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Showing papers on "Grain size published in 1986"


Journal ArticleDOI
TL;DR: In this article, a two-phase mixture theory is presented which describes the deflagration-to-detonation transition (DDT) in reactive granular materials, based on the continuum theory of mixtures formulated to include the compressibility of all phases and the compaction behavior of the granular material.

1,155 citations


Journal ArticleDOI
TL;DR: In this paper, the influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas medium high-pressure deformation apparatus.
Abstract: The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas-medium high-pressure deformation apparatus. The specimens used were hot-pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single-crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10-mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.

814 citations


Journal ArticleDOI
TL;DR: In this paper, the authors developed a Fracture mechanics model to interpret thermal cracking in the Oak Hall limestone and found that the internal stresses due to thermal expansion anisotropy are relaxed by plastic flow in this relatively fine grained limestone.
Abstract: Samples of Westerly granite, Frederick diabase, and Oak Hall limestone were thermally cracked at room pressure to various peak temperatures. Scanning electon microscopy (SEM) was performed on ion-thinned samples. The thermally induced crack density is dependent upon the temperature, thermal expansion mismatch, thermal expansion anisotropy, initial crack porosity, and grain size. The mode of propagation is different for grain boundary and intragranular cracks. Crack densities in the granite were quantified using stereological techniques. The thermally induced crack surface area per unit volume apparently has a quadratic dependence on the temperature increase, a physical interpretation for which can be formulated on the basis of energetic balance. Fracture mechanics models are developed to interpret thermal cracking. The predictions concerning thermal crack initiation temperature and crack propagation and arrest behavior agree well with observations for the granite and the diabase. The model predicts significant thermal cracking for the Oak Hall limestone, which contradicts the SEM observation. A possible explanation for this discrepancy is that the internal stresses due to thermal expansion anisotropy are relaxed by plastic flow in this relatively fine grained limestone.

382 citations


Journal ArticleDOI
TL;DR: In this article, the detrimental aging phenomenon observed in ZrO2-Y2O3 materials was investigated with respect to the gaseous aging environment and the Y 2O3 and SiO2 content of the material and it was shown that the aging phenomenon is caused by water vapor and that intergranular silicate glassy phases play no significant role.
Abstract: The detrimental aging phenomenon observed in ZrO2-Y2O3 materials, which causes tetragonal ZrO2 to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y2O3 and SiO2 content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter-granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO2 to produce clusters of small (20 to 50 nm) crystallites of α-Y(OH)3. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y2O3) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.

360 citations


Journal ArticleDOI
TL;DR: In this article, a model for the packing density of multisized grains is presented, which is expressed as a function of the fractional solid volume of each grain size present.

346 citations


Journal ArticleDOI
TL;DR: In this article, the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized.
Abstract: Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

306 citations


Journal ArticleDOI
TL;DR: In this paper, a stability theory is presented which describes the conditions under which thin films rupture, and it is found that holes in the film will either grow or shrink, depending on whether their initial radius is larger or smaller than a critical value.
Abstract: A stability theory is presented which describes the conditions under which thin films rupture. It is found that holes in the film will either grow or shrink, depending on whether their initial radius is larger or smaller than a critical value. If the holes grow large enough, they impinge to form islands; the size of which are determined by the surface energies. The formation of grooves where the grain boundary meets the free surface is a potential source of holes which can lead to film rupture. Equilibrium grain boundary groove depths are calculated for finite grain sizes. Comparison of groove depth and film thickness yields microstructural conditions for film rupture. In addition, pits which form at grain boundary vertices, where three grains meet, are another source of film instability.

298 citations


Journal ArticleDOI
TL;DR: Hall-petch strengthening has been observed for the microhardness of electrodeposited nickel material extending to a finest grain diameter of approximately 12 nm as mentioned in this paper, which is the smallest grain size known.

292 citations


Journal ArticleDOI
TL;DR: In this article, a Monte Carlo computer simulation technique is applied to the problem of recrystallization, in which a continuum system is modeled employing a discrete lattice, and the nucleation rate is chosen as either constant or site saturated.

263 citations


Journal ArticleDOI
TL;DR: In this article, the effect of MgO as a solid-solution additive in the sintering of Al2O3 was studied and the separate effects of the additive on densification and grain growth were assessed.
Abstract: The effect of MgO as a solid-solution additive in the sintering of Al2O3 was studied. The separate effects of the additive on densification and grain growth were assessed. Magnesia was found to increase the densification rate during sintering by a factor of 3 through a raising of the diffusion rate. The grain-size dependence of the densification rate indicated control primarily by grain-boundary diffusion. Magnesia also increased the grain growth rate during sintering by a factor of 2.5. The dependence of the grain growth rate on density and grain size suggested a mechanism of surface-diffusion-controlled pore drag. It was argued, therefore, that MgO enhanced grain growth by raising the surface diffusion coefficient. The effect of MgO on the densification rate/grain growth rate ratio was, therefore, found to be minimal and, consequently, MgO did not have a significant effect on the grain size/density trajectory during sintering. The role of MgO in the sintering of alumina was attributed mainly to its ability to lower the grain-boundary mobility.

194 citations


Journal ArticleDOI
TL;DR: In this article, measured variations of pivoting angles with grain size, shape (reliability and angularity) and imbrication are employed in analyses of grain threshold to examine how these factors influence selective grain entrainment and sorting.
Abstract: Measured variations of pivoting angles with grain size, shape (‘reliability’ and angularity) and imbrication are employed in analyses of grain threshold to examine how these factors influence selective grain entrainment and sorting. With a bed of uniform grain sizes, as employed experimentally to establish the standard threshold curves such as that of Shields, the threshold condition depends on grain shape and fabric. The analysis demonstrates quantitatively that there should be a series of nearly-parallel threshold curves depending on grain pivoting angles. For a given grain size, the order of increasing flow strength required for entrainment is spheres, smooth ellipsoids (depending on their ‘reliability’), angular grains, and imbricated ellipsoids (depending on their imbrication angles). The relative threshold values for these different grain shapes and fabric are predicted according to their respective pivoting angles, but remain to be directly tested by actual threshold measurements. The pivoting angle of a grain also depends on the ratio of its size to those it rests upon. This dependence permits an evaluation of selective entrainment by size of grains from a bed of mixed sizes, the condition generally found in natural sediments. The pivoting model predicts systematic departures from the standard threshold curves for uniform grain sizes. Such departures have been found in recent studies of gravel threshold in rivers and offshore tidal currents. The pivoting model is compared with those threshold data with reasonable agreement. However, more controlled measurements are required for a satisfactory test of the model. It is concluded that variations in pivoting angles for grain entrainment are significant to the processes of selective sorting by grain size and shape.

Journal ArticleDOI
TL;DR: In this article, the average grain size of ZrO2(+Y, o) materials sintered at 1400°C was observed to depend significantly on the Y2O3 content.
Abstract: The average grain size of ZrO2(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y2O3 content. The average grain size decreased by a factor of 4 to 5 for Y2O3 contents between 0.8 and 1.4 mol% and increased at Y2O3 contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y2O3-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y2O3. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.

Journal ArticleDOI
TL;DR: In this paper, a theory of grain growth was developed in which textural effects were taken into account by introduction of orientation dependent grain boundary energies and mobilities, and it was shown that grain growth leads to pronounced texture changes which are accompanied by characteristic changes of the scattering of the grain size distribution, which then not even approximately follows a t 1 2 - law.

Journal ArticleDOI
TL;DR: In this article, the effect of small uniaxial stresses on the sintering of CdO powder compacts was studied using a loading dilatometer, where compacts of two different green densities were sintered at 1123 K and subjected to stresses between 0 and 0.25 MPa.
Abstract: The effect of small uniaxial stresses on the sintering of CdO powder compacts was studied using a loading dilatometer. Compacts of two different green densities were sintered at 1123 K and subjected to stresses between 0 and 0.25 MPa. Densification and creep occur simultaneously, and the effects of these two processes can be separated. Between relative densities of 0.5 and 0.9, the dependence of the uniaxial creep rate on density can be described in terms of a stress intensification factor which depends exponentially on the porosity but is independent of the grain size. Comparison of the densification and creep rates permits definition of the sintering stress, which is found to decrease with increasing density, and verification of the Zener relation. The stress and grain size dependence of the creep rate, and the grain size dependence of the densification rate, support grain-boundary diffusion as the rate-controlling step in both processes.

Journal ArticleDOI
TL;DR: In this article, a dynamic calculation involving the formation of a non-equilibrium crack in the carbide is used to explain the experimental observations, which indicates a grain size dependence.

Journal ArticleDOI
01 Apr 1986-Icarus
TL;DR: Theoretical models for the thermal conductivity of grains smaller than 30 microns diameter and over 1 mm diameter were developed in this paper to aid in the understanding of the thermal inertia of the surface regolith of Mars.

Journal ArticleDOI
TL;DR: In this article, the role of dislocations and grain boundaries in martensite nucleation was investigated and the authors concluded that grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei.
Abstract: In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (Ms) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations, work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation with decreasing grain size is the main factor to depress Ms in fine-grained specimens.

Journal ArticleDOI
TL;DR: The results of an Einstein Observatory imaging proportional counter investigation of the halos produced by the scattering of X-rays from interstellar grains of four compact Galactic and two extragalactic X-ray sources are reported in this article.
Abstract: The results of an Einstein Observatory imaging proportional counter investigation of the halos produced by the scattering of X-rays from interstellar grains of four compact Galactic (low-latitude) and two extragalactic (high-latitude) X-ray sources are reported. It is found that the intensity of these halos correlates well with the amount of visual extinction and the distance through the Galaxy's dust layer: quantities which measure the column density of grains to a given source. From this result, and from the size and shape of the halos from the Galactic sources, a number of grain parameters are derived in the context of two alternate grain size distributions: the Mathis-Rumpl-Nordsieck (MRN) and the Oort-van de Hulst distributions, either of which is capable of producing the observed halos. Though no single grain size is capable of producing the shapes observed for these halos, a mean size of about 0.1 micron and a number density of about 10 to the -12th grains/cu cm produce the correct halo intensities. From the additional parameters determined from the size and shape of the halos, results concerning the amount of material in grains which are in general agreement with the observed depletion of the elements from the gas phase of the interstellar medium, as well as with the detailed predictions of the MRN size distribution, are found.

Journal ArticleDOI
TL;DR: In this article, a series of experiments have been undertaken to determine how this angle depends on grain shape (rollability and angularity), on the ratio of the size of the pivoting grain to those beneath, and on factors such as imbrication.
Abstract: Important to grain entrainment by a flowing fluid is the pivoting angle of the grain about its contact point with an underlying grain. A series of experiments has been undertaken to determine how this angle depends on grain shape (rollability and angularity), on the ratio of the size of the pivoting grain to those beneath, and on factors such as imbrication. The experiments involved gravel-sized spheres (ball-bearings and marbles), natural pebbles selected for their approximately triaxial ellipsoid shapes, and angular crushed basalt pebbles. The pivoting angles for these grains were measured on an apparatus consisting of a board which can be progressively inclined, the angle of the board being equal to the pivoting angle at the instant of grain movement. The pivoting angles of spheres showed reasonable agreement with a theoretically derived equation, showing much better agreement than in previous studies which utilized sand-sized spheres. A series of measurements with spheres ranging from sand to gravel sizes reveals that the pivoting angles decrease with increasing particle size. Our results are therefore consistent with the earlier studies limited to sand-size spheres. The cause of this size dependence is unknown since moisture and electrostatic binding can be ruled out. Similar size dependencies are also found for the ellipsoidal pebbles and angular gravel. The experiments with ellipsoidal pebbles demonstrated a strong shape dependence for the pivoting angle, being a function of the ratio of the pebble's smallest to intermediate axial diameters. This ratio controls the grain's ability to roll and pivot; with small ratios of these diameters the pebbles tended to slide out of position, whereas with ratios closer to unity (circular cross-section) true pivoting took place and the angles were smaller. Experiments with flat pebbles placed in an imbricated arrangement yielded much larger angles than when the pebbles lay in a horizontal position, the pivoting angle being increased approximately by the imbrication angle. The angular crushed gravel also required high pivoting angles, apparently due to interlocking of the grains resulting from their angularity. Other factors being equal, the measurements of pivoting angles demonstrate that the order of increasing difficulty of entrainment is spheres, ellipsoidal grains, angular grains, and imbricated grains. The results obtained here make possible the quantitative evaluation of these shape effects on grain threshold, as well as evaluation of the selective entrainment of grains from a bed of mixed sizes.

Journal ArticleDOI
TL;DR: In this article, the kinetics of grain growth in a Zn-22% Al alloy during static annealing and superplastic deformation (SPD) were studied.

Journal ArticleDOI
TL;DR: In this paper, the authors showed that grain growth in polycrystalline caicite at 800°C and 300-MPa confining pressure is inhibited by the addition of Al2O3 particles; volume fractions of second phase (f) ranged from 0.02 to 10.0 vol%, and several powders with different average particle size were used.
Abstract: Grain growth in polycrystalline caicite (CaCO3) at 800°C and 300-MPa confining pressure is inhibited by the addition of Al2O3 particles; volume fractions of second phase (f) ranged from 0.02 to 10.0 vol%, and several powders with different average particle size were used. The stable grain size reached during heat treatment was inversely proportional to l/fm where m varied from 0.3 to 0.55 — agreeing with results from other grain growth experiments in ceramics and metals, and with models developed for particles located at grain boundaries and grain corners, but not agreeing with models developed for random dispersions. The dependence of the stable grain size on second-phase particle size in these experiments was less systematic, possibly because of variations in the particle size distributions used, or because of particle agglomeration effects. In the single-phase aggregates, the growth kinetics were consistent with a normal grain growth equation with n=3, although uncertainties were large.

Journal ArticleDOI
TL;DR: Secondary grain growth in thin Au films on SiO2 substrates is reported in this article, which indicates that surface energy anisotropy provides selectivity in the driving force for growth of secondary grains.
Abstract: Secondary grain growth in thin Au films on SiO2 substrates is reported. Secondary grains have {111} texture which minimizes the surface energy. This indicates that surface energy anisotropy provides selectivity in the driving force for growth of secondary grains. In thin Au films on SiO2, surface‐energy‐driven secondary grain growth occurs at room temperature as soon as a film becomes continuous. This mode of grain growth is, most likely, responsible for the development of the frequently observed {111} deposition texture in thin Au films.

Journal ArticleDOI
TL;DR: In this article, two new approaches to modeling magnetization data are described: the first approach models the magnetic properties of natural materials in terms of mixtures of possible source materials, such as magnetite and haematite crystals of varying concentrations and grain sizes.

Journal ArticleDOI
TL;DR: In this paper, the authors studied the thickness dependence of transparent conducting indium-doped ZnO films and found that the density of trap states due to chemisorbed oxygen at the grain boundaries was found to depend on the orientation of grains which depended on the film thickness.

Journal ArticleDOI
TL;DR: In this article, a polycrystalline alumina having an average grain size of 1.6 μm was deformed to true strains of up to 60% in simple uniaxial compression at 1693 K without significant cavitation.
Abstract: Dense polycrystalline alumina having an average grain size of 1.6 μm was deformed to true strains of up to 60% in simple uniaxial compression at 1693 K without significant cavitation. A strain rate of ∼2 × 10−4 s−1 was obtained at a uniaxial stress of ∼30 MPa. Such a reasonable flow stress at such high strain rates and a relatively low temperature raises a real possibility of isothermal superplastic forging of alumina ceramics. The mechanism of deformation was identified to be diffusion creep with grain boundaries as the dominant diffusion path. The specimens suffered considerable grain growth during deformation. Part of the grain growth resulted from time and temperature, but the majority was induced by deformation strain.

Book ChapterDOI
01 Jan 1986
TL;DR: In this article, a model for the microstructural development during liquid phase assisted sintering of Si3N4 and the relation between micro-structural characteristics and mechanical properties is presented.
Abstract: Microstructural development during liquid-phase assisted densification of Si3N4 powder compacts is a function of powder properties, type and amount of additives as well as of processing conditions (time, temperature, pressure). Varying these parameters results in—besides different densities and phase compositions—a different mean grain size and especially grain morphology of the β-grains. These characteristics mainly determine the mechanical properties at temperatures >1000°C. In order to optimize dense Si3N4 materials by correlating microstructural characteristics with mechanical properties, quantitative analysis of the grain structure is necessary. Thus, a method has been developed to quantify microstructures consisting of elongated, rod-like β-grains. By means of these characteristics it is possible to discuss the interdependence between starting composition, processing conditions, microstructure and resulting mechanical properties of dense Si3N4 materials. This leads to a model for the microstructural development during liquid- phase assisted sintering of Si3N4 and the relation between microstructural characteristics and mechanical properties.

Journal ArticleDOI
TL;DR: In this article, the slow plastic flow properties of a series of binary B2 FeAl intermetallics at elevated temperatures, and ranging in aluminum content from 39.8 to 48.7 at.

Journal ArticleDOI
TL;DR: On etudie l'influence de la dimension de grain sur la tenacite et la transformation t→m d'une zircone polycristalline quadratique stabilisee par Y 2 O 3.
Abstract: On etudie l'influence de la dimension de grain sur la tenacite et la transformation t→m d'une zircone polycristalline quadratique stabilisee par Y 2 O 3

Journal ArticleDOI
TL;DR: In this paper, an optimized seed selection through ion channeling (SSIC) process was proposed for enhancing the grain size and texture of polycrystalline Si films grown by low-pressure chemical vapor deposition on SiO2.
Abstract: This work outlines an optimized seed selection through ion channeling (SSIC) process for enhancing the grain size and {110} texture of polycrystalline Si films grown by low‐pressure chemical vapor deposition on SiO2. These films, 0.44 μm thick, were self‐implanted at normal incidence to various doses (1–20 × 1014 cm−2) and subsequently recrystallized at 600 °C. An enhanced average grain diameter resulted after implantation and annealing, ranging from 0.10 to 2.0 μm (versus the as‐deposited 0.080 μm) and increasing with the implant dose. The grain size versus implant dose behavior may be explained by a previously proposed stochastic model. An enhanced {110} texture was also observed after processing for implant doses of 6–14 ×1014 cm−2. The optimal dose was 11 × 1014 cm−2, for which the {220} diffracted x‐ray intensity was 30 times the as‐deposited value, and the 〈110〉 directions were confined to within ±4° (versus the as‐deposited ±20°) of the surface normal. This experiment demonstrates the effectiveness...

Journal ArticleDOI
TL;DR: In this article, high purity polycrystalline iron of two different grain sizes was deformed at room temperature in tension to strains of up to 0.27, and the deformation mechanisms operative at selected strain levels identified by examining both the bulk microstructure by transmission electron microscopy and features developed on prepolished surfaces using optical microscopy.