Large Number Of Grains Research Articles

Dispersion of an acoustic pulse passing through a large-grained polycrystalline film

Propagation of a short acoustic pulse through a polycrystalline film comprised of large randomly oriented elastically anisotropic grains is analyzed theoretically. For average grain size much larger than the film thickness, a short acoustic pulse launched normally into the film will traverse each grain in a time determined by the acoustic slowness in the direction normal to the film, which will depend on the local grain orientation. A typical measurement averages over a large number of grains resulting in the broadening of the composite output pulse. The resulting pulse shape is characterized by distinct features related to stationary values of the directionally dependent acoustic slowness of the crystalline material. Maxima and minima in the slowness yield discontinuities in the pulse shape, while saddle points yield logarithmic singularities. For cubic and hexagonal crystals, power law singularities result from cones of directions in which the slowness is a maximum or minimum. Numerical results, taking into account Gaussian broadening of the input pulse, are presented for thin film materials commonly encountered in picosecond ultrasonic experiments, such as copper, gold, and aluminum.

EFEITOS DA ADIÇÃO DO INOCULANTE Al-3%Ti-1%B ÀS LIGAS DO SISTEMA Al-Si

Grain refinement of the as-cast macrostructure by the addition of the inoculant Al-3%Ti-1%B is examined for Al-3%Si, Al-7%Si, and Al-11%Si alloys using thermal and Fourier analyses as well as quantitative metalography. The effects of inoculant addition on the cooling curves, on the grain size, on the solid fraction evolution with time, and on the latent heat of transformation are investigated. The results show that the cooling curves are changed significantly with the inoculant addition only at the beginning of primary solidification. The evolution of solid fraction with time, however, occurs more slowly with the addition of inoculant. For the Al-3%Si alloy, the time rate of solid formation decreases from the beginning of solidification to a solid fraction of 50%. The average grain size decreases substantially with the addition of the inoculant, suggesting the presence of a larger number of grains during solidification

Variant Selection and Texture in an AISI 301LN Stainless Steel

There has recently been significant interest in the problem of variant selection in the strain-induced transformation of austenite to α’-martensite in metastable austenitic stainless steels. Previous work has highlighted our poor understanding of the mechanisms leading to this transformation, in particular the role that the macroscopic stress plays in the transformation. In this work, we have sought to perform detailed experiments aimed at developing a statistical grain level view of variant selection in one particular grade of austenitic stainless steel. EBSD measurements made over a large number of grains as well as macroscopic texture measurements made at different levels of imposed plastic strain allow for comparison against various approaches for predicting variant selection based on the Patel-Cohen interaction energy.

Nucleation of silicon on Si 3N 4 coated SiO 2

Control of the nucleation during directional solidification of solar cell silicon is important in order to be able to control the growth and number of grains formed. A certain amount of undercooling is required to obtain dendritic growth with faceted twins (which has shown promising results for structure control), but a too high undercooling will lead to extensive nucleation which will oppose the positive effect of a small number of large grains with controlled growth directions. In the present experiments, the nucleation undercooling of silicon on Si 3N 4 coated SiO 2 with variation in coating parameters has been investigated. Experiments were performed with the sessile drop method, and with differential thermal analysis, with a cooling rate of 20 K/min. There were no significant differences in nucleation undercooling between the different variations in coating. The undercooling does not seem to be dependent on coating thickness, oxygen concentration, wetting angle or roughness at the given cooling rate.

Large-scale 3D random polycrystals for the finite element method: Generation, meshing and remeshing

A methodology is presented for the generation and meshing of large-scale three-dimensional random polycrystals. Voronoi tessellations are used and are shown to include morphological properties that make them particularly challenging to mesh with high element quality. Original approaches are presented to solve these problems: (i) “geometry regularization”, which consists in removing the geometrical details of the polycrystal morphology, (ii) “multimeshing” which consists in using simultaneously several meshing algorithms to optimize mesh quality, and (iii) remeshing, by which a new mesh is constructed over a deformed mesh and the state variables are transported, for large strain applications. Detailed statistical analyses are conducted on the polycrystal morphology and mesh quality. The results are mainly illustrated by the high-quality meshing of polycrystals with large number of grains (up to 10 5), and the finite element method simulation of a plane strain compression of ε = 1.4 of a 3000-grain polycrystal. The presented algorithms are implemented and distributed in a free (open-source) software package: Neper.

Investigation of argon nanocrystalline thermal conductivity by molecular dynamics simulation

The molecular dynamics simulation (MD) method has been widely applied to the investigation of the thermal conductivity of single crystalline materials during last decade but no report on the thermal conductivity of polycrystalline solids by direct MD simulation with three-dimensional polycrystalline structure is found so far. In this paper, the periodic configuration of nanocrystalline argon for MD simulation is constructed based on the Voronoi tessellation method. The thermal conductivity of both perfect single crystal and nanocrystalline bulk argon are calculated by the Green–Kubo method. The results are compared with experiments as well as MD simulation results reported by others. The thermal conductivity of nanocrystalline argon with the average grain size of 5 nm is much lower than that of the corresponding perfect single crystal at temperatures between 10 and 70 K. The reduction can be attributed to grain boundary thermal resistance due to large number of grain interfaces in polycrystalline argon, as well as their limitation on the phonon mean free path. The magnitude of estimated grain boundary thermal resistance at different temperatures is on the scale of 10−9 m2 K/W. The grain boundary thermal resistance is larger at lower temperature. This report demonstrates that the thermal property of polycrystalline materials can be investigated by MD simulation.

SUBDOSES DE HERBICIDA E POTÁSSIO EM COBERTURA NO SISTEMA INTEGRAÇÃO LAVOURA-PECUÁRIA

The objective of this study was to evaluate soybean and Brachiaria forage yields, under a crop-livestock integration system, in terms of increasing covering herbicide and potassium rates. The experiment was carried out in a randomized complete blocks design, in a split-split-plot scheme, with three replications. The treatments were: cultivars (CD 216 and VMax), for plots; herbicide rates (0 g ha -1 , 0.24 g ha -1 ; and 0.96 g ha -1 of haloxyfop-methyl), for split-plots; and potassium doses (30 kg ha -1 and 60 kg ha -1 of K 2 O), for split-split-plots. VMax presented the highest number of pods per plant, while CD 216 presented the largest number of grains per pods. There was interaction among cultivars and herbicide doses, concerning the one hundred grains mass. For the split-plot, in comparison with VMax, CD 216 showed a higher one hundred grains mass rate. T here were no significant levels of herbicide and K for soybean yield under crop-livestock integration. However, the highest CD 216 and VMax yield rates were obtained at 0 g i.a. ha -1 and 0.96 g i.a. ha -1 of the herbicide and 60 kg ha -1 and 30 kg ha -1 of K 2 O. The K doses which resulted in a higher dry biomass yield rate for Brachiaria brizantha were 30 kg ha -1 of K 2 O, in consortium with the CD 216 cultivar, and 60 kg ha -1 of K 2 O, in consortium with VMax. KEY-WORDS: Brachiaria brizantha ; Glycine max ; herbicides; intercropping; haloxyfop-methyl.

Influence of the Cluster Orientation on the Epitaxy: Deposition of Co Nanoclusters on Cu(001) Surfaces

Deposition at low energy of 147-atom icosahedral Co nanoclusters on Cu(001) substrates is studied by molecular-dynamics simulations. Atomic interactions were mimicked by a many-body potential based on the tight-binding second-moment approximation. Clusters were rotated by using the two first Euler angles, in the so-called "x-convention," and subsequently, they were deposited on the substrate. The dependence of the degree of epitaxy on these angles has been obtained. Epitaxy is also related to the initial number of (001)-oriented atoms, especially for extreme values of this latter quantity. A better epitaxial matching is connected with a larger spreading index. The explanation of the epitaxial behavior of the supported clusters resides mainly in the dynamical interaction between grains during approximately the first 40 ps. Whenever the newly-formed (001)-oriented grain competes against a large number of grains after the collision, a very low epitaxial matching is obtained.

The response of rate and duration of grain filling to long-term selection for yield in Italian durum wheats

Genetic advance in durum wheat (Triticum turgidum subsp. durum) grain yield in Italy has been achieved by bringing forward flowering time, achieving a larger number of grains per unit area, and altering the pattern of senescence. The performance, in the absence of any moisture stress, of a set of 6 Italian durum wheat cultivars released over the past 100 years was compared under 4 environments and 2 nitrogen rates, to ascertain whether the changes brought about by selection for yield have also indirectly affected the rate and duration of grain filling. Grain filling lasted 35–36 days in all cultivars except ‘Ichnusa’ (39 days), although modern cultivars flowered earlier than older ones. The lack of any breeding effect on grain-filling duration also meant that the later old cultivars were not negatively affected by the higher ambient temperatures during their grain filling. The maximum rate of grain filling ranged from 2.4 to 3.3 mg/day and showed a highly significant negative correlation with the year of cultivar release (r = –0.91*). The variation in grain weight, significant but not correlated with the year of release, was associated with the rate of grain filling, which was in turn related to the grain number per unit area. A compensating variability still exists among modern Italian cultivars in both grain number and grain-filling rate, which demonstrates that durum wheat grain yield can be increased while also preserving high grain weights.

Probing intra-granular deformation by micro-beam Laue diffraction

Deformation in polycrystalline materials does not occur uniformly in the constituent grains, but shows strong inter- and intra-granular variations. The local deformation response depends on lattice orientation, anisotropic elastic-plastic properties, hardening and damage behaviour and the microstructural neighbourhood. Understanding of these complex interactions is vital for the construction and validation of crystal plasticity models, the introduction of mesoscopic deformation effects into polycrystal models and the fundamental understanding of the material’s deformation behaviour.Micro-beam Laue diffraction provides an excellent tool for the study of inter- and intra-granular deformation, allowing, unlike other microscopy methods, measurements within the bulk of the material. A focused “pink” beam is used to probe the interior of individual grains within a polycrystalline sample. The diffracted radiation forms a pattern of Laue spots which is captured by an area detector. From the spot positions, lattice orientation and elastic strains can be deduced. The intensity distribution within individual spots, can be interpreted in terms of the dislocation arrangement within the gauge volume.In this paper we present a novel setup for microbeam Laue diffraction recently developed on the B16 test-beamline at the Diamond Light Source, UK, with the aim of collecting a high quality dataset characterising mesoscopic deformation behaviour within a polycrystal containing a small number of large grains. Such a dataset was collected from a large grained Ni sample in which a number of maps covering several grains were recorded for a sequence of deformation steps. In this paper some first results from this investigation are presented. Ultimately the dataset is intended to allowe a direct comparison between crystal plasticity models and experimental behaviour.

Modeling of high-density compaction of granular materials by the Discrete Element Method

Cold compaction of metal powders is now commonly studied at a microscopic scale, to further our understanding of contact mechanics between grains. The Discrete Element Method (DEM) is therefore, a good compromise between calculation time and precision. DEM simulations are in general limited to a relative density of about 0.8, because the existing contact laws do not reproduce all the physical phenomena involved in the densification of granular media. Local contact mechanics can be studied by finite element analyses on meshed distinct elements (MDEM, Meshed Distinct Element Method). However, this method is too time-consuming when in the presence of a large number of grains. In the following work, a new analytical contact law will be formulated with MDEM which will subsequently be used to validate the DEM model. Thus, it will be possible with DEM modeling to reproduce high-density compaction of random packings up to a relative density of about 0.95. By introducing a local relative density parameter in the force–displacement relationship, the incompressibility effects which rule high-density behaviors can be introduced in the modeling of powder compaction.

Microstructure and texture evolution during the drawing of tungsten wires

Tungsten wires develop during their forming process a pronounced fibre texture that causes anisotropic deformation of single grains. The aim of this work is to simulate the crystallographic texture and microstructure evolution that arises during wire drawing using two different texture models. A visco-plastic self-consistent model that allows simulations using a large number of grains is compared with a crystal plasticity finite element model that provides a more detailed insight into the wire’s microstructure. Texture predictions of both models are discussed and quantitatively compared with experimental texture measurements obtained by neutron diffraction. The developed fibre texture causes plane strain deformation of single grains, which induces grain curling. The prediction of grain curling is of importance because it allows studying the residual stresses that trigger splits, at the grain level.

C4 rice: a challenge for plant phenomics.

There is now strong evidence that yield potential in rice (Oryza sativa L.) is becoming limited by 'source' capacity, i.e. photosynthetic capacity or efficiency, and hence the ability to fill the large number of grain 'sinks' produced in modern varieties. One solution to this problem is to introduce a more efficient, higher capacity photosynthetic mechanism to rice, the C4 pathway. A major challenge is identifying and engineering the genes necessary to install C4 photosynthesis in rice. Recently, an international research consortium was established to achieve this aim. Central to the aims of this project is phenotyping large populations of rice and sorghum (Sorghum bicolor L.) mutants for 'C4-ness' to identify C3 plants that have acquired C4 characteristics or revertant C4 plants that have lost them. This paper describes a variety of plant phenomics approaches to identify these plants and the genes responsible, based on our detailed physiological knowledge of C4 photosynthesis. Strategies to asses the physiological effects of the installation of components of the C4 pathway in rice are also presented.

Cumulative damage model based on two-mode fatigue damage bounds

Based on earlier work by the author, a macro-model of damage accumulation is developed that circumvents crack tip micro-mechanics through introduction of two-mode damage curves. In such model, the actual microcrack configuration is replaced by an ‘equivalent’ crack while the actual heterogeneous medium is replaced by an effective homogeneous and isotropic medium in which the crack is embedded. By the simultaneous presence of slip and opening-mode damage during each cycle, the two-mode damage curves reflect conditions at a crack tip in the effective medium. Such curves are obtained from conditions imposed at four discrete microstructure barriers. With all barriers established in terms of the fatigue limit. The fatigue process is thus divided into three stages, the analogues of the three creep stages under constant stress. Two length scales are defined from order of magnitude considerations: a macroscale defining a representative volume element (RVE) and a microscale defining a much smaller volume of grain size order; both volumes surrounding the ‘equivalent’ crack tip. The presence of a sufficiently large number of grains of random size and orientation makes the RVE representative of the medium as a whole; in such a medium, the location and orientation of the ‘equivalent’ crack becomes immaterial. While the RVE is dominated by monotonic slip, the microvolume represents a region of cyclic slip; both volumes surround the crack tip, in line with fracture mechanics concepts. This model is then applied to cumulative damage and bounds on mean residual lifetime are obtained for two-stage high–low (H–L) and low–high (L–H) cycling. The presence of microstructure barriers proves essential to damage irreversibility in L–H cycling, leading to a division of the entire region behind the S-N curve into three regions dominated by part or by all three fatigue stages. A statistical analysis based on the two-mode bounds leads to a median residual lifetime and to bounds on the variance. It is argued that the variance can only be bracketed by the present model but not uniquely defined, unless ‘instantaneous’ cyclic hardening is introduced as an additional random variable. Bounds on crack growth rate under constant amplitude cycling are also derived that effectively coincide in the macrocrack regime, thus explaining the small crack rate scatter observed. An experimental method is proposed whereby long fatigue lifetimes, including the fatigue limit, can be indirectly determined from a crack growth rate drop that occurs at the transition from a predominantly slip to a predominantly opening-mode interval of crack growth.

Measurement of wheat gluten and barley hordeins in contaminated oats from Europe, the United States and Canada by Sandwich R5 ELISA

We have investigated the extent of contamination with wheat, barley, rye or a mixture of these cereals in a large number of grains and commercial oats. We have also attempted to identify the type of cereal contaminant. Sandwich R5 ELISA (using either gliadins or hordeins as standards), western blot, matrix-assisted laser desorption/ionization time-of-flight mass spectrometric and quantitative real-time PCR (Q-PCR) techniques have been used to analyze a total of 134 oats, comprising grains and commercial oat products collected from Europe, the United States and Canada. Twenty-five of the 134 pure, uncontaminated oat varieties were found to have undetectable levels of gluten, whereas most of the 109 grains and commercial oat products were mainly contaminated with mixtures of wheat, barley and rye, barley being the predominant contaminant. The percentages of these cereals in the oat samples have been calculated by specific wheat, barley and rye Q-PCR systems. The oat samples were grouped according to the avenin spectra determined by the mass spectrometric technique. The data confirmed that contaminated oat foods, based on the same variety, could have different levels of wheat, barley and rye contamination. This study has verified that contamination with wheat gliadins or barley hordeins in oat samples can be measured by the Sandwich R5 ELISA, using either gliadins or hordeins as standards, and also the importance of using confirmatory techniques (such as western blot, Q-PCR and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry) to confirm that most oats are contaminated with mixtures of wheat, barley and rye.

An effective computer generation method for the composites with random distribution of large numbers of heterogeneous grains

An effective computer generation method is presented in this paper to more perfectly and rapidly generate the random distribution domains with large numbers of grains (pores). At first, the geometries of heterogeneous grains and the stationary random distribution model with large numbers of grains are defined. Second, the effective computer generation method, including compactness algorithm and selection algorithm, is described in detail. Then the effectiveness of the generation method and the comparison with the take-and-place method are given, and some examples with different geometries of grains in 2- and 3-dimension cases are illustrated. The computer generation method in this paper has been applied to the computation of effective heat transfer behavior for the composites of the random distribution with large numbers of grains, and some numerical results are demonstrated. The generation method in this paper is able to make the generated samples hold better stochastic property, and it is also suitable to generating samples subjected to non-uniform probability model.

Effect of Sn on the microstructure and compressive deformation behavior of the AZ91D aging alloy

The effect of Sn on the microstructure and compressive deformation behavior of aging AZ91D magnesium alloy was investigated. The results show that the discontinuous precipitation occurs near the grain boundary in the form of lamellar precipitates of the Mg 17Al 12 phase. These precipitates alternatively lie in the α-Mg matrix perpendicular to the grain boundaries. With an increase in the Sn content, the cellular discontinuous precipitation can be efficiently suppressed, such that no discontinuous precipitation is observed when the Sn content is increased to 2 wt.%. The AZ91D aging alloy has a higher ultimate fracture strength than the base 2%Sn alloy due to the large numbers of grains containing the cellular discontinuous precipitation forming nearby the grain boundaries. In these regions the β phase can effectively hinder the dislocation movement. Conversely, the soft behavior induced by a high fraction of twins is the main reason that the strength of 2%Sn alloy is lower than that of the AZ91D alloy.

Recent advances in simulations of grains in a rotating drum.

The discrete elements method (DEM) has been widely used in the past decade to study a wide variety of granular systems. The use of numerical simulations constitutes an interesting alternative to the experiment as they can shed new light on a phenomenon as they can overcome experimental obstacles. A lot of granular phenomena can be studied in 2D or with a limited number of grains but the peculiar phenomenon of axial segregation (or banding) is 3-dimensional by nature and requires a large number of grains. Only very recently has it been made possible to simulate 3D systems on a large scale. This highlight reviews recent work on this topic and attempts to show what knowledge is gained from DEM numerical simulations. The perspectives on the future benefit of this method as well as the challenges it faces are discussed.

GIS-based detection of grain boundaries

The recognition of grain boundaries in deformed rocks from images of thin-sections or polished slabs is an essential step in describing and quantifying various fabric elements and strain. However, many of the methods in use today require labor-intensive manual digitization of grain boundary information. Here, we propose an automated, GIS-based method to detect grain boundaries and construct a grain boundary database in which the shape, orientation, and spatial distribution of grains can be quantified and analyzed in a reproducible manner. The proposed method includes a series of operations and functions to identify grain boundaries and construct the grain boundary database. These processes are integrated into a GIS model using ArcGIS ModelBuilder; thus, little or no operator intervention is required to perform the entire analysis. The method was evaluated using thin section images taken from three sandstone samples. The results indicate that the proposed method can correctly identify >70% of grains recognized manually without any intervention and is especially suitable for analyses where large numbers of grains are required.

Relações de causa e efeito em espigas de milho relacionadas aos tipos de híbridos

O peso de grãos é um dos componentes determinantes do rendimento de grãos de milho, que apresenta relações complexas com várias características morfológicas da espiga. O objetivo deste trabalho foi verificar como o peso dos grãos das espigas de milho está relacionado diretamente ou indiretamente com as características morfológicas das espigas e se esta relação depende do tipo de híbrido: simples, triplo ou duplo. O experimento de milho foi realizado no ano agrícola de 2004/05, na Universidade Federal de Santa Maria, com seis tratamentos, que foram compostos de dois híbridos simples, dois híbridos triplos e dois híbridos duplos. O delineamento utilizado foi o de blocos completos ao acaso, com três repetições, com parcelas de duas filas de cinco metros de comprimento com 0,8 m entre filas e densidade de 55.000 plantas ha-1. As correlações entre as características morfológicas da espiga e o peso de grãos foram desdobradas em efeitos diretos e indiretos. O número de filas de grãos por espiga está correlacionado com o peso de grãos apenas nos híbridos duplos (0,5298), e esta correlação se deve ao efeito indireto positivo do número de grãos por espiga (0,8875) e negativo do peso de 100 grãos (- 0,3795). A seleção de espigas com maior peso de 100 grãos e maior número de grãos por espiga tem efeito direto sobre o aumento do peso de grãos por espiga nos híbridos simples e triplo, enquanto que, no híbrido duplo, apenas o número de grãos por espiga tem efeito. As relações entre as características de espigas são dependentes dos genótipos, o que deve complicar um pouco mais a tarefa dos melhoristas de plantas ao selecionar genótipos para maior peso de grão por espiga.