Two-dimensional Grain Research Articles

Validation of three-dimensional diffraction contrast tomography reconstructions by means of electron backscatter diffraction characterization

Microstructure reconstructions resulting from diffraction contrast tomography data of polycrystalline bulk strontium titanate were reinvestigated by means of electron backscatter diffraction (EBSD) characterization. Corresponding two-dimensional grain maps from the two characterization methods were aligned and compared, focusing on the spatial resolution at the internal interfaces. The compared grain boundary networks show a remarkably good agreement both morphologically and in crystallographic orientation. Deviations are critically assessed and discussed in the context of diffraction data reconstruction and EBSD data collection techniques.

Transmitting, emitting and controlling light: Processing of transparent ceramics using current-activated pressure-assisted densification

We review recent advances in using current-activated pressure-assisted densification (CAPAD) for fabricating polycrystalline ceramics that are transparent to visible and infrared light. In addition, we discuss important one-dimensional (vacancies), two-dimensional (grain boundaries) and three-dimensional (pores) defects that can affect optical properties. We pay special attention to procedures whereby the benefits of CAPAD can be used to ensure the precise control of microstructure that facilitates transparency and other functionalities.

Two-Dimensional Observation of Grain Movements in Elongated and Aligned Grain Structure during High Temperature Deformation

It is understood that grains move by grain boundary sliding, and change their relationship to each other during superplastic deformation. Ideal two-dimensional observation of grain movements from the specimen surface is difficult even in the shear deformation because grains move three-dimensionally according to the stress distribution against the specimen surface. In this study, ODS steel with elongated grains aligned along one direction was deformed perpendicular to the aligned axis to achieve ideal two-dimensional grain movements. Surface height profiles with a laser microscope showed small amount of three-dimensional grain movements, while two-dimensional grain movements and rotations were appeared by observations before and after the deformation with SEM-EBSD.

High-Performance Polycrystalline Silicon Thin-Film Transistors with Two-Dimensional Location Control of the Grain Boundary via Excimer Laser Crystallization

High-performance low-temperature polycrystalline silicon (Poly-Si) thin-film transistors (TFTs) have been fabricated with two-dimensional (2-D) location-controlled grain boundaries using excimer laser crystallization (ELC). By locally increased thickness of the amorphous silicon (a-Si) film that was served as the seed crystals with a partial-melting crystallization scheme, the cross-shaped grain boundary structures were produced between the thicker a-Si grids. The Poly-Si TFTs with one parallel and one perpendicular grain boundary along the channel direction could therefore be fabricated to reach excellent field-effect mobility of 530 cm2/V-s while the conventional ones exhibited field-effect mobility of 198 cm2/V-s. Furthermore, the proposed TFTs achieved not only superior electric properties but also improved uniformity as compared with the conventional ones owing to the artificially controlled locations of grain boundaries.

Monte Carlo simulation of influence of reheat temperature on austenite grain growth in C–Mn steel

The evolution and topology of a two-dimensional austenite grain growth of C–Mn steel are simulated by the Monte Carlo (MC) model during the reheating process. Simulated microstructural maps have been generated in a square lattice (400×400) at various MC steps and temperatures. Normalised grain size distribution has also been computed at different reheating temperatures, assuming a uniform temperature distribution in the slab. Two important model parameters, namely, the grain growth exponent and the model constant, have been estimated to predict the average grain size. The activation energy for grain growth has been calculated using the Arrhenius relationship. The grain growth behaviour as a function of both MC and physical time has been computed, and the effect of temperature on the rate of growth has been analysed. The predicted temporal evolutions of grain growth and model parameters have been validated with the published literature and found to be in good agreement.

Two-Dimensional Grain Growth Simulation by Local Curvature Multi-Vertex Model

A local curvature multi-vertex model was developed. This model is the straightforward two-dimensional topological network model based on the physical principles which are the curvatures of grain boundaries and the grain boundary tensions at triple junctions. The model was applied to the artificial random microstructure under some conditions of grain boundary characters. The misorientation distribution was changed very little under constant grain boundary energy and mobility, but it was change much under grain boundary character dependent on misorientation. Therefore, in order to discuss actual textures, it is important to take grain boundary characters into account.

Role of Inclination Dependent Anisotropy on Boundary Populations during Two-Dimensional Grain Growth

A Monte Carlo model is presented, in which the interface anisotropy is primarily inclination dependent. A dual grid method is used to determine boundary inclination in discretized digital microstructures. Evolution of the grain boundary character distribution (GBCD) in polycrystalline systems, from an initially random distribution, is inversely correlated with the anisotropy in interfacial energy, as expected.

Self-Similar Solutions to a Kinetic Model for Grain Growth

We prove the existence of self-similar solutions to the Fradkov model for two-dimensional grain growth, which consists of an infinite number of nonlocally coupled transport equations for the number densities of grains with given area and number of neighbours (topological class). For the proof we introduce a finite maximal topological class and study an appropriate upwind-discretization of the time dependent problem in self-similar variables. We first show that the resulting finite dimensional differential system has nontrivial steady states. Afterwards we let the discretization parameter tend to zero and prove that the steady states converge to a compactly supported self-similar solution for a Fradkov model with finitely many equations. In a third step we let the maximal topology class tend to infinity and obtain self-similar solutions to the original system that decay exponentially. Finally, we use the upwind discretization to compute self-similar solutions numerically.

Controlled Poisson Voronoi tessellation for virtual grain structure generation: a statistical evaluation

A controlled Poisson Voronoi tessellation (CPVT) model has been developed for producing two-dimensional virtual grain structures that are statistically equivalent to metallographic observations of polycrystalline materials in terms of the tessellation's regularity and grain size distribution. The descriptive fitting model, which is critical to link the grain size distribution parameter to the regularity parameter, has been improved in this work; previously, the descriptive model was poor for large values of the distribution parameter c. A set of four physical parameters is involved in uniquely determining the grain size distribution properties and configuring the CPVT system. Emphasis is devoted to examining the effectiveness and robustness of the CPVT system in generating virtual grain structures with specified properties. Two series of statistical tests are performed to validate the agreement between the prescribed regularity and that of the resultant tessellations, and to investigate the details of the overall grain size distribution. In order to explore sample size effects, three statistical tests were conducted for a range of regularity values. In addition, a materials modelling system for crystal plasticity finite element analysis is demonstrated, which implements the CPVT model for grain structure generation. Two real microscopic images with different grain size distribution features are employed to examine the capability of the system to generate virtual grain structures that match physical measurements.

Granular packings of cohesive elongated particles

We report numerical results of effective attractive forces on the packing properties of two-dimensional elongated grains. In deposits of non-cohesive rods in 2D, the topology of the packing is mainly dominated by the formation of ordered structures of aligned rods. Elongated particles tend to align horizontally and the stress is mainly transmitted from top to bottom, revealing an asymmetric distribution of local stress. However, for deposits of cohesive particles, the preferred horizontal orientation disappears. Very elongated particles with strong attractive forces form extremely loose structures, characterized by an orientation distribution, which tends to a uniform behavior when increasing the Bond number. As a result of these changes, the pressure distribution in the deposits changes qualitatively. The isotropic part of the local stress is notably enhanced with respect to the deviatoric part, which is related to the gravity direction. Consequently, the lateral stress transmission is dominated by the enhanced disorder and leads to a faster pressure saturation with depth.

Phase field crystal simulations of nanocrystalline grain growth in two dimensions

We study two-dimensional grain growth at the nanoscale using the phase field crystal (PFC) model. Our results show that for circular grains with large misorientations the grain area decreases linearly with time, in good agreement with classical grain growth theory. For circular grains with small initial misorientations, grain rotation occurs as a result of the coupled motion between the normal motion of the grain boundary and the tangential motion of the adjacent grains. Despite this rotation and its effect on the grain boundary energy, the grain area decreases linearly with time. In addition, for intermediate initial grain misorientations, we find a repeating faceting–defaceting transition during grain shrinkage and a different relationship between the grain area and time, which suggests a different grain growth mechanism than that for small and large misorientations. For a circular grain embedded between a bicrystal with a symmetric tilt boundary, we find that the evolution of the embedded grain closely depends on dislocation reactions at triple junctions.

Charge Transport Model Based on Single-Layered Grains and Grain Boundaries for Polycrystalline Pentacene Thin-Film Transistors

The charge transport process in organic thin-film transistors (OTFTs) has always been under intensive research while the mechanism is still controversial, even in the small-molecule OTFTs. In this study, we report on a transport model for interpreting the charge transport mechanism in polycrystalline pentacene TFTs. By investigating the growth mode of pentacene films and the distribution of charge carriers in the channel of OTFTs, a transport model involving two-dimensional single-layered grains and grain boundaries (GBs), namely the two-dimensional grain boundary (TDGB) model, is proposed for describing the charge transport process in polycrystalline pentacene TFTs. An analytical expression for the field-effect mobility could be obtained from the model. The model is applied to explain the mobility dependence on the grain sizes at the first monolayer of pentacene films, temperature and on gate voltage in polycrystalline pentacene TFTs, with good agreement obtained. In addition, the values of some crucial parameters are given by the simulations based on the model.

Star Grain Regression under Spin Induced Acceleration Effect

Spinning is used in some of solid rocket motors to increase the flight trajectory precision or for stability requirements. The angular acceleration due to the spin effect increases the burning rate of solid propellant and changes the motor performance by increasing the operating pressure and decreasing the burning time. So it is important to know the grain regression taken place in the solid propellant rocket motor in the acceleration field. In this study, we represent the grain regression analysis of two-dimensional axis-symmetric star grain configuration of the solid propellant rocket motor under spin induced acceleration effect to study how the spin affects on the internal ballistics of the solid rocket motor. Grain regression is done by two methods - geometrical approach and numerical approach. The burning rates on the propellant surface are different with its radial distance, acceleration vector angle and surface slope when the rocket is spinning. With the different burn rates on the propellant surface, the propellant surface perimeter and port area are computed by using the numerical method, and the results are compared with that of constant burn rate.

001]-oriented nonepitaxial growth in L1-ordered FePt thin film by SiO2 addition and rapid thermal annealing

A highly L10-ordered FePt film with a crystalline [001] orientation was prepared by SiO2 addition and rapid thermal postannealing on an oxidized Si substrate. In order to clarify the mechanism of L10 ordering and the crystalline orientation of a nonepitaxial FePt film, we investigated the effect of the preparation conditions on the film structure. The crystalline quality was strongly dependent on the additive amount of SiO2; a high chemical order without the distribution of the chemical order and a nearly perfect [001] orientation were obtained at a SiO2 composition of 10 vol %. The nucleation of L10 grains with a high chemical order and a [001] orientation was observed from an initial phase transformation stage at a low annealing temperature. The as-deposited FePt–SiO2 film consisted of a nanocrystalline structure in which SiO2 segregated at the grain boundaries; SiO2 diffused with the grain growth during thermal annealing. Because the in-plane tensile stress among the grains was a dominant origin for the [001] texture formation, two-dimensional grain growth with the annihilation of grain boundaries played a crucial role in the [001] ordering in nonepitaxial FePt films. It was also crucial to maintain a high stress during annealing, and the [001] ordering was enhanced by rapid heating.

A Microstructural Study of Grain Boundary Engineered Alloy 800H

A series of grain boundary engineering (GBE) procedures was performed on samples of 800H, a common iron-nickel base alloy. The effects of varying the GBE parameters (deformation percentage and annealing temperature/time) were examined using electron backscatter diffraction orientation mapping. GBE samples exhibited up to a 70 pct increase in Σ3n boundary length fraction compared to the as-received condition. The incorporation of these Σ3n boundaries was also measured using a simplified model of two-dimensional grain boundary transport and its electrical analogy. The results showed that GBE resulted in increased resistance to grain boundary diffusion compared to the as-received condition. The evolution of a GBE microstructure was also studied on a per-cycle basis, and the results are shown to be consistent with previous studies.

On the analytical solution for self-similar grain size distributions in two dimensions

In a recent publication an analytical solution of the Fokker–Planck continuity equation for the grain size distribution for two-dimensional grain growth in the long time limit (self-similar state) was provided. It used von Neumann–Mullins law and the results of Rios and Glicksman, but was based on a stochastic formulation first proposed by Pande. In this paper this analytical solution is compared with experimental and computer simulation distributions. It is found that grain size distribution, as obtained by simulations of two-dimensional grain growth, although in agreement with our analytical results, may in fact differ from experimentally obtained grain size distributions in thin films. It is also shown mathematically that in the two limiting cases the general solution is reduced to the Hillert or Rayleigh distributions.

Von Neumann–Mullins equation in the Potts model of two-dimensional grain growth

In this study, the validity of von Neumann–Mullins equation was examined for the Potts model of two-dimensional grain growth. Simple geometric consideration of the lattice in the model confirmed that the Potts model used in this study naturally satisfies von Neumann–Mullins equation, and provided the exact value of the grain boundary mobility. Monte Carlo simulation of a single grain growth demonstrated that the grain area growth rate is statistically constant for a given vertex number. The simulated grain boundary mobility agreed with its analytical prediction. The kinetics of individual grains in a polycrystalline structure was also investigated. The area change rate of the individual grains was not uniform over a short time interval, and then yielded large errors of the grain boundary mobility. Some of estimation techniques were tried to reduce the errors. Regardless of the methods we exploited, the average value of the simulated mobilities was a good approximation of the results from analytical prediction or single grain growth simulations.

DART: a robust algorithm for fast reconstruction of three-dimensional grain maps

A novel algorithm is introduced for fast and nondestructive reconstruction of grain maps from X-ray diffraction data. The discrete algebraic reconstruction technique (DART) takes advantage of the intrinsic discrete nature of grain maps, while being based on iterative algebraic methods known from classical tomography. To test the properties of the algorithm, three-dimensional X-ray diffraction microscopy data are simulated and reconstructed with DART as well as by a conventional iterative technique, namely SIRT (simultaneous iterative reconstruction technique). For 100 × 100 pixel reconstructions and moderate noise levels, DART is shown to generate essentially perfect two-dimensional grain maps for as few as three projections per grain with running times on a PC in the range of less than a second. This is seen as opening up the possibility for fast reconstructions in connection within situstudies.

Direct Observation of Grain Rotation-Induced Grain Coalescence in Two-Dimensional Colloidal Crystals

A spatially modulated laser is used to produce multiple localized thermal gradients in a colloidal sample placed above a gold surface. We use an optical microscope to observe real time dynamics of the resulting two-dimensional colloidal crystal grains and find that grain rotation-induced grain coalescence (GRIGC) occurs with the rotation of both grains before coalescence. Control over the grain size shows that the time scale for grain boundary annealing in our system is in good agreement with theoretical expressions formulated for nanocrystal growth.

Flexible Magnetic Nanoparticles–Reduced Graphene Oxide Composite Membranes Formed by Self‐Assembly in Solution

A facile and robust route for the pre-synthesized Fe(3)O(4) nanoparticles (NPs) exclusively assembled on both sides of reduced graphene oxide (RGO) sheets with tunable density forming two-dimensional NPs composite membranes is developed in solution. The assembly is driven by electrostatic attraction, and the nanocomposite sheets display considerable mechanical robustness, such as it can sustain supersonic and solvothermal treatments without NPs falling off, also, can freely float in solution and curl into a tube. The obtained two-dimensional composite grain membranes exhibit superparamagnetic behavior at room temperature but responds astutely to an external magnetic field. In addition, these magnetic composite membranes show an enhanced absorption capability for microwaves. The grain sheets are attractive for biomedical, sensors, environmental applications and electric-magnetic devices benefited from large surfaces, high magnetization moment, and superparamagnetic properties. The effective integration of oxide nanocrystals on RGO sheets provides a new way to design semiconductor-carbon nanocomposites for nanodevices or catalytic applications.