Clusters Of Grains Research Articles

X-ray diffraction lineshape analysis of pulsed laser deposited ZnO nano-structured thin films

In this work, we report on the synthesis of highly textured c-oriented zinc oxide (ZnO) thin films on single crystal c-plane sapphire substrates using pulsed laser deposition (PLD). The growth experiments were performed at different substrate temperatures and oxygen pressures. The crystalline properties and surface morphology of the resulting films were studied using XRD peak analysis, rocking curve measurements, and SEM imaging. By varying deposition pressure and temperature, we observe different surface morphologies of the ZnO thin films, from smooth surfaces to clusters of grains and nanorods. The surface and volume structure of the films were found to be correlated. Within the range of experimental parameters used in this work, a growth temperature of 825 °C and a pressure of 10 mTorr lead to nearly epitaxial ZnO thin films having the largest grain size and the lowest strain perpendicular to the substrate surface i.e. along the growth axis.

Structural and optical properties of polyaniline-green silver nanocomposite

ABSTRACTA facile synthesis of nanocomposite of polyaniline-silver nanoparticles (PANI-AgNP) has been studied here. The PANI-Ag nanocomposites were synthesized by in-situ polymerization of aniline using ammonium peroxydisulphate (APS) as oxidizing agent with varying concentration of Ag nanoparticles (0, 5 and 10 ml). AgNP were green synthesized separately using live cell filtrate of Aspergillus foetidus. The characterization of developed nanocomposite have been done by following XRD, FT-IR, FESEM, TGA and UV-Vis for their structure, surface morphology respectively. The analysis of XRD demonstrates that both PANI and nanocomposites is amorphous in nature and the possibility of interaction between PANI and AgNP is present as observed from FT-IR spectroscopy. The microstructural study of nanocomposites shows clustering of grains, changes in morphology with aggregation after incorporation of green AgNP in PANI. Various spectroscopic and microscopic techniques reveal that though structurally different the nanocomposites have same band gap as that of pure PANI with the composites having better thermal stability as compared to the PANI. The obtained results can be useful for further development of conducting polymer-green nanocomposite.

Characterization of texture and grain boundary character distributions of selective laser melted Inconel 625 alloy

Select laser melting (SLM) is one of the practical metal additive manufacturing processes to fabricate complex structures. The inhomogeneous microstructure and high residual stress are typically resulted from a non-uniform melting and rapid solidification during a SLM process, which has the significant impact on part dimensional instability, corrosion resistance, and mechanical properties. The bulk of current research is limited to the general characterization of grain size, texture, and mechanical properties. However, grain boundary character distribution (GBCD) and its relationship with texture are yet to be understood. This study has shown that the preferred 〈110〉 along building direction and 〈100〉 along laser scanning direction were produced. The high density of low angle grain boundary (LAGB) can be an indicator of residual stress levels in the as-SLMed samples. The newly-developed ∑3, ∑9 and ∑27 grain boundaries were heavily populated, and the cluster of grains interfaced by ∑3n (n = 0, 1, 2…) was introduced at the expense of LAGB only at high recrystallization annealing (1150°C or over). The research results suggest that GBCD optimization be possible in the SLMed Inconel 625 alloy via suitable post annealing, which allows improving intergranular corrosion resistance and enhancing mechanical properties.

Bending transition in the penetration of a flexible intruder in a two-dimensional dense granular medium.

We study the quasistatic penetration of a flexible beam into a two-dimensional dense granular medium lying on a horizontal plate. Rather than a buckling-like behavior we observe a transition between a regime of crack-like penetration in which the fiber only shows small fluctuations around a stable straight geometry and a bending regime in which the fiber fully bends and advances through series of loading and unloading steps. We show that the shape reconfiguration of the fiber is controlled by a single nondimensional parameter L/L_{c}, which is the ratio of the length of the flexible beam L to L_{c}, a bending elastogranular length scale that depends on the rigidity of the fiber and on the departure from the jamming packing fraction of the granular medium. We show, moreover, that the dynamics of the bending transition in the course of the penetration experiment is gradual and is accompanied by a symmetry breaking of the granular packing fraction in the vicinity of the fiber. Together with the progressive bending of the fiber, a cavity grows downstream of the fiber and the accumulation of grains upstream of the fiber leads to the development of a jammed cluster of grains. We discuss our experimental results in the framework of a simple model of bending-induced compaction and we show that the rate of the bending transition only depends on the control parameter L/L_{c}.

Inertial shear flow of assemblies of frictionless polygons: Rheology and microstructure.

Motivated by the understanding of shape effects in granular materials, we numerically investigate the macroscopic and microstructural properties of anisotropic dense assemblies of frictionless polydisperse rigid pentagons in shear flow, and compare them with similar systems of disks. Once subjected to large cumulative shear strains their rheology and microstructure are investigated in uniform steady states, depending on inertial number I, which ranges from the quasistatic limit ([Formula: see text]) to 0.2. In the quasistatic limit both systems are devoid of Reynolds dilatancy, i.e., flow at their random close packing density. Both macroscopic friction angle [Formula: see text], an increasing function of I , and solid fraction [Formula: see text], a decreasing function of I, are larger with pentagons than with disks at small I, but the differences decline for larger I and, remarkably, nearly vanish for [Formula: see text]. Under growing I , the depletion of contact networks is considerably slower with pentagons, in which increasingly anisotropic, but still well-connected force-transmitting structures are maintained throughout the studied range. Whereas contact anisotropy and force anisotropy contribute nearly equally to the shear strength in disk assemblies, the latter effect dominates with pentagons at small I, while the former takes over for I of the order of 10-2. The size of clusters of grains in side-to-side contact, typically comprising more than 10 pentagons in the quasistatic limit, very gradually decreases for growing I.

Influence of differential speed rolling ratio on the ridging behavior of ultra purified 17%Cr ferritic stainless steel

The effect of the differential speed rolling on the ridging resistance and the crystallographic texture evolution of ultra purified 17%Cr ferritic stainless steel was investigated in order to select the optimal procedure for improving the surface quality. In the present study, the three different cold rolling processes were carried out: a conventional cold rolling and two different speed ratios asymmetric rolling. In the conventional cold rolling, the ridging of ferritic stainless steel was pronounced due to the band-like clusters of grains with similar crystallographic orientations. For differential speed rolling, ridging depth decreased and ridging was less pronounced when a higher different speed ratio was applied. The results show that severe ridging is attributed to the presence of band-like clusters of γ-fiber (〈111〉//ND) oriented grains. The shear bands introduced by differential speed rolling can enhance the nucleation for recrystallization both in grain interiors and grain boundaries, which can fragment the band-like clusters of similar orientation effectively, promoting the random grain orientation and improving surface quality.

Vortices Enhance Diffusion in Dense Granular Flows.

This Letter introduces unexpected diffusion properties in dense granular flows and shows that they result from the development of partially jammed clusters of grains, or granular vortices. Transverse diffusion coefficients D and average vortex sizes ℓ are systematically measured in simulated plane shear flows at differing inertial numbers I revealing (i)a strong deviation from the expected scaling D∝d^{2}γ[over ˙] involving the grain size d and shear rate γ[over ˙] and (ii)an increase in average vortex size ℓ at low I, following ℓ∝dI^{-1/2} but limited by the system size. A general scaling D∝ℓdγ[over ˙] is introduced that captures all the measurements and highlights the key role of vortex size. This leads to establishing a scaling for the diffusivity in dense granular flow as D∝d^{2}sqrt[γ[over ˙]/t_{i}] involving the geometric average of shear time 1/γ[over ˙] and inertial time t_{i} as the relevant time scale. Analysis of grain trajectories is further evidence that this diffusion process arises from a vortex-driven random walk.

Nonlinear softening of unconsolidated granular earth materials

Unconsolidated granular earth materials exhibit softening behavior due to external perturbations such as seismic waves, namely, the wave speed and elastic modulus decrease upon increasing the strain amplitude. In this letter, we describe a theoretical model for such behavior. The model is based on the idea that shear transformation zones (STZs)—clusters of grains that are loose and susceptible to contact changes and rearrangement—are responsible for plastic deformation and softening of the material. We apply the theory to experiments on simulated fault gouge composed of glass beads, and demonstrate that the theory predicts nonlinear resonance shifts and reduction of the P-wave modulus, in agreement with experiments. The theory thus offers insights on the nature of the critical state prior to failure on earthquake faults.

Nonlinear softening of unconsolidated granular earth materials

AbstractUnconsolidated granular earth materials exhibit softening behavior due to external perturbations such as seismic waves, namely, the wave speed and elastic modulus decrease upon increasing the strain amplitude above dynamics strains of about 10−6 under near‐surface conditions. In this letter, we describe a theoretical model for such behavior. The model is based on the idea that shear transformation zones—clusters of grains that are loose and susceptible to contact changes, particle displacement, and rearrangement—are responsible for plastic deformation and softening of the material. We apply the theory to experiments on simulated fault gouge composed of glass beads and demonstrate that the theory predicts nonlinear resonance shifts, reduction of the P wave modulus, and attenuation, in agreement with experiments. The theory thus offers insights on the nature of nonlinear elastic properties of a granular medium and potentially into phenomena such as triggering on earthquake faults.

In situ EBSD observation of grain boundary character distribution evolution during thermomechanical process used for grain boundary engineering of 304 austenitic stainless steel

Electron backscatter diffraction (EBSD) was used to examine the microstructural evolution in a one-step thermomechanically processed 304 austenitic stainless steel specimen during the thermomechanical process of grain boundary engineering. Solution-treated materials were cold-rolled to 3% reduction and subsequently annealed at 1220K for different annealing times. The EBSD observation of the specimen showed an increase in the frequency of coincident site lattice (CSL) boundaries and a decrease in the percolation probability of random boundaries. Additionally, the specimen exhibited heterogeneous growth of clusters of grains that contained a high frequency of CSL boundaries. These clusters of grains were developed in the entire observed area by strain-induced grain growth according to the results of grain orientation spread analysis. The details of the growth of the clusters and the disconnection of random boundaries were successfully observed in situ using EBSD and a heating stage. The frequency of CSL boundaries increased with the growth of the clusters. Disconnection of random boundaries between the clusters was achieved by the formation of annealing twins through the impingement of the growing clusters during the thermomechanical process. Twin variant selection to introduce CSL boundaries into a random boundary network was observed by the in situ EBSD observation.

A geometrical texture model for multi-mineral liberation prediction

We present a novel methodology for numerically reconstructing a three-dimensional (3D) texture for simulating multi-mineral rock and estimating the particle properties and liberation characteristics of its breakage products. A stochastic geometrical model has been developed to describe the intact structure of ores at any scale, potentially containing several mineral types, each with unique grade and grain size distribution parameters. Additional controls over the intact texture include the ability to vary the proportion of grain inclusions within grains of a different mineral phase and by varying the degree of spatial clustering of grains within the texture. Repeated random 2D and 3D sectioning of the 3D texture to simulate particle production and to calculate their compositions is used to generate outputs comparable to reports from mineral liberation analysis systems; including density distributions, liberation distributions, and particle class distributions. These outputs enable texture parameters to be fitted to corresponding measured liberation data and is demonstrated for a four-mineral sulphide ore, yielding a realistic distribution of particle properties, essential for accurately modelling particle separations.

Pore configuration landscape of granular crystallization

Uncovering grain-scale mechanisms that underlie the disorder–order transition in assemblies of dissipative, athermal particles is a fundamental problem with technological relevance. To date, the study of granular crystallization has mainly focussed on the symmetry of crystalline patterns while their emergence and growth from irregular clusters of grains remains largely unexplored. Here crystallization of three-dimensional packings of frictional spheres is studied at the grain-scale using X-ray tomography and persistent homology. The latter produces a map of the topological configurations of grains within static partially crystallized packings. Using numerical simulations, we show that similar maps are measured dynamically during the melting of a perfect crystal. This map encodes new information on the formation process of tetrahedral and octahedral pores, the building blocks of perfect crystals. Four key formation mechanisms of these pores reproduce the main changes of the map during crystallization and provide continuous deformation pathways representative of the crystallization dynamics.

Electromechanical Response of Polycrystalline Barium Titanate Resolved at the Grain Scale

Ferroic materials are critical components in many modern devices. Polycrystalline states of these materials dominate the market due to their cost effectiveness and ease of production. Studying the coupling of ferroic properties across grain boundaries and within clusters of grains is therefore critical for understanding bulk polycrystalline ferroic behavior. Here, three‐dimensional X‐ray diffraction is used to reconstruct a 3D grain map (grain orientations and neighborhoods) of a polycrystalline barium titanate sample and track the grain‐scale non‐180° ferroelectric domain switching strains of 139 individual grains in situ under an applied electric field. The map shows that each grain is located in a very unique local environment in terms of intergranular misorientations, leading to local strain heterogeneity in the as‐processed state of the sample. While primarily dependent on the crystallographic orientation relative to the field directions, the response of individual grains is also heterogeneous. These unique experimental results are of critical importance both when building the starting conditions and considering the validity of grain‐scale modeling efforts, and provide additional considerations in the design of novel ferroic materials.

Fracture reveals clustering in cohesive granular matter

We report an experimental study of the morphology of fractures in cohesive granular materials. Cohesion is introduced by equilibrating the grains with a humid atmosphere. The setup allows to produce a controlled crack in a thin layer of a glass beads assembly, and observe with an extremely high resolution the edge of the fracture at the free surface of the layer. The detailed multi-scale analysis of the fracture profile reveals the presence, in the bulk of the material, of clusters of grains whose size increases monotonically with the relative humidity. These results are important because the formation of clusters, resulting in a heterogeneity of the cohesion force, governs the mechanical properties of cohesive granular matter in contact with a humid atmosphere.

Mode of occurrences and geochemistry of amphibole in the Kolihan-Chandmari copper deposits, Rajasthan, India: insight into the ore-forming process

The Proterozoic Aravalli-Delhi Fold Belt in western India hosts some of the most important base metal sulfide deposits in the country, among which the Khetri group of deposits, North Delhi Fold Belt, is well known for its huge Cu repository. Copper mineralization in the Kolihan-Chandmari deposit, North Khetri Copper Belt, is hosted mainly by amphibole-bearing feldspathic quartzite and garnetiferous chlorite schist/quartzite. This study deals with mode of occurrences, textures and major and trace element geochemistry of amphiboles in amphibole-bearing feldspathic quartzite and their ore-genetic implications. Optically and geochemically different amphiboles, namely colorless A1, green A2 and blue A3, occur in these deposits. While the A1 amphiboles belong to Fe-Mg-Mn group (cummingtonite-grunerite), the A2 (magnesio-ferri-hornblende) and A3 (hastingsite, sadanagaite, tschermakite) amphiboles mostly belong to calcic group. The A1 amphiboles commonly occur as disseminated grains and clusters of grains while the A2 and A3 amphiboles occur mostly as veins and pockets. Amphiboles in the veins and pockets are generally randomly oriented, commonly with radial arrangement of the grains, implying a late-/post-deformation origin. Based on mode of occurrences and mineral replacement textures, the relative timing of formation is established as A1→A2→A3, A1 being the oldest. The A1 amphiboles may be metamorphic or hydrothermal in origin. However, the A2 and A3 amphiboles are entirely epigenetic hydrothermal in origin. Textural and geochemical characteristics demonstrate that the Na-Ca-poor, Fe-Mg-Mn-bearing A1 amphiboles were replaced by the Ca-rich A2 amphiboles, both of which in turn were replaced by the Na-Ca-K-Cl-rich A3 amphiboles. This is evidence of Fe (-Mg) metasomatism followed by Ca-Na and finally by Na-K (-Ca) metasomatism. Based on their high Cl and Na contents and ubiquitous association with Cl-rich marialitic scapolite (in contrast to A1 and A2 amphiboles), the A3 amphiboles are proposed to have crystallized from a hydrothermal fluid with a significant evaporite or basinal brine component. Calculated Fe3+ contents and Eu and Ce anomalies collectively indicate that the fluid parental to the A3 amphiboles was more oxidized compared to those related to the other two amphibole types. Sulfide mineralization, represented by chalcopyrite-pyrrhotite-magnetite-pyrite±uraninite±allanite, is associated only with this alkali and Cl-rich hydrothermal A3 amphibole veins and pockets. The mode of occurrences, mineralogy of the host rock and the mineralized veins in tandem with the geochemisty of amphibole suggests that the mineralization associated with the A3 amphiboles is the consequence of epigenetic hydrothermal mineralization from a Na-K-Cl-rich fluid. Copper was likely transported as Cu-chloride complexes and precipitated as Cu-sulfide due to destabilization of chloride complexes during precipitation of Cl-rich amphibole and Cl-rich marialitic scapolite. The A3 amphiboles, associated with sulfide mineralization, are characteristically enriched in Cu, Ni and most other trace elements compared to the A1 and A2 amphiboles. This study thus suggests that the major and trace element compositions of amphibole can be used in differentiating mineralized vs. non-mineralized systems. Involvement of high salinity oxidized brine in mineralization, abundant Na-K (-Ca) alteration, ubiquitous presence of magnetite in the ores, and other circumstantial evidence indicate that the epigenetic hydrothermal sulfide mineralization associated with the A3 amphibole veins and pockets in the Kolihan-Chandmari deposits has many characters akin to iron oxide copper gold (IOCG) style mineralization.

A method of coupling discrete dislocation plasticity to the crystal plasticity finite element method

A method of concurrent coupling of planar discrete dislocation plasticity (DDP) and a crystal plasticity finite element (CPFE) method was devised for simulating plastic deformation in large polycrystals with discrete dislocation resolution in a single grain or cluster of grains for computational efficiency; computation time using the coupling method can be reduced by an order of magnitude compared to DDP. The method is based on an iterative scheme initiated by a sub-model calculation, which ensures displacement and traction compatibility at all nodes at the interface between the DDP and CPFE domains. The proposed coupling approach is demonstrated using two plane strain problems: (i) uniaxial tension of a bi-crystal film and (ii) indentation of a thin film on a substrate. The latter was also used to demonstrate that the rigid substrate assumption used in earlier DDP studies is inadequate for indentation depths that are large compared to the film thickness, i.e. the effect of the plastic substrate modelled using CPFE becomes important. The coupling method can be used to study a wider range of indentation depths than previously possible using DDP alone, without sacrificing the indentation size effect regime captured by DDP. The method is general and can be applied to any problem where finer resolution of dislocation mediated plasticity is required to study the mechanical response of polycrystalline materials, e.g. to capture size effects locally within a larger elastic/plastic boundary value problem.

Heterogeneous grain-scale response in ferroic polycrystals under electric field.

Understanding coupling of ferroic properties over grain boundaries and within clusters of grains in polycrystalline materials is hindered due to a lack of direct experimental methods to probe the behaviour of individual grains in the bulk of a material. Here, a variant of three-dimensional X-ray diffraction (3D-XRD) is used to resolve the non-180° ferroelectric domain switching strain components of 191 grains from the bulk of a polycrystalline electro-ceramic that has undergone an electric-field-induced phase transformation. It is found that while the orientation of a given grain relative to the field direction has a significant influence on the phase and resultant domain texture, there are large deviations from the average behaviour at the grain scale. It is suggested that these deviations arise from local strain and electric field neighbourhoods being highly heterogeneous within the bulk polycrystal. Additionally, the minimisation of electrostatic potentials at the grain boundaries due to interacting ferroelectric domains must also be considered. It is found that the local grain-scale deviations average out over approximately 10–20 grains. These results provide unique insight into the grain-scale interactions of ferroic materials and will be of value for future efforts to comprehensively model these and related materials at that length-scale.

Effects of Annealing Process on the Structural, Optical and Electrical Properties of Copper Oxide Thin Films Grown by Immersion Technique

The synthesized and characterization on the growth of copper oxide thin films on fluorine-doped tin oxide (FTO) coated glass with annealing and without annealing process has been studied by immersion techniques. Furthermore, ZnO layer has been used in order to improved the absorption spectrum of CuO films. The copper oxide films were analyzed on the morphological, structural, optical and electrical by field emission scanning electron microscopy (FESEM), X-ray diffractometer (XRD), UV-Vis spectroscopy (absorbance) and I-V characteristics instruments. The atomic force microscope (AFM) was used in order to characterize the surface imaging of copper oxide films and the thicknesses were measured using a surface profiler. The AFM studies revealed that the roughness of the CuO films increased after annealing was applied this is due to the formation of large clusters of grains from the merging of small clusters grains. The CuO films thicknesses also becomes two times higher than the CuO films without annealing process.

Assessment of residual stress fields at deformation twin tips and the surrounding environments

Stress fields close to twin tips and the associated local neighbourhoods of a hexagonal close-packed (HCP) polycrystal were studied in detail. For this purpose, a coarse grain textured Zircaloy-2 sample was firstly strained uniaxially in a macroscopic direction that favours tensile twin formation. The sample was then unloaded and residual elastic strains and lattice rotations measured using the high-resolution electron backscatter diffraction (HR-EBSD) technique. Measured elastic strain maps of various clusters of grains including parent and twin pairs were then analysed. Stress, dislocation density, and their associated concentrations close to twin tips, within twins, in the immediate neighbouring grain, at the intersection of two twins, and within parent grains were investigated. It is shown that the stress field at the twin tips varies as a function of local neighbourhood. High stress, lattice rotation, and dislocation density concentrations were generally observed close to twin tips both within twins and within the immediate neighbouring grains. It is shown that dislocation density concentration is maximum at the intersection of two twins which can potentially provide susceptible site for crack nucleation.

On the correlation of meso-scale local texture and roping profile in AA6xxx sheet alloys

Roping as a heterogeneous plastic deformation is generally attributed to the occurrence of the meso-scale clustering of grains with similar orientations. Large-scale electron backscattered diffraction (EBSD) orientation maps are customarily used to correlate the orientation topography with the roping profile. The most common way of investigating this phenomenon is to extract the predominant texture components and then to correlate them with the roping profile, since grains belong to different texture components lead to different plastic responses. Instead of using a microscopic representative volume element in the length scale of the grain size, the present work proposes a moving window mechanical model to use a representative volume element of the meso-scale, corresponding to a grain cluster, to simulate roping. For a tensile test in the transverse direction, a quantitative prediction of surface roping profile can be obtained. For an artificial EBSD orientation map, the proposed model can yield both roping wavelength and amplitude.