu3Ti4O12 ceramics in comparison with undoped CaCu3Ti4O12 ceramics dielectric constant ɛr = 1.1 × 105 at 50 Hz and temperature 500 °C. Complex impedance analysis indicates that the existence of a non-Debye type of relaxation phenomenon of the material, which is further substantiated from the electrical modulus spectrum. Dielectric relaxation time for various temperatures of undoped and neodymium-doped CaCu3Ti4O12 ceramics were calculated from both imaginary parts of impedance spectrum and electric modulus spectrum. The activation energy was considerably varied 0.2102–0.1910 eV for undoped and 0.2741–0.2425 eV for Neodymium-doped CaCu3Ti4O12 ceramics, which suggests that the electrical conduction behaviour was primarily originated with the mobility of ionized oxygen defects.

Continuum damage model for low-cycle fatigue of metals: An overview

This work aims to give a comprehensive and accurate view of those micromechanical models that have been developed over the past two decades by our research team. We hereby refer to the distinct capabilities and theoretical difficulties of such models published in various journals by discussing them in a more integrated manner. We also believe that our community can find a significant benefit from this work through below mentioned discussions, which answer many of the questions asked by researchers and those interested in this field. Returning to this work topic, the low-cycle fatigue (LCF) life is highly affected by microstructural instabilities, inhomogeneity and shear bands formation. The first type of damage is related to the nuclei of fatigue microcracks, for several FCC and BCC metals generally governed by continuous irreversible slips within the intensive slip bands. The second type of damage concerns ductile damage caused by cavitation induced by plastic strain and hydrostatic stress. Thereby, fatigue-failure is a relevant topic that requires further explanations to better understand the plausible damaging mechanisms of microcracks and/or microvoids at lower levels of observation. To model the microcracks initiation, some attempts were conducted via micromechanical models assuming, for example, that the microcracks initiate at the crystallographic slip system level describing the LCF response of metals under simple and complex loading paths. In this overview, the latest development of mixed approach with double character called Micromechanical Determinist-Probabilistic Model (MDPM) coupled with damage will be presented with its generalized structure.

Landau damping in one dimensional periodic inhomogeneous collisionless plasmas

Landau damping in a collisionless plasma is a well-known example of wave particle interaction. In the past, this phenomenon was addressed for homogeneous equilibria in the linear and non-linear limit of the perturbation amplitude. However, in reality, equilibria are almost always inhomogeneous or non-uniform in space. Considering a one dimensional, collisionless, unmagnetized, electrostatic plasma with stationary ions and kinetic electrons in a periodic inhomogeneous exact equilibrium of scale k0−1 as the starting point, the fate of a small amplitude (linear) perturbation of scale k−1 is investigated using a Vlasov–Poisson solver. Three different spatial regimes, namely, k0 > k, k0 ∼ k, and k0 < k, are addressed. In the k0 > k regime, long wavelength perturbation k is found to generate (k ± Nk0) modes (where N is an integer), which allows damping of long wavelength perturbation in an inhomogeneous plasma and formation of phase-space vortices at phase velocities vϕ = ω/(k ± Nk0). Perhaps for the first time, novel phenomena such as “inhomogeneity induced Landau damping arrest” and “inhomogeneity induced plasma echo” are observed in k0 ∼ k and k0 < k regimes, respectively. New scaling laws as a function of inhomogeneity amplitude are also reported.

Understanding the Selective Deposition of Li Metal on Nonuniform Electrode Surfaces Using Atomic Force Microscopy

The use of lithium metal in secondary batteries has been impeded by its tendency to form dendrites: branching conductive structures of metal that can lead to capacity loss and, ultimately, internal shorts in the battery. Patterned electrodes, in addition to artificially increasing the current density of cells by increasing the surface area available for reaction, also generate a nonuniform electric field in the vicinity of the electrode surface. This nonuniform electric field, though rapidly screened by the electrolyte, can promote inhomogeneous deposition and Solid Electrolyte Interphase formation. As the consequence of these effects is not theoretically apparent since Solid Electrolyte Interphase volume and conductivity changes can, in principle, offset variations in local current density, we have performed experiments to examine the deposition of Li on nonuniform electrode surfaces using Atomic Force Microscopy. We measure the local variations in topography, SEI thickness, and composition, and discuss their implications for the formation of dendrites in Li metal.

The influence of inhomogeneous hydrate formation on permeability anisotropy of porous media

Hydrate formation is an inhomogeneous process, which results in permeability anisotropy, i.e., permeability differs in different directions. Permeability anisotropy of hydrate-bearing sands is analyzed based on CT (computed tomography) scanning and pore network modeling in this study. A digital sample is divided into eight different parts based on hydrate distribution. Eight equivalent micropore structures are extracted by the maximum ball method. The absolute and relative permeabilities of each part in three different directions (x, y, and z) are calculated by a permeability simulator. Before hydrate formation, the sand pack permeability in the different directions is nearly the same. After hydrate formation, the sample pore structures become more complex. Both the absolute and relative permeabilities vary in the different directions. When hydrate saturation exceeds a threshold, the degree of permeability anisotropy is weakened. Hydrate formation induces a more significant relative gas permeability anisotropy compared to relative water permeability anisotropy. The degree of relative water permeability anisotropy increases at high water saturations.

Improvement of Sweep Efficiency in a Heterogeneous Reservoir

ABSTRACT As it is known, many reservoir formations have an irregular permeability profile. Such heterogeneity leads to irregular profiles of the injected water progression and its unequal distribution, if traditional technology is used. High heterogeneity of the reservoir leads to early water breakthroughs and clogging of production wells, in addition: some areas containing naphtha remain difficult to access. The relevance of the issue under consideration is due to the demand to increase oil recovery from reservoirs. The purpose of the paper is to analyze methods of influencing a non-uniform formation. This paper provides an overview of studies aimed at greater reservoir coverage by exposure when developing non-uniform reservoirs; The main parameters of the formation that affect the decrease in the coverage of the reservoir by exposure are identified, and the improvement of the impact on the inhomogeneous formation by changing the density of perforation holes is considered. The materials of the paper are of practical value, since hydrodynamic modeling was carried out to determine the effect of changes in the density of perforations on the coverage of the formation.