Anisotropic Grain Research Articles

Extreme Clusters of Grains in Random Microstructures of Polycrystals

It was experimentally observed that in polycrystalline materials under low macro loading of the specimen the first sites of failure initiation take place in the specific clusters of few grains. In some grains of these extreme clusters, the local (meso-) strains and stresses are high enough to cause first damages or plastic slips. In the stochastic microstructure of polycrystals, the formation of an extreme cluster is random and rare. Nevertheless, they govern the failure process initiation and can severely affect the reliability of polycrystalline machine parts. It is time and resource consuming to search and investigate extreme clusters on the real specimens of polycrystalline materials experimentally. A theoretical tool is desirable. Here we present the powerful computational method to look for extreme clusters, to investigate their possible patterns, and to evaluate the absolute maximums of local strains/stresses that can be achieved in these clusters. The experimentally observed clusters consist of few (3-4) preferably oriented neighboring grains or even of one big supergrain. The strain and stress bursts arise due to an interaction of the grains. One can expect that in bigger clusters, larger local bursts of fields can be generated. We found the typical forms of the extreme clusters (small and big) in four different polycrystals with grains of a weak and strong anisotropy for the case of uniaxial tension. In all regarded cases, the extreme clusters have the forms of the symmetrical patterns. In big clusters of highly anisotropic grains, the maximum of mesostrain exceeds the macrostrain by several times. In clusters of weakly anisotropic grains, the local strain concentration is rather moderate (tens of percents).

Hydrothermal synthesis of ABi2Ta2O9Aurivillius phase: A comparative study of A-site cation size on structure, dielectric, optical properties

In this study, the double-layered Aurivillius phases CaBi2Ta2O9 (CBT) and PbBi2Ta2O9 (PBT) were prepared through a hydrothermal route with NaOH as a mineralizer. XRD analysis confirmed that the CBT and PBT compounds were successfully formed and adopted an orthorhombic crystal structure with an [Formula: see text]21am symmetry. Le Bail refinements of XRD data indicated that the unit cell volume of CBT was smaller than PBT and is associated with the smaller ionic radius of [Formula: see text] compared to [Formula: see text]. The surface morphology of both samples, as determined using SEM, demonstrated plate-like grains with anisotropic grain growth. It was found that the different ionic radii of [Formula: see text]-site cations ([Formula: see text] and [Formula: see text] strongly affected the structural, optical and electrical properties of the Aurivillius phase. The occupation of smaller [Formula: see text] cations induced a higher structural distortion, which resulted in higher bandgap ([Formula: see text] energy and ferroelectric transition temperature ([Formula: see text] of CBT, compared to those of PBT.

Real-time super-resolution mapping of locally anisotropic grain orientations for ultrasonic non-destructive evaluation of crystalline material

Estimating the spatially varying microstructures of heterogeneous and locally anisotropic media non-destructively is necessary for the accurate detection of flaws and reliable monitoring of manufacturing processes. Conventional algorithms used for solving this inverse problem come with significant computational cost, particularly in the case of high-dimensional, nonlinear tomographic problems, and are thus not suitable for near-real-time applications. In this paper, for the first time, we propose a framework which uses deep neural networks (DNNs) with full aperture, pitch-catch and pulse-echo transducer configurations, to reconstruct material maps of crystallographic orientation. We also present the first application of generative adversarial networks (GANs) to achieve super-resolution of ultrasonic tomographic images, providing a factor-four increase in image resolution and up to a 50% increase in structural similarity. The importance of including appropriate prior knowledge in the GAN training data set to increase inversion accuracy is demonstrated: known information about the material’s structure should be represented in the training data. We show that after a computationally expensive training process, the DNNs and GANs can be used in less than 1 second (0.9 s on a standard desktop computer) to provide a high-resolution map of the material’s grain orientations, addressing the challenge of significant computational cost faced by conventional tomography algorithms.

Anisotropic grain boundary area and energy distributions in tungsten

Describing microstructure evolution in tungsten requires a quantitative description of the anisotropic grain boundary energy. We present a grain boundary energy function for tungsten that specifies the energy of an arbitrary boundary given its five macroscopic crystallographic parameters. A comparison of measured grain boundary areas and the grain boundary energies given by the function at the Σ11, Σ17b, and Σ33a misorientations, which are problematic to determine by measurement or atomistic calculations, reveals inverse correlations that are similar to what have been observed in other metals.

Microstructure formation of α/β-sialons prepared via an aqueous processing route and from cost effective silicon nitride powders

High quality α/β-Sialon materials were reproducible prepared using cost effective silicon nitride powders and an aqueous processing route. The influence of the powder quality (SN-E10, Silzot HQ, SicoNide P95H) on sintering and phase formation was investigated. With all 3 powders dense Sialon materials with nominal composition RxSi12-(m + n)Alm + nOnN16-n and m = 0.5 and n = 1 and 4 wt% excess rare earth additives could be prepared. The material based on Silzot HQ exhibits the highest α-Sialon content and subsequently the highest hardness due to the lower oxygen content of the starting powder.The microstructural analysis using XRD, FESEM including EDX- and EBSD-mapping reveal, that the α-Sialon grains are formed on existing α-Si3N4 grains of the starting powder. Anisotropic grain growth of the α-Sialon takes place therefore the differences in the microstructure depend strongly on the oxygen content but not from the α/β-content of the starting powders.

Abnormal grain growth in ultrafine grained Ni under high-cycle loading

Abnormal grain growth can occur in polycrystalline materials with only a fraction of grains growing drastically to consume other grains. Here we report abnormal grain growth in ultrafine grained metal in a rarely explored high-cycle loading regime at ambient temperature. Abnormal grain growth is observed in electroplated Ni microbeams with average initial grain sizes less than 640 nm under a large number of loading cycles (up to 109) with low strain amplitudes (< 0.3%). Such abnormal grain growth occurs predominantly in the family of grains whose <100> orientation is along the tensile/compressive loading direction. Micromechanics analysis suggests that the elastic anisotropy of grains dictates the thermodynamic driving force of abnormal grain growth, such that the lowest strain energy density of the <100> oriented grain family dominates grain growth. This work unveils a unique type of abnormal grain growth that may be harnessed to tailor grain microstructures in materials.

Viscoplastic behavior of a porous polycrystal with similar pore and grain sizes: Application to nuclear MOX fuel materials

This study deals with the viscoplastic behavior of a porous polycrystal with pores and grains of similar sizes. Such a microstructure can be encountered in irradiated nuclear Mixed OXide (MOX) fuel materials. MIcronized MASter blend (MIMAS) MOX are multi-phase materials mainly composed of two or three phases depending on their fabrication process. One of these phases corresponds to plutonium-rich agglomerates which strongly evolve during irradiation. The large Pu-rich agglomerates become highly porous due to the accumulation of fission gases and to the apparition of irradiation bubbles. In a past study, Wojtacki et al. (2020) showed that pores distributed inside the Pu-rich clusters have a strong impact on the overall viscoplastic behavior of MOX fuel, when considering a purely isotropic behavior for the Pu-rich clusters. In the present study, the impact of pores similar in size to the surrounding anisotropic grains on the overall viscoplastic behavior is studied in details through numerical full-field simulations. A crystal plasticity model recently developed by Portelette et al. (2018) is used to describe the anisotropic behavior of the polycrystalline matrix with dislocation glide mechanisms. Three-dimensional full-field simulations are performed by a method based on Fast Fourier Transforms (FFT) to compare the behavior of porous materials with that of dense materials. These simulations show that, in the case of spherical pores, their relative size with respect to that of the grains plays a minor role in the overall viscoplastic behavior. However, in the case of polyhedral pores, the relative size effect is more pronounced. With fixed porosity, decreasing the relative size of the cavities with respect to the size of grains leads to a softening of the material and a decrease of the viscoplastic flow stress.

Novel estimation method for anisotropic grain boundary properties based on Bayesian data assimilation and phase-field simulation

Utilizing the data assimilation and multi-phase-field grain growth model, this study proposes a novel framework of measuring anisotropic (nonuniform) grain boundary energy and mobility. The framework can evaluate a large number of boundary properties from typical observations of grain growth without requiring specifically designed experiments or calculations. In this method, by optimizing the multi-phase-field model parameters such that the simulation results are in good agreement with the observation data, the energies and mobilities of multiple individual boundaries are directly and simultaneously estimated. To validate the method, numerical tests on boundary property estimation were performed using synthetic microstructure dataset generated from grain growth simulations with a priori assumed property values. Systematic tests on simple tricrystal systems confirmed that the proposed method accurately estimates each boundary energy and mobility within an error of only several % of their assumed true values even for conditions with strong property anisotropy and grain rotation. Further numerical tests were conducted on a more general multi-grain system, showing that our method can be successfully applied to complicated polycrystalline grain growth. The obtained results demonstrate the potential of the proposed method in extracting a large dataset of grain boundary properties for arbitrary boundaries from actual grain growth observations.

A 2D Front-Tracking Lagrangian Model for the Modeling of Anisotropic Grain Growth.

Grain growth is a well-known and complex phenomenon occurring during annealing of all polycrystalline materials. Its numerical modeling is a complex task when anisotropy sources such as grain orientation and grain boundary inclination have to be taken into account. This article presents the application of a front-tracking methodology to the context of anisotropic grain boundary motion at the mesoscopic scale. The new formulation of boundary migration can take into account any source of anisotropy both at grain boundaries as well as at multiple junctions (MJs) (intersection point of three or more grain boundaries). Special attention is given to the decomposition of high-order MJs for which an algorithm is proposed based on local grain boundary energy minimisation. Numerical tests are provided using highly heterogeneous configurations, and comparisons with a recently developed Finite-Element Level-Set (FE-LS) approach are given. Finally, the computational performance of the model will be studied comparing the CPU-times obtained with the same model but in an isotropic context.

Enhanced ionic conductivity of aluminum tungstate by crystallographic orientation in a strong magnetic field

AbstractThe ionic conduction of multiply charged ions, rather than singly charged ions, is beneficial for energy storage and sensor applications. The low mobility of multiply charged ions is one important obstacle to the implementation of these applications. Chemical methods, such as doping and solid solution formation, have been used to improve ionic conductivity. However, the apparent performance of ceramic electrolytes can be improved by the crystallographic alignment of anisotropic grains. In this study, crystal‐oriented aluminum tungstate ceramics were processed by slip casting in a strong magnetic field. The b‐axis‐ and c‐axis‐oriented aluminum tungstate ceramics can be produced by this technique. The orientation of grains along the b‐axis could enhance ionic conductivity by at least 1.77 times compared to that of a randomly oriented sample and 2.13 times compared to that of the c‐axis‐oriented sample. The results of this study suggest that this method can improve the ionic conductivity of an anisotropic material using polycrystalline processing instead of difficult single‐crystal synthesis techniques.

The influence of microstructural anisotropy on the hot deformation of wire arc additive manufactured (WAAM) Inconel 718

Hybrid additive manufacturing, incorporating additive manufacturing (AM) and other thermo-mechanical processes, has been developed to improve AM mechanical properties by modifying the as-deposited microstructure and eliminating defects. Additive manufactured parts present strong anisotropic properties, as shown by the anisotropic columnar grain morphology and texture. Samples of AM Inconel 718 were tested at high temperature and under uniaxial compression over a range of conditions. The evolution of microstructural anisotropy and the viscoplastic behaviour under these hot deformation processes was studied. The microstructure and texture evolution were characterised with optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that the initial anisotropic microstructure had a negligible effect on flow stress and slip system activation during the hot deformation. The shape of original grains did, however, play a predominant role in determining the final microstructure. When the compression direction was perpendicular to the longitudinal of columnar grains, a more uniform microstructure was obtained under high-flow-stress conditions. This preferred compression direction provides guidance for hot deformation in hybrid additive manufacturing practice. Furthermore, for the nickel alloy studied, controlling the deformation direction to achieve a fine grain structure at a lower temperature (950 °C, lower than δ-solvus) brings practical benefits in the form of possible further δ grain refinement and less demanding thermal conditions during subsequent deformation processes.

Comparative Study and Limits of Different Level-Set Formulations for the Modeling of Anisotropic Grain Growth.

In this study, four different finite element level-set (FE-LS) formulations are compared for the modeling of grain growth in the context of polycrystalline structures and, moreover, two of them are presented for the first time using anisotropic grain boundary (GB) energy and mobility. Mean values and distributions are compared using the four formulations. First, we present the strong and weak formulations for the different models and the crystallographic parameters used at the mesoscopic scale. Second, some Grim Reaper analytical cases are presented and compared with the simulation results, and the evolutions of individual multiple junctions are followed. Additionally, large-scale simulations are presented. Anisotropic GB energy and mobility are respectively defined as functions of the mis-orientation/inclination and disorientation. The evolution of the disorientation distribution function (DDF) is computed, and its evolution is in accordance with prior works. We found that the formulation called “Anisotropic” is the more physical one, but it could be replaced at the mesoscopic scale by an isotropic formulation for simple microstructures presenting an initial Mackenzie-type DDF.

Ultrafine Cemented Carbides with Cobalt and Iron Binders Prepared via Reactive &lt;i&gt;In Situ&lt;/i&gt; Sintering

Reactive sintering of cemented carbides involves mechanical and thermal activation of precursor elemental powders, followed by in-situ synthesis of tungsten carbide. This approach promotes formation of ultrafine microstructure favored in many cemented carbide applications. Our study focuses on the effect of mechanical activation (high-energy milling) on the properties of powder and following thermal activation (sintering) on the microstructure characteristics and phase composition. Reactive sintering proved effective – an ultrafine grained microstructure of cemented carbides with Co and Fe binders was achieved. Formation of tungsten carbide grains was complete at low temperature during reactive spark plasma sintering, resulting in textured microstructure with anisotropic grain formation and growth.

Mechanical properties of quartz sand and gypsum powder (plaster) mixtures: Implications for laboratory model analogues for the Earth's upper crust

Granular materials are a useful analogue for the Earth's crust in laboratory models of deformation. Constraining their mechanical properties is critical for such model's scaling and interpretation. Much information exists about monomineralic granular materials, such as quartz sand, but the mechanical characteristics of bimineralic mixtures, such as commonly-used quartz sand mixed with gypsum powder (i.e. plaster), are largely unconstrained. We used several mechanical tests (density, tensile, extension, shear) to constrain the failure envelope of various sand-plaster mixtures. We then fitted linear Coulomb and parabolic Griffith failure criteria to obtain cohesions and friction coefficients. Tests of the effects of emplacement technique, compaction and humidity demonstrated that the most reproducible rheology is given by oven-drying, pouring and mechanically compacting sand-plaster mixtures into their experimentation container. As plaster content increases, the tensile strength of dry sand-plaster mixtures increases from near zero (pure quartz sand) to 166 ± 24 Pa (pure plaster). The cohesion increases from near zero to 250 ± 21 Pa. The friction coefficient varies from 0.54 ± 0.08 (sand) to 0.96 ± 0.08 (20 wt% plaster). The mechanical behaviour of the resulting mixtures shifts at 20–35 wt% plaster from brittle Coulomb failure along a linear failure criterion, to more complex brittle-plastic Coulomb-Griffith failure along a non-linear failure criterion. With increasing plaster content, the brittle-plastic transition occurs at decreasing depth within a pile of sand-plaster mixture. We infer that the identified transitions in mechanical behaviour with increasing plaster content relate to (1) increasing porosities, (2) increasing grain size distributions, and (3) a decrease in sand-sand grain contacts and corresponding increase in contacts of anisotropic gypsum-gypsum grains. The presented characterisation enables a more quantitative scaling of the mechanical behaviour of sand-plaster mixtures, including their tensile strength. Sand-plaster mixtures can thereby realistically simulate brittle-plastic properties of the Earth's crust in scaled laboratory models.

Performance comparison of phased array transducers for inspection of dissimilar welds on nuclear reactor components

Trimetallic weld joint of steam generator is crucial and most critical component of Prototype Fast Breeder Reactor (PFBR). Ultrasonic examination of trimetallic welds are more challenging due to the coarse anisotropic grain structure of the weld metal and the mismatch of the acoustic impedance at the weld metal interface, which contributes to increase scattering, sound beam deviation and distortion, reflection and refraction of the incident sound field. This paper presents a comparative study on phased array ultrasonic examination of dissimilar weld joints (P91 – Alloy 800) using linear array (LA) and dual matrix array (DMA) transducer. Prior to the experimental investigations, wave propagation through the dissimilar weld joint and the defect sensitivity is understood by carrying out simulation based on ray-tracing and semi-analytical methods. Weld joint sample with artificial defect such as lack of side wall fusion was fabricated and studied using both transducer of Phased Array Ultrasonic Testing (PAUT).

Formation of anisotropic grains and modified ferroelectric properties in Cr‐doped Bi5FeTi3O15 thin films

AbstractAurivillius phase Bi5CrxFe1−xTi3O15 (0≤ x ≤1) thin films are prepared by the chemical solution deposition method, and the effect of Cr content on the microstructure, ferroelectric property, and electric transport behavior of Bi5CrxFe1−xTi3O15 films is investigated. X‐ray diffraction analysis shows that all of Bi5CrxFe1−xTi3O15 films are complete solid solution and maintain the Aurivillius structure. The replacement of Fe3+ with smaller Cr3+ decreases anisotropy and lattice aspect ratio in a‐b plane, which is minimized at the composition of Bi5Cr0.5Fe0.5Ti3O15. This changes the grain shape from sphere to plate, and Bi5Cr0.5Fe0.5Ti3O15 film consists of only plate‐like grains. Cr doping increases saturated polarization (Pm) and decreases coercive field (Ec). Cr doping increases Pm of Bi5CrxFe1−xTi3O15 film to 35 μC/cm2, but decreases Ec down to 125 kV/cm. A decrease in the lattice aspect ratio of a‐b plane promotes the alignment of ferroelectric dipoles under electric field. The frequency‐dependent dielectric property and the leakage current show that the plate‐like grains of Cr‐rich Bi5CrxFe1−xTi3O15 films suppress the transport of carriers from grains to grains and prevents a dramatic leakage current increase. The results of this study provide a design rule to control the ferroelectricity of Aurivillius phase Bi5CrxFe1−xTi3O15 thin films by modifying the composition and lattice aspect ratio.

Evolution on phase composition and properties of alumina-based ceramics fabricated from high-titania special-grade natural bauxite micropowder

In this study, Al2O3-based ceramics were fabricated with natural bauxite powder as the raw material. The phase compositions evolution behavior during the heat treatment and its influence on the properties of fabricated ceramics were investigated. With increasing heat treatment temperature, the sintering degree of high-titania special-grade bauxite became better. The samples showed decreased porosity, increased bulk density, Vickers hardness, fracture toughness and flexural strength. However, when the heat temperature increased to 1650 °C, decomposition of tieillite occurs in sample, leading to increased Al2O3 and TiO2 content in the liquid phase. Corundum and mullite grains with anisotropic growth appeared in the samples, leading to a decrease in the density, the Vickers hardness and flexural strength of samples decreased consequently. However, those anisotropic grains could prolong the crack propagation path and improve the fracture toughness of the material.

La3+ substitution induced structural transformation in CaBi4Ti4O15 Aurivillius phases: Synthesis, morphology, dielectric and optical properties

In this work, La-substituted CaBi4Ti4O15 Aurivillius phase, CaBi4-xLaxTi4O15 (x = 0, 0.5, 1, 1.5, and 2), were synthesized by a molten salt method. The effects of La3+ substitution on structure, morphology, dielectric and optical properties were systematically investigated. Refinement of XRD data confirm that a ferroelectric orthorhombic structure with space group A21am was adopted for x = 0, 0.5, 1, and 1.5. In terms of x = 2, the structure transformed to an intermediate paraelectric orthorhombic structure of the Amam space group. The anisotropic plate-like grains were observed across all the samples, which decreased in size with an increase in La3+ content. With La3+ substitution, the ferroelectric transition temperature (Tc) significantly decreased since the structural distortion as the effect of 6s2 lone pair electron of Bi3+ was reduced with the substitution of La3+. Additionally, the reduced structural distortion was responsible for a decrease in the bandgap (Eg) energy. A diffused phase transition of ferroelectric characterized by the broadened Tc peak was induced by La3+ substitution since the increased disorder of A-site cation (Ca/Bi/La). The ferroelectric phase with a lower Tc and the lower Eg energy proposes the sample of x = 1.5 as a potential material for electrocaloric and solar cell applications.

A fully dynamic bridging approach for modeling the intergranular failure mechanisms in 2D polycrystalline materials

This paper presents a multiscale approach to model the dynamic transition between macro, micro, and homogenized atomistic scales in 2D polycrystalline materials. At the macroscale, the material is assumed as an isotropic homogeneous medium. The material at the microscale, in contrast, is characterized by anisotropic grains with stochastic morphologies and random crystalline orientations. The Boundary Element Method (BEM) is used to model both macro and micro scales using isotropic and anisotropic formulations, respectively. To connect both scales, the macro transient responses at internal points are prescribed as boundary conditions to the microscale analysis. The mechanical behavior at the interfaces between grain boundaries is assessed using Cohesive Interface Elements (CIEs). Based on the Multiscale Cohesive Zone Model (MCZM), this approach transfers the homogenized strain energy evaluated in the CIE during the microscale analysis to an atomistic arrangement. In order to define a failure criterion from the atomistic interpretation, the extended Finnis–Sinclair potential is applied to describe the interactions between atoms. Finally, we present some examples of intergranular crack propagation resulting from the rupture of atomic bonds due to the interatomic forces.

Bismuth layer‐structured ferroelectrics with non‐sheet‐like polyhedral microstructures

AbstractBismuth layer‐structured ferroelectrics (BLSFs) are promising candidates for a variety of applications because of their high Curie temperature and environmentally benign nature as compared to their lead contained counterparts. Featured by the mica‐like grain growth habit and spontaneous polarization along crystallographic a‐b plane, it is difficult to obtain excellent ferroelectric properties for most of the BLSFs fabricated by the conventional ceramic route. Herein, a new approach is reported to obtain Bi2WO6 and Bi4Ti3O12 BLSF ceramics with non‐sheet‐like polyhedral microstructures. The intriguing ferroelectric hysteresis behavior, dielectric tunability, and domain structures are successfully observed. The Bi2WO6 ceramic with a unique single‐domain structure in each grain shows a typical ferroelectric polarization‐electric field hysteresis loop with a remnant polarization of ~17.8 μC cm−2 and a coercive field of ~43 kV cm−1. These values have never been obtained in Bi2WO6 ceramics with sheet‐like microstructure. The Bi4Ti3O12 ceramic with irregular‐shaped and curved tiny domains exhibits an antiferroelectric‐like behavior, which is significantly different from the reported Bi4Ti3O12 counterparts that have plate‐like anisotropic grains. The present study would stimulate microstructure design and property modulation for other layer‐structured materials.