Chains Of Grains Research Articles

Закономірності формування структури приповерхневого рекристалізованого шару в процесі відновлювальної термічної обробки нікелевих жароміцних сплавів

In modern nickel nickel-based high-temperature strength alloys of the ZhS32 type with a directional and single-crystal structure on surfaces that were previously subjected to mechanical treatment, with the subsequent isothermal exposure in the vicinity of the homogenization temperature lasting more than 15 minutes a subsurface recrystallized layer up to 40-65 μm deep is formed such us a chain of grains. The formation of this layer is a side effect that restrained the industrial application of vacuum heat treatment to restore local operational degradation ("raft"-structure) on the overheated inlet edges of non-bandage shelved gas-cooled high-pressure turbine blades of some modern aircraft turbine engines. Given the technical complexity of reliable removal of this recrystallized layer from the tract surface of thin-walled gas-cooled blades, it is important to develop technological measures to translate grain boundaries in unremoved residues of recrystallized near-surface layer into a safer structural state. The regularities of recrystallized layer formation in the process of 3-stage reductive vacuum treatment has been investigated by the methods of raster electron microscopy and X-ray structural (EDX) microanalysis at magnification up to × 2000 on fragments of TVT blades with ZhS26-VI alloy (directional structure) and ZhS32-VI alloy (monocrystalline structure). It has been established that the recrystallized layer formed in the process of aging 1.25-1.5 hours at the temperature of alloy homogenization, is a chain of grain with a tightly packed γ׳-phase with layers at their boundaries, which consist of the non-reinforcing γ׳-phase up to 1-3 μm wide with the carbide phases presence. It has been shown that the rational choice of the temperature of high-temperature aging is an effective technological control of the grain boundaries structural state in a near-surface recrystallized layer of the considered high-temperature strength alloys. Assigning a temperature of 1050°C for isothermal exposure after homogenization heat treatment for ZhS26 and ZhS32 alloys allows to reduce the grain boundaries width in the recrystallized layer to 1-2 μm, keep them intermittent and avoid systematic release of carbide particles at the grain boundaries. On the basis of established in the Paton welding institute of regularities of formation and control of the structure of the near-surface recrystallized layer developed technological recommendations for optimizing the vacuum heat treatment modes to restore the structure of non-bandage shelved gas-cooled high-pressure turbine blades with type ZhS32 high-temperature strength alloys. This technology has passed experimental and practical testing during the next maintenance cycle of flight operation on one of the modern turbojet double-circuit gas turbine engines with afterburner combustion chamber. Keywords: nickel nickel-based high-temperature strength alloys, vacuum heat treatment, restoration of alloys structure, subsurface recrystallized layer, raster electron microscopy.

Globalization and Restructuring of the Agricultural Production in the Cerrados Biome of Central-North Brazil (MATOPIBA)

An important expansion of agricultural and livestock supply chains has occurred in Brazil in the last decades, which consolidated regional economies associated with several agroforestry products. This phenomenon has been encouraged by government policies, which not only have enforced the financial and technical modernization of the sector but have contributed decisively to the expansion of the so-called Brazilian agricultural frontier, a space of agricultural modernization in the interior of the country. The Cerrado biome region of central-north Brazil (currently known as MATOPIBA), which is the subject of this article’s analysis, has gained prominence under agribusiness rule, transforming itself into a space that favors the supply chain of grains. This phenomenon has produced relevant changes in the regional economy by triggering new dynamics in economic, spatial, social, and labor relations. Also, it has been responsible for various contradictions, considering the new economy’s innumerable impacts on the environment and local population’s livelihood.

Formation of chain structures in systems of charged grains interacting via isotropic pair potentials

Conditions for the formation of chain structures of charged grains confined in the gravitational field by external electric fields are studied analytically and numerically. The relationships between the parameters of the pair interaction potential, the number of grains, and the electric field gradient in the trap are found. A criterion for the violation of stable equilibrium in a quasi-one-dimensional chain of grains and the formation of a new configuration in the system is proposed.

Anisotropic Thermal Conductivity Model for Dry Snow

Fabric tensors quantify the directional arrangement of a granular material's microstructure. In treating snow as such they are beneficial in characterizing morphologies that exhibit a distinct directional arrangement—like chains of grains in depth hoar and potentially other weak layers. Microstructural variables, including the directional arrangement of bonds, impact thermo-mechanical properties like strength, stiffness, and conductivity of the granular material. The conductivity model proposed here incorporates a fabric tensor, linking the textural arrangement of a granular assembly to a material property. In this work, dry dense snow was subject to temperature gradients (100 and 50 K/m) in a lab. The resulting morphology was driven by temperature gradient metamorphism. Underpinning the importance of microstructure, the observed heat transfer coefficient (EHC) increased in the direction of the applied gradient without appreciable changes in density. Periodic tomography yielded measurable microstructural data used to calculate a fabric tensor and the evolving conductivity tensor. Through the fabric tensor the analytical conduction model accounts for ~ 43% of the observed increase in EHC. The model also calculates a decrease in conductivity in the plane orthogonal to the temperature gradient due to a developing anisotropy. Snow metamorphism models parameterized by density alone cannot predict such directionally-dependent behavior because they are strictly valid for only isotropic materials.

On the properties of “short” granular magnets with disordered chains of grains: A field between grains

The values and functional exponential form of the demagnetizing factor of “short” cylindrical samples of the polyspherical medium and samples (elements) of this medium, i.e., chains of spheres, have been established. The tortuosity factor of chains formed by spheres (as effective elementary conductors of the magnetic flux in a magnetized granular medium) has been analyzed. It has been demonstrated that the tortuosity factor for chains in a filling medium is insignificant. The strength of the field between spheres in the chain, like the magnetic permeability of the effective magnetization channel, has an extremum (bell-shaped) profile. It has been revealed (by using magnetophoresis of finely dispersed ferroparticles) that the gradient of this strength exhibits a two-extremum profile.

Albedo of summer snow on sea ice, Ross Sea, Antarctica

Surface all‐wave albedo and physical parameters (grain radius, mass density, surface temperature, and stratification) of an austral summer snow cover on sea ice were measured in the Ross Sea during January‐February 1999. It was observed that (1) from north to south the snow surface temperature decreases, albedo increases, snow mass density decreases, snow composite grain radius decreases, and number density of composite grains increases; (2) the size of the clusters of snow grains (i.e., melt clusters and well‐bonded chains of grains), rather than the size of the individual snow grains, is the dominant parameter controlling albedo (the smaller the snow composite grain radius and the larger the number density are, the higher the albedo is); and (3) in determining the snow albedo, grain size dominates over the number density and the upper layers dominate over the lower layers. A comparison of the statistical relation between albedo and the averaged grain size shows that statistical significance is independent of the selection of the grain size averaging method. Suggestions for snow albedo parameterization are made.

Coulomb effects and hopping transport in granular metals

We investigate effects of Coulomb interaction and hopping transport in the insulator phase of granular metals and quantum dot arrays. We consider a spatially periodic as well as an irregular array, including disorder in a form of a random on-site electrostatic potential. We study the Mott transition between the insulating and metallic states in the regular system and find the dependence of the Mott gap upon the intergranular coupling. The conductivity of a strictly periodic array has an activation form with the Mott gap as an activation energy. Considering irregular systems we concentrate on the transport properties in the dielectric, low coupling limit and derive the Efros-Shklovskii law for hopping conductivity. In the irregular arrays electrostatic disorder results in the finite density of states on the Fermi level giving rise to the variable range hopping mechanism. We develop a theory of tunneling through a chain of grains and discuss in detail both elastic and inelastic cotunneling mechanisms; the former dominates at very low temperatures and/or very low applied electric fields, while the inelastic mechanism controls tunneling at high temperature/fields. Our results are obtained within the framework of the new technique based on the mapping of quantum electronic problem onto the classical gas of Coulomb charges. The processes of quantum tunnelling of real electrons are represented in this technique as trajectories (world lines) of charged classical particles in $d+1$ dimensions. The Mott gap is related to the dielectric susceptibility of the Coulomb gas in the direction of the imaginary time axis.

Disorders in granular matter

A major issue in material science is the formulation of a macroscopic description of material properties based on local characteristics. We consider here the case of a granular medium, for which we show that the applicable treatments depend not only on the nature of the disorder but also on the specific property under consideration. The ordinary transport properties of a porous medium are adequately described by classical homogenization techniques introduced long ago in the study of electrical composites. Poorly connected porous systems or multi-phase flows, where a wide range of scale effects are present, require descriptions—such as percolation or fractals—which take into account large-scale disorder. We consider extensions of this approach to other properties—e.g., hydrodynamic dispersion. In a different context, the properties of solid, unconsolidated granular media cannot be treated by homogenization techniques because of the sensitivity of the properties to the local disorder at contact zones; an example of the latter is that the very broad distribution of stresses in a compressed granular medium is found to depend very sensitively on the local ordering and leads to a very heterogeneous distribution of local forces along continuous chains of grains.

Gelation, Shear-Thinning and Shear-Thickening in Cement Slurries

Gelation, Shear-Thinning and Shear-Thickening in Cement Slurries — The rheological behavior of cement slurries may be described in terms of a few elementary behaviors: - the rapid formation of a gel, at rest; - the collapse of this gel under a critical stress directly related to the strength of the interparticle forces; - the progressive destruction of the gel fragments under moderate shear rate, with an increasingly shear-thinning behavior as the elementary particles gets smaller; - the reconstruction, at high shear rate, of shear-resistant structures, probably as chains of grains at contact; - complete jamming, possibly. We analyze the transition mechanisms from one step to the next one. We show how the addition of a dispersing agent favors shear-thickening at the expense of shear-thinning and how the control of the particle surface state and of interparticle friction restricts shear-thickening and the risk of jamming.

Nonlinear Dynamics of Grains in a Liquid-Saturated Soil

A new kind of nonlinearity of inertial type caused by accelerated motionof interacting particles is described. The model deals with an ensembleof grains immersed into a vibrating fluid. First, the nonlinearvibration of two connected grains is studied. The temporal behaviours ofdisplacement and velocity, as well as spectrum of vibration, areanalysed. Numerical simulations are performed. Then an infinite chain ofgrains is considered and the corresponding differential-differenceequation is derived. For the continuum limit the inhomogeneous nonlinearwave equation is solved and temporal profiles are calculated. A newresonant phenomenon is described and the resonant curves areconstructed.

Secondary solitary wave formation in systems with generalized Hertz interactions

We consider a chain of monodisperse elastic grains of radius R where the grains are barely in contact. The grains repel upon contact via the Hertz-type potential, V proportional to delta(n), n > 2, where delta > or = 0, is the grain-grain overlap, delta identical with 2R-(u(i+1)-u(i)), where u(i) denotes the displacement of grain i from its original equilibrium position. This being a computational study, we consider n to be arbitrary. Our dynamical simulations build on several earlier studies by Nesterenko, Coste, and Sen and co-workers that have shown that an impulse propagates as a solitary wave of fixed spatial extent, infinity < L(n) < 1, through a chain of grains. Here, we develop on a recent study by Manciu, Sen, and Hurd [Phys. Rev. E 63, 016614 (2001)] that shows that colliding solitary waves in the chains of interest spawn a well-defined hierarchy of multiple secondary solitary waves (SSWs) that carry approximately 0.5% or less of the energy of the original solitary waves. We show that the emergence of SSWs is a complex process where nonlinear forces and the discreteness of the grains lead to the partitioning of the available energy into hierarchies of SSWs. The process of formation of SSWs involves length scales and time scales that are controlled by the strength of the nonlinearity in the system. To the best of our knowledge, there is no formal theory that describes the dynamics associated with the formation of SSWs. Calculations for cases where the Hertz-type potential can be symmetric in the overlap parameter delta, i.e., where delta can be both positive and negative, suggest that the formation of secondary solitary waves may be a fundamental property of certain discrete, nonlinear systems.

Sintering of unequal grains

This theory shows that, as the ratio of the grain sizes increases, the boundary between the grains becomes more highly curved and the smaller grain becomes more deeply embedded in the larger one. At a ratio of only 1.57, the exposed portion of the small grain is too small to allow it to bond to a third, larger grain, and it cannot effectively connect to a chain of grains. For equal grains the theory reduces to the earlier theory but, as the size ratio increases, the stress gradient in the grain boundary decreases and the grain boundary becomes more tightly curved. These two factors decrease the rate of bonding, which decreases as either the ratio of the sizes, or their absolute sizes, increase.

Generalized Drude model: unification of ballistic anddiffusive electron transport

For electron transport in parallel-plane semiconducting structures, a model isdeveloped that unifies ballistic and diffusive transport and thus generalizesthe Drude model. The unified model is valid for arbitrary magnitude of themean free path and arbitrary shape of the conduction band edge profile.Universal formulas are obtained for the current-voltage characteristic in thenondegenerate case and for the zero-bias conductance in the degenerate case,which describe in a transparent manner the interplay of ballistic and diffusivetransport. The semiclassical approach is adopted, but quantum correctionsallowing for tunnelling are included. Examples are considered, in particularthe case of chains of grains in polycrystalline or microcrystallinesemiconductors with grain size comparable to, or smaller than, the mean freepath. Substantial deviations of the results of the unified model from those ofthe ballistic thermionic-emission model and of the drift-diffusion model arefound. The formulation of the model is one-dimensional, but it is argued thatits results should not differ substantially from those of a fullythree-dimensional treatment.

Seasonal contrasts in snow-cover characteristics on Ross Sea ice floes

AbstractSnow-cover characteristics on ice floes in the Ross Sea, Antarctica, were examined during cruises in autumn 1998 and summer 1999. The autumn snow cover was shallower, colder and had higher and more variable salinity, and smaller single and composite grain-sizes than the summer snow cover. The autumn snow cover was dominated by rounded particles in chains of grains and clusters, while the summer snow cover was composed primarily of melt clusters. There was extensive flooding of the summer snow cover at the snow/ice interface. The summer snow cover was nearly isothermal and close to the melting point. It exhibited obvious signs of melting and refreezing in the form of ice lenses, pipes and superimposed ice, although no melt ponds were evident. Many of the ice lenses were located directly above the saline standing water found on most of the summer ice floes.

Unified model of ballistic and diffusive carrier transport: Application to photovoltaic materials

A unified model of one-dimensional ballistic and diffusive carrier transport in semiconductors, valid for arbitrary mean free path and arbitrary shape of the band edge profiles, is applied in a study of the effect of grain boundaries on the transport properties of photovoltaic materials. Adopting the trapping model to describe the grain boundaries, dark conductivities and photoconductivities are calculated as a function of donor density for samples consisting of chains of grains with length comparable to the mean free path. The results of the unified model are found to deviate substantially from those of the purely ballistic and purely diffusive limits.

Impulse propagation in granular media

Grains can be described as elastic objects. When two grains are pushed against one another, they repel via Hertz’ nonlinear force law, F∝δn, n&amp;gt;2, δ≥0, being the overlap between the grains. We show that the propagation of an impulse of any magnitude in a 1D chain of grains at zero loading can be described as a solitary wave. The width of the solitary wave, L(n)→1 as n→∞, L∼5 grain diameters for typical granular contacts and L(n)→∞ when n→2. The condition δ≥0 leads to the formation of secondary solitary waves when two identical solitary waves propagating in opposite directions collide. Randomness in grain densities and sizes and restitutional losses lead to approximately exponential decays in distance of the energy of a propagating solitary wave. It turns out that impulses can be exploited to fingerprint a buried impurity mass in a 1D chain with and without gravity. In closing, we shall discussthe backscattering of an impulse from a buried object in 3D beds at shallow depths. Implications of this work with respect to humanitarian demining and related applications will be presented. [Work supported in part by U.S. Army Corps of Engineers and Sandia National Laboratories. Work done in collaboration with Marian Manciu (SUNY-Buffalo) and Alan J. Hurd (Sandia National Labs).]

The propagation and backscattering of soliton-like pulses in a chain of quartz beads and related problems. (I). Propagation

We confirm that for vanishingly small loading and large impact condition, it may be possible to generate solitons in a chain of grains that are characterized by Hertzian contacts. For uniform or progressive loading conditions throughout the chain, one generates soft-solitons which are weakly dispersive in space and time. Under conditions of weak impact, one generates acoustic pulses through the chain. We describe the displacements, velocities and accelerations suffered by the individual grains when subjected to solitons, soft-solitons and acoustic pulses and describe the effects of restitution on the propagating pulse.

Existence criterion of solitary waves in a chain of grains

We extend the proof given by MacKay [Phys. Lett. A 251 (1999) 191] to the general case of arbitrary power-law type contact force f∝ δ p . We find that the criterion for the existence of solitary waves in the horizontal granular chain is p>1.

Subsídios à tomada de decisão da escolha da modalidade para o planejamento dos transportes no estado do Paraná

Os canais de comercialização de grãos e farelo de soja convivem com a separação geográfica entre produção e consumo ou exportação no Estado do Paraná. Nesse sentido, o aprimoramento da oferta dos serviços de transporte é instrumento particularmente interessante na construção da competitividade das commodities, em cujos mercados prevalecem como padrão de concorrência o preço. Esse estudo utilizou modelos de racionalização do uso da infra-estrutura de transporte, com base nos fretes, para balizar a avaliação entre a distribuição modal socialmente desejável e aquela efetivamente observada. Os resultados obtidos sinalizaram para o incremento muito significativo na utilização das ferrovias. Em decorrência do desvio em relação à situação efetivamente observada, sugere-se que haja esforços dos órgãos de planejamento dos transportes para o aprimoramento de outros aspectos associados ao transporte ferroviário, tais como pontualidade, perdas e condições gerais de acondicionamentos, o que talvez tenha mais facilidade de ser implementado, em face da gestão privada das ferrovias paranaenses. Isso poderia fazer convergir interesses particulares dos usuários e interesses públicos.

Magnetic coupling in boron-rich NeFeB nanocomposites

In this article, we report types of magnetic domain patterns caused by either dipolar coupling or exchange coupling inside a heat-treated boron-rich NdFeB melt-spun ribbon. The average grain size of the sample was about 38 nm. Single particle domains were commonly observed in Fresnel images all through the specimen, suggesting little exchange coupling among the grains in these regions. Snake-shaped long interactive domains consisting of chains of grains were found in many Foucault images at the regions where exchange coupling is weak. These domains are caused by dipolar coupling along the chains. Wider domains consisting of many grains were also observed by Fresnel images, although they were much less common than the single particle domains. The grains inside a wider domain are believed to be exchange coupled with each other. The magnetic properties of such boron-rich NdFeB composites are influenced more by the dipolar coupling than by exchange coupling between the nanocrystalline grains.