Polycrystalline Materials Research Articles

Elastic films of single-crystal two-dimensional covalent organic frameworks.

The properties of polycrystalline materials are often dominated by defects, and two-dimensional (2D) crystals can even be divided and disrupted by a line defect1-3. In contrast, 2D crystals are often required to be processed into films, which are inevitably polycrystalline and contain numerous grain boundaries, and therefore are brittle and fragile, hindering application in flexible electronics, optoelectronics and separation1-4. Moreover, similar to glass, wood, and plastics, they suffer from trade-off effects between mechanical strength and toughness.5, 6 Here, we report a method to produce highly strong, tough and elastic films of an emerging class of 2D crystals - 2D covalent organic frameworks (COFs) composed of single-crystal domains connected by interwoven grain boundary on water surface using an aliphatic bi-amine as a sacrificial go-between. Films of two 2DCOFs were demonstrated, which showed Young's moduli and breaking strength of 56.7 ± 7.4 GPa and 73.4 ± 11.6 GPa, and 82.2 ± 9.1 N/m and 29.5 ± 7.2 N/m, respectively. We envisage the sacrificial go-between guided synthesis method and the interwoven grain boundary will inspire grain boundary enigineering of various polycrystalline materials, endowing them with new properties, enhancing their current applications and paving the way for new applications.

Fracture mechanisms and suppressed temperature effects of YBCO superconducting materials dominated by grain boundaries

YBa2Cu3O7-δ (YBCO) superconducting materials are typical polycrystalline materials, and the presence of intrinsic grain boundaries (GBs) is the main reason that affects superconductivity, in which high-angle GBs lead to low critical current density. There is no doubt that GBs also play an important role in the mechanical properties and fracture mechanisms of YBCO superconducting materials. In this work, the uniaxial tensile molecular simulations have been performed to investigate the fracture mechanisms of YBCO superconducting materials dominated by different types of GBs. The (100) single crystal and (110) twinning boundary models have been employed as comparative references. The results show that the low-angle GBs of YBCO consist of discontinuous dislocation cores, while the high-angle GBs are composed of a series of structural unit arrangements. Meanwhile, Young's modulus and ultimate tensile strength of YBCO can be enhanced by twin boundaries, while the other GBs have different effects on Young's modulus and ultimate tensile strength. In addition, the expansion of dislocation cores and the deformation of characteristic structural units are the main fracture mechanisms. More significantly, a novel temperature effect has been found to be significantly suppressed under the high-angle GBs. These findings not only provide a deep understanding of the GB structures, but may also provide guidance for modulating the mechanical properties of YBCO superconducting materials.

The evolution of intergranular networks during grain growth and its effect on percolation behavior

Triple junctions (TJs) are line defects in three-dimensional (3D) polycrystalline materials where three grains meet. Transport along the TJs depends on the connectivity between them. Here, we investigate the connectivity of more than 6000 TJs in a 3D microstructure of pure iron (Fe) through the lens of bond percolation. Our efforts are made possible by synchrotron-based x-ray diffraction tomography, which allows us to resolve the TJ network both temporally and spatially, during grain growth. In the framework of standard percolation theory, we determine a percolation threshold of the TJs, above which there exists a continuous pathway of TJs that travels infinitely far. Our experimental results indicate a surprisingly different percolation threshold (by 16%) compared to models and theories. The reason for the discrepancy is that the real TJ network has a topological disorder that is not present in the idealized microstructures considered in prevailing models and theories. Leveraging the wealth of time-resolved data we trace the origin of this disorder by following the topological transitions that accompany grain growth. Overall, the insights obtained in this study can help guide the design of polycrystalline materials via defect engineering.

An alternate approach for estimating grain-growth kinetics

Rate of grain growth, which aides in achieving desired properties in polycrystalline materials, is conventionally estimated by measuring the size of grains and tracking its change in micrographs reflecting the temporal evolution. Techniques adopting this conventional approach demand an absolute distinction between the grains and the interface separating them to yield an accurate result. Edge-detection, segmentation and other deep-learning algorithms are increasingly adopted to expose the network of boundaries and the associated grains precisely. An alternate approach for measuring grain-growth kinetics, that curtails the need for advanced image-processing treatment, is presented in this work. Grain-growth rate in the current technique is ascertained by counting the number of triple-( and quadruple-) junctions, and monitoring its change during the microstructural evolution. The shifted focus of this junction-based treatment minimises the significance of a well-defined grain-boundary network, and consequently, the involvement of the sophisticated techniques that expose them. A regression-based object-detection algorithm is extended to realise, and count, the number of junctions in polycrystalline microstructures. By examining the change in the number of junctions with time, the growth rate is subsequently determined. Growth kinetics estimated by the present junction-based approach, across a wide-range of multiphase polycrystalline microstructures, agree convincingly with the outcomes of the conventional treatment. Besides offering a novel technique for grain-growth measurement, the analysis accompanying the current work unravels a trend, compatible with the topological events, in the progressive evolution of the triple-junctions count. The present approach, through its underlying algorithm, provides a promising option for monitoring grain-growth during in-situ investigations.

Electrical resistivity and magnetoresistance of Sr3Fe2+xMo1–xO9–3x/2 (x = 0.45, 0.60, and 1.00) prepared by the solid-state reaction

In this paper, we focus on understanding the behavior of the temperature dependence of electrical resistivity (ρ) and magnetoresistance (MR) in Sr3Fe2+xMo1–xO9–3x/2 perovskites with x = 0.45, 0.60, and 1.00. Three polycrystalline materials were synthesized using conventional solid-state reaction techniques. Rietveld analysis of room-temperature X-ray diffraction data reveals that all three compositions crystallize in tetragonal symmetry; where x = 0.45 and 0.60 undergo an I4/mcm space group, while x = 1.00 adopts a P4/mmm space group. Electrical resistivity measurements carried out at magnetic fields (0 and 1 T) reveal the presence of a transition from the ferromagnetic phase to the paramagnetic phase in the vicinity of the TMI. The metal-insulator transition temperature (TMI) decreases with increasing Fe content and rises slightly with changing magnetic field from 0 to 1 T. The magnetoresistance (MR) of the SrFeO3–δ sample (x = 1.00) has a large value of about 63.6%, indicating that it can be exploited for certain devices. A variety of theoretical models are used to study the behavior of electrical resistivity in various temperature ranges, showing excellent agreement with experimental results.

Synergistic adsorption and photocatalysis study of TiO2 and activated carbon composite

The discharge of organic pollutants by the textile and dyeing industries presents an escalating threat to aquatic environments, necessitating the development of effective remediation strategies. This study introduces the utilization of graphite-like structured activated carbon (AC), derived from highland barley straw—a biomass unique to the Plateau regions of China, including Tibet, Qinghai, and Gansu—as a support material for the TiO2 catalyst. TiO2/AC composites with different TiO2 loadings were synthesized by ultrasonic impregnation. The TiO2/AC composites were found to be polycrystalline materials composed of anatase and rutile phases. The TiO2 nanoparticles are well-dispersed over the surface of the AC. The photocatalytic activity of these composites was evaluated through their capacity to degrade a methylene blue (MB) solution upon irradiation. It was observed that the inclusion of TiO2 increases the number of adsorption sites and active sites for methylene blue, with the photocatalytic activity being notably higher at a 3-wt% TiO2 loading, achieving a remarkable 99.6 % degradation efficiency for 100 mg/L MB within 100 min. The experimental kinetic data for the photocatalytic process follow the pseudo-first-order kinetic model. Furthermore, TiO2/AC retains high photocatalytic activity after five reaction cycles. This research provides valuable insights into the application of biomass-derived materials for the purification of water, offering a sustainable solution to both pollution and agricultural waste challenges in Plateau areas of China.

An atomistic survey of shear coupling in asymmetric tilt grain boundaries and interpretation using the disconnections framework

Grain Boundaries (GB) play an important role in determining the behavior of polycrystalline materials. While the mechanisms of motion and associated shear response for symmetric tilt grain boundaries (STGBs) are well studied, the same is not true for asymmetric tilt grain boundaries (ATGBs) despite their greater prevalence in polycrystals. This study aims to investigate the shear response of a large collection of asymmetric tilt grain boundaries (ATGBs) using molecular dynamics (MD) simulations and interpret the data using a discrete disconnections model that works remarkably well for STGBs. MD simulations of shear-driven ATGBs show that the plastic shear (shear coupling factor) in the region swept by a GB exhibits a complex dependence on the inclination angle, and this dependence changes with the misorientation of the GB. In addition, the shear response was observed to be highly sensitive to the applied shear rate and temperature. Recognizing the spatial and temporal scale limitations of MD simulations, we extended the discrete disconnections mesoscale model of Khateret al. (2012) to calculate the nucleation barriers of disconnection modes and predict the effective shear coupling of an ATGB. We observed that the mesoscale model’s predictions of the shear coupling factor of ATGBs do not agree with those observed in MD simulations. Finally, we examine the hypotheses of our mesoscale model that contribute to disagreements between MD simulations and the mesoscale model and propose improvements to the mesoscale model for future work.

In-situ rapid nondestructive characterization of multiple irregular three-dimensional grain boundary structures and electrical properties by nanorobot with SEM in ZnO

To address the problem of spatial quantification of irregular grain boundaries, a unique approach for the nondestructive characterization of multiple irregular three-dimensional grain boundaries is proposed using nanorobots with scanning electron microscopy (SEM) to accomplish in-situ rapid synchronized quantification of structures and electrical properties. By constructing a three-dimensional trigonometric geometric mapping relationship, multiple irregular three-dimensional grain boundary structures and electrical properties can be rapidly quantified. Experimental results demonstrate that it is feasible to accomplish in-situ rapid nondestructive characterizations. The two-dimensional profile shapes of the targeted grain boundaries at each layer exhibit irregular and non-uniform features in the Y-Z plane. The hypotenuse length between the virtual space tilting lines presents a three-dimensional characteristic in the X-Z plane. The spatial tilt angle of the virtual space tilting line can be calculated using the right-triangle relation. The unequal numbers of grains along the same tilted hypotenuse direction with equal lengths directly illustrate the existence of irregular and nonlinear grain boundaries in bulk ZnO ceramics. The relatively uniform spatial sizes of the grain boundary illustrate the coupled spatial characteristics existing in the irregular micro-structures. The relatively uniform equivalent circuit fitting parameters confirmed the existence of relatively uniform doping components in bulk ZnO ceramics. Furthermore, this method provides a unique approach for the rapid nondestructive quantitative analysis of multiple three-dimensional irregular grain boundaries in bulk polycrystalline materials.

Comparison of the relationship between hardness and wear resistance of polycrystalline diamond and cubic boron nitride

AbstractHardness is an important indicator of the wear resistance of tool materials. The study of the effect of hardness on the wear resistance of tool materials has significant practical applications. In this experimental work, a series of polycrystalline cubic boron nitride (PcBN) and diamond (PCD) compacts with different hardness are sintered under high temperature and pressure, and these samples were fashioned into turning tools, used for edge wear testing in the granite turning process. Based on the measurements above, the relationship between hardness and wear resistance in two materials with identical grain sizes (1–2 µm) was examined, and the results reveal a significant positive correlation between the hardness and wear resistance of the two materials. Remarkably, the relationship between hardness and wear resistance in two materials complies with the same type of fitting function. But incredibly, the wear resistance of the two materials exhibits inconsistency as the hardness increases. Specifically, below 40 GPa Vickers hardness, both materials exhibit comparable wear resistance. However, above 40 GPa, the difference in wear resistance between the two materials becomes more pronounced with increasing hardness. At the Vickers hardness of 70 GPa, the abrasion ratio of the PCD is approximately 3.5 times that of the PcBN, and it is anticipated that the disparity between them will continue to expand within the broader range of hardness. Further analysis reveals that the intrinsic hardness of the crystal grains significantly impacts the wear resistance of polycrystalline materials.

Structural, microstructural, dielectric, electrical, and magnetic properties of a scandium-modified bismuth ferrite

ABSTRACT This paper reports on the synthesis method and characterisation of a binary polycrystalline material (Pb0.6Bi0.2Sc0.2)(Fe0.4Ti0.6)O3 ceramic. The room temperature XRD pattern confirms an orthorhombic crystal structure with average crystallite size of 25.3 nm. Study on SEM micrograph reveals the homogeneous dispersal of grains and well-defined grain boundaries and also the compactness of the grains implies the development of a highly dense material. The purity and composition of this material are verified using EDX, and confirms presence of all the constituent elements (Pb, Bi, Sc, Fe, Ti, and O) both in weight and atomic percentage. According to research on dielectric spectra as a function of temperature range of 25°C to 500°C and frequency range of 1 kHz to 1 MHz, there may be Maxwell–Wagner-type dispersion at low frequencies, which considerably decreases towards higher frequencies because of decreasing space charge polarisation. The negative temperature coefficient of resistance behaviour is observed from impedance spectroscopy. The research of modulus spectroscopy demonstrates the existence of a non-Debye type of relaxation. The thermally activated charge carriers regulate the study of the conduction mechanism. The semicircular arcs found both in Nyquist and Cole–Cole plots provide an evidence for the semiconducting nature. The magnetic study reveals the superparamagnetic nature of the sample.

Uniaxial deformation behaviors and mechanism of Ti–6Al–4V alloy with bi-oriented grains

It is of great importance to investigate the deformation and damage mechanisms of polycrystalline materials with soft crystal orientations, considering the soft grains are usually the fatigue crack initiation sites. In this study, a Ti–6Al–4V alloy fully consisted of <01 1‾ 0> and <1‾ 21‾ 0> α (soft) grains was prepared and its uniaxial deformation behavior was investigated. Interestingly, no significant asymmetry on the tension-compression yield strength was found in this alloy with bi-grain orientations, though mechanical asymmetry was generally found in the traditional Ti alloys with strong texture. After high temperature solution treatment, the preferred bi-orientations were retained and the asymmetry degree of yield strength did not change, showing an excellent stability in texture and mechanical performance. Deformation mechanisms during tension and compression were studied and discussed. This work provides a new understanding on the deformation and damage mechanisms in the grains with soft crystal orientations, and may guide the microstructural design of Ti alloy for pursuing high mechanical and fatigue performances in the future.

Direct Single Crystal to Amorphous Transformation and Memory Effect in AlPO4-17.

Pressure induced amorphization provides a distinct route to prepare novel amorphous materials. Single crystals of the porous aluminophosphate AlPO4-17 directly transform to an amorphous state beginning at 0.6 GPa, without fragmentation into polycrystalline material. Apart from a reduction in dimensions, the amorphous material retains the form of the initial single crystal. Remnant crystalline domains in the amorphous material also preserve the initial orientation of the single crystal. X-ray diffraction indicates the compression of the structure around the empty pores in the xy plane and such an amorphization mechanism is consistent with a direct structural relationship between the single crystal and amorphous forms. The collapse of the initial pore volume is almost complete at 2.5 GPa. A memory effect is observed in the amorphous form, which strongly expands on decompression. The present process opens the way for the synthesis of topologically ordered amorphous materials approaching "perfect glasses" with improved mechanical properties.

An analytical representation of the 2d generalized balanced power diagram

Tessellations are an important tool to model the microstructure of cellular and polycrystalline materials. Classical tessellation models include the Voronoi diagram and the Laguerre tessellation whose cells are polyhedra. Due to the convexity of their cells, those models may be too restrictive to describe data that includes possibly anisotropic grains with curved boundaries. Several generalizations exist. The cells of the generalized balanced power diagram are induced by elliptic distances leading to more diverse structures. So far, methods for computing the generalized balanced power diagram are restricted to discretized versions in the form of label images. In this work, we derive an analytic representation of the vertices and edges of the generalized balanced power diagram in 2d. Based on that, we propose a novel algorithm to compute the whole diagram.

Phase-field modeling of stored-energy-driven grain growth with intra-granular variation in dislocation density

We present a phase-field (PF) model to simulate the microstructure evolution occurring in polycrystalline materials with a variation in the intra-granular dislocation density. The model accounts for two mechanisms that lead to the grain boundary migration: the driving force due to capillarity and that due to the stored energy arising from a spatially varying dislocation density. In addition to the order parameters that distinguish regions occupied by different grains, we introduce dislocation density fields that describe spatial variation of the dislocation density. We assume that the dislocation density decays as a function of the distance the grain boundary has migrated. To demonstrate and parameterize the model, we simulate microstructure evolution in two dimensions, for which the initial microstructure is based on real-time experimental data. Additionally, we applied the model to study the effect of a cyclic heat treatment (CHT) on the microstructure evolution. Specifically, we simulated stored-energy-driven grain growth during three thermal cycles, as well as grain growth without stored energy that serves as a baseline for comparison. We showed that the microstructure evolution proceeded much faster when the stored energy was considered. A non-self-similar evolution was observed in this case, while a nearly self-similar evolution was found when the microstructure evolution is driven solely by capillarity. These results suggest a possible mechanism for the initiation of abnormal grain growth during CHT. Finally, we demonstrate an integrated experimental-computational workflow that utilizes the experimental measurements to inform the PF model and its parameterization, which provides a foundation for the development of future simulation tools capable of quantitative prediction of microstructure evolution during non-isothermal heat treatment.

Extending intergranular normal-stress distributions using symmetries of linear-elastic polycrystalline materials

AbstractIntergranular normal stresses (INS) are critical in the initiation and evolution of grain boundary damage in polycrystalline materials. To model the effects of such microstructural damage on a macroscopic scale, knowledge of INS is usually required statistically at each representative volume element subjected to various loading conditions. However, calculating INS distributions for different stress states can be cumbersome and time-consuming. This study proposes a new method to extend the existing INS distributions to arbitrary loading conditions using the symmetries of a polycrystalline material composed of randomly oriented linear-elastic grains with arbitrary lattice symmetry. The method relies on a fact that INS distributions can be accurately reproduced from the first (typically) ten statistical moments, which depend trivially on just three stress invariants and a few material invariants due to assumed isotropy and material linearity of the polycrystalline model. While these material invariants are complex averages, they can be extracted numerically from a few existing INS distributions and tabulated for later use. Practically, only three such INS distributions at properly selected loadings are required to provide all relevant material invariants for the first 11 statistical moments, which can then be used to reconstruct the INS distribution for arbitrary loading conditions. The proposed approach is demonstrated to be accurate and feasible for an arbitrarily selected linear-elastic material under various loading conditions.

Prediction and Elucidation of Physical Properties of Polycrystalline Materials Using Multichannel Machine Learning of Electron Backscattering Diffraction

AbstractThe application of machine learning in materials science has yielded several benefits, including the prediction of physical properties and the improvement of experimental efficiency. However, with complex models, such as convolutional neural networks (CNN), learning has become a black box, from which no universal physical knowledge can be obtained. In this study, a highly accurate prediction of the electrical properties of polycrystalline semiconductor thin films is achieved by learning multichannel CNN models from electron backscattering diffraction (EBSD) data, that is band contrasts, grain boundaries, and inverse pole figures. In addition, it examines how the CNN model learned the correlation between the crystallinity, grain boundaries, crystallographic orientation, and carrier mobility by polarizing certain EBSD data and checking the predicted changes in carrier mobility. Physical parameters affecting carrier mobility can be extracted, which is challenging via human image recognition. The methods proposed in this study will not only enable the prediction of electrical properties from EBSD data for all materials but also will contribute to the discovery of complex physical phenomena beyond the limits of human analysis.

A microstructure-sensitive electro-chemo-mechanical phase-field model of pitting and stress corrosion cracking

An electro-chemo-mechanical phase-field formulation is developed to simulate pitting and stress corrosion in polycrystalline materials. The formulation incorporates dependencies of mechanical properties and corrosion potential on crystallographic orientation. The model considers the formation and charging dynamics of an electric double layer through a new general boundary condition for the solution potential. The potential of the model is demonstrated by simulating corrosion in polycrystalline materials with various grain morphology distributions. The results show that incorporating the underlying microstructure yields more extensive defects, faster defect kinetics, and irregular pit and crack shapes relative to a microstructurally-insensitive homogeneous material scenario.

Decoupling the nonthermal effect of current by electrically assisted crystal plasticity modeling of Ni-based single crystal superalloys

The electrically assisted (EA) deformation process has received considerable attention in recent years, accompanied by research on current-induced deformation mechanisms. However, there are still challenges in eliminating thermal effects, which have prevented a comprehensive understanding of the underlying current-induced mechanisms. Opting for a single crystal (SC) in research provides advantages in decoupling the nonthermal effect of electric current at smaller scales and eliminating the complex interactions that exist in polycrystalline materials. Therefore, the innovation of this work lies in decoupling the nonthermal effect of electric current and conducting a comprehensive analysis of anisotropic deformation and mechanisms within a Ni-based SC with different crystallographic axes and various current directions during electrically assisted tensile simulation. A significant tension axis direction in the SC during EA tension was induced by the combination of a higher current direction factor (|cosθ|) and a dimensionless factor for the current density (|Jα/J0α|) along the [100] axis. The stress drop within the SC due to the nonthermal effect of electric current generally increased with increasing current direction. This was attributed to the increased dislocation density differences and decreased temperature. The increased stress anisotropy of the SC at a current direction of 45° was attributed to fewer activated (111) slip systems and the pinning effect of more dislocations within these systems. This study advances our understanding of the thermal and nonthermal effects of electric current and offers valuable insights for the informed application of EA deformations in industrial and aerospace settings with SC superalloys.

Localized plastic strain accumulation in shape memory ceramics under cyclic loading

The premature failure of shape memory ceramics (SMC)s under cyclic loading is a critical issue limiting their applications as actuators and thermal protection layers. Martensitic phase transformation (MPT), essential for superelasticity and shape memory functionalities in SMCs, induces localized plastic deformations due to phase expansion. In polycrystalline materials, the accumulation of localized plastic strain serves as the primary mechanism for fatigue crack initiation under cyclic loading. In this research, a phase-field (PF) phase transformation model coupled with a viscoplasticity model is presented to study the effects of microstructural features, engineered pores, and sample size on plastic strain accumulation (PSA) during cyclic loading of SMCs. Our findings highlight that the grain boundaries (GB)s are critical regions with high PSA, with noticeable reductions observed by decreasing the grain boundary density (GBD). Additionally, we found that engineered pores effectively reduced cyclic PSA, however, we identified a threshold on the volume fraction of pores. Also, textured microstructures with certain characteristics demonstrate significant influence on cyclic PSA. A cross-correlation data analysis approach is employed to study the relationship between the studied microstructural features and PSA to facilitate the identification of the most important factors controlling the irreversible PSA during compressive cyclic loading. By mitigating the rate of PSA, significant enhancements in cyclic life before fatigue crack initiation can be achieved, enabling superior performance in practical applications.

A parallel and performance portable implementation of a full-field crystal plasticity model

We have developed a parallel implementation of an Elasto-Viscoplastic Fast Fourier Transform-based (EVPFFT) micromechanical solver to enable computationally efficient crystal plasticity modeling for polycrystalline materials. Our primary focus lies in achieving performance portability, allowing a single EVPFFT implementation to run optimally on various homogeneous architectures, including multi-core Central Processing Units (CPUs), as well as on heterogeneous computer architectures comprising multi-core CPUs and Graphics Processing Units (GPUs) from different vendors. To accomplish this goal, we have leveraged MATAR, a C++ software library that simplifies the creation and utilization of multidimensional dense or sparse matrix and array data structures. These data structures are designed to be portable across diverse architectures through the use of Kokkos, a performance-portable library. Additionally, we have employed the Message Passing Interface (MPI) to efficiently distribute the computational workload among processors. The heFFTe (Highly Efficient FFT for Exascale) library is used to facilitate the performance portability of the fast Fourier transforms (FFTs) computation. The computational performance of EVPFFT is evaluated and presented in terms of parallel scalability and simulation runtime on different high-performance computing (HPC) architectures. The utility of the developed framework to efficiently simulate the micro-mechanical fields in polycrystalline microstructures in engineering applications is discussed. Program summaryProgram Title: EVPFFTCPC Library link to program files:https://doi.org/10.17632/2k8579fyyv.1Developer's repository link:https://github.com/lanl/FierroLicensing provisions: BSD 3-Clause LicenseProgramming language: C++External routines/libraries: MPI, Kokkos, MATAR, HeFFTe, HDF5Nature of problem: EVPFFT is a crystal plasticity code designed to compute micro-mechanical fields within a polycrystalline representative volume element (RVE) and predict the macroscale response of the RVE.Solution method: EVPFFT uses the periodic Green's function method in Fourier space to solve the field equations of static stress equilibrium in a periodic spatial domain.