Showing papers on "Grain size published in 2006"
TL;DR: In this paper, the authors reviewed the current understanding of the electrical properties of the grain boundaries of acceptor-doped zirconia and ceria, with an emphasis on the grain-boundary defect structure.
617 citations
TL;DR: In this article, the influence of texture and grain size on work hardening behavior and dynamic recovery of magnesium alloys was studied, in addition to the direct effect of texture through the change in the orientation factor for basal and prismatic slip, effects were found on dynamic recovery and the appearance of stage II of workhardening.
598 citations
TL;DR: In this article, the microstructural evolution during cryomilling, consolidation and processing, the thermal stability of the microstructure, and mechanical properties of consolidated materials are compared to those results for powders and consolidated materials generated by mechanical alloying, milling at ambient temperatures and other means to produce fine grained materials.
516 citations
25 Nov 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the microstructure and the strength of lath martensite in Fe 0.2C and Fe 1.2Mn alloys were analyzed as a function of the prior austenite grain size.
Abstract: The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels.
496 citations
TL;DR: In this article, two different mechanisms for plastic strain-induced grain refinement in pure Cu were identified, corresponding to different levels of strain rate, and the minimum size of refined grains via this process is about 100 nm.
446 citations
TL;DR: In this article, the effect of grain size on NiFe 2 O 4 nanoparticles has been studied in detail using X-ray diffraction (XRD) data and the strain graph and it was found that the specific magnetization of the nanosized NiFe O 4 powders was lower than that of the corresponding coarse-grained counterparts and decreased with a decrease in grain size.
435 citations
TL;DR: In this paper, isothermal and constant-grain-size sintering was carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature.
Abstract: Isothermal and constant-grain-size sintering have been carried out to full density in Y2O3 with and without dopants, at as low as 40% of the homologous temperature. The normalized densification rate follows Herring's scaling law with a universal geometric factor that depends only on density. The frozen grain structure, however, prevents pore relocation commonly assumed in the conventional sintering models, which fail to describe our data. Suppression of grain growth but not densification is consistent with a grain boundary network pinned by triple-point junctions, which have a higher activation energy for migration than grain boundaries. Long transients in sintering and grain growth have provided further evidence of relaxation and threshold processes at the grain boundary/triple point.
338 citations
TL;DR: In this article, microstructural analyses of peridotite mylonites from the oceanic lithosphere indicate that shear localization results from the combined effects of grain size reduction, grain boundary sliding and second phase pinning during deformation.
338 citations
TL;DR: In this paper, the authors investigated the preparation of bulk dense nanocrystalline BaTiO 3 and Ni-Cu-Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%.
Abstract: We investigated the preparation of bulk dense nanocrystalline BaTiO 3 and Ni-Cu-Zn ferrite ceramics using an unconventional two-step sintering strategy, which offers the advantage of not having grain growth while increasing density from about 75% to above 96%. Using nanosized powders, dense ferrite ceramics with a grain size of 200 nm and BaTiO 3 with a grain size of 35 nm were obtained by two-step sintering. Like the previous studies on Y 2 O 3 , the different kinetics between densification diffusion and grain boundary network mobility leaves a kinetic window that can be utilized in the second-step sintering. Evidence indicates that low symmetry, ferroelectric structures still exist in nanograin BaTiO 3 ceramics, and that saturation magnetization is the same in nanograin and coarse grain ferrite ceramics.
303 citations
15 Oct 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: SiC particles were uniformly dispersed into an AZ31 matrix by friction stir processing (FSP) and the SiC particles promoted the grain refinement of the AZ 31 matrix by FSP as discussed by the authors.
Abstract: SiC particles were uniformly dispersed into an AZ31 matrix by friction stir processing (FSP). The SiC particles promoted the grain refinement of the AZ31 matrix by FSP. The mean grain size of the stir zone with the SiC particles was obviously smaller than that of the stir zone without the SiC particles. The microhardness of the stir zone with the SiC particles was reached about 80 Hv due to the grain refinement and the distribution of the SiC particles. Additionally, the SiC particle/AZ31 region showed fine grains even at elevated temperatures (∼400 °C) resulting in the pinning effect by the SiC particles. In contrast, the microhardness was significantly decreased attributed to the abnormal grain growth of the FSPed AZ31 without the SiC particles.
277 citations
TL;DR: In this article, the basis for the use of fine grain size parameters for inference of paleoflow speeds is reviewed, based on data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition.
Abstract: The basis for, and use of, fine grain size parameters for inference of paleoflow speeds is reviewed here. The basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field, experimental data on suspended sediment transport and deposition, and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed. In the deep sea, sorting events occur under resuspension/deposition events in benthic storms. At flow speeds below 10–15 cm s−1, size in the noncohesive “sortable silt” (10–63 μm) range is controlled by selective deposition, whereas above that range, removal of finer material by winnowing also plays a role. The best particle size instruments to measure a flow speed–related grain size employ the settling velocity method, while laser diffraction sizers can yield misleading results because of particle shape effects. Potential problems, including source effects, downslope supply on continental margins, spatial variability of flow over bedforms, and influence of ice-rafted detritus, are examined. A number of studies using the sortable silt flow speed proxy are reviewed, and inverse modeling of grain size distributions is examined. Outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed.
TL;DR: The Hillert theory can be regarded as an accurate description for the 3D ideal grain growth, and the large-scale phase-field simulation confirms the3D growth law obtained from the Surface Evolver simulations in smaller scales.
Abstract: We developed an efficient computation scheme for the phase-field simulation of grain growth, which allows unlimited number of the orientation variables and high computational efficiency independent of them Large-scale phase-field simulations of the ideal grain growth in two-dimensions (2D) and three-dimensions (3D) were carried out with holding the coalescence-free condition, where a few tens of thousands grains evolved into a few thousand grains By checking the validity of the von Neumann-Mullins law for individual grains, it could be shown that the present simulations were correctly carried out under the conditions of the ideal grain growth The steady-state grain size distribution in 2D appeared as a symmetrical shape with a plateau slightly inclined to the small grain side, which was quite different from the Hillert 2D distribution The existence of the plateau stems from the wide separation of the peaks in the size distributions of the grains with five, six, and seven sides The steady-state grain size distribution in 3D simulation of the ideal grain growth appeared to be very close to the Hillert 3D distribution, independent of the initial average grain size and size distribution The mean-field assumption, the Lifshitz-Slyozov stability condition, and all resulting predictions in the Hillert 3D theory were in excellent agreement with the present 3D simulation Thus the Hillert theory can be regarded as an accurate description for the 3D ideal grain growth The dependence of the growth rate in 3D simulations on the grain topology were discussed The large-scale phase-field simulation confirms the 3D growth law obtained from the Surface Evolver simulations in smaller scales
TL;DR: In this paper, a model for the low temperature tensile response of copper polycrystals with grain sizes in the range of 2-50μm was proposed and the initial work hardening behavior was strongly grain size dependent and was considered to arise from a combination of kinematic and isotropic hardening due to dislocation-grain boundary interactions.
TL;DR: In this article, a colloidal mixture of Al2O3/ZrO2 composite powders was mixed and consolidated by a colloid method, sintered to >98% theoretical density at 1550°C, and subsequently heat-treated at temperatures up to 1700°C for grain-size measurements.
Abstract: Alumina and Al2O3/ZrO2 (1 to 10 vol%) composite powders were mixed and consolidated by a colloidal method, sintered to >98% theoretical density at 1550°C, and subsequently heat-treated at temperatures up to 1700°C for grain-size measurements. Within the temperature range studied, the ZrO2 inclusions exhibited sufficient self-diffusion to move with the Al2O3 4-grain junctions during grain growth. Growth of the ZrO2, inclusions occurred by coalescence. The inclusions exerted a dragging force at the 4-grain junctions to limit grain growth. Abnormal grain growth occurred when the inclusion distribution was not sufficiently uniform to hinder the growth of all Al2O3 grains. This condition was observed for compositions containing ≤2.5 vol% ZrO2, where the inclusions did not fill all 4-grain junctions. Exaggerated grains consumed both neighboring grains and ZrO2, inclusions. Grain-growth control (no abnormal grain growth) was achieved when a majority (or all) 4-grain junctions contained a ZrO2 inclusion, viz., for compositions containing ≥5 vol% ZrO2. For this condition, the grain size was inversely proportional to the volume fraction of the inclusions. Since the ZrO2 inclusions mimic voids in all ways except that they do not disappear, it is hypothesized that abnormal grain growth in single-phase materials is a result of a nonuniform distribution of voids during the last stage of sintering.
TL;DR: In this article, the deformation twins were observed in polycrystalline Cu with grain sizes varying from micrometers to nanometers during the process of equal channel angular pressing at room temperature and low strain rate.
TL;DR: In this article, the piezoelectric properties of 0.95NKN-0.05BT powders during sintering were investigated and improved with increasing relative density and grain size.
Abstract: For 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 (0.95NKN-0.05BT) ceramics sintered at 1040–1075°C, abnormal grain growth occurred but the grain size decreased when the sintering temperature exceeded 1075°C. The dielectric constant (ϵ3T∕ϵ3), electromechanical coupling factor (kp), and piezoelectric constant (d33) were considerably increased with increasing relative density and grain size. Evaporation of Na2O deteriorated the piezoelectric properties by decreasing the resistivity. To minimize Na2O evaporation, specimens were muffled with 0.95NKN-0.05BT powders during the sintering. Improved piezoelectric properties of d33=225pC∕N, kp=36%, and ϵ3T∕ϵ3=1058 were obtained for specimen sintered at 1060°C for 2h with muffling.
TL;DR: In this paper, the hydrogen desorption properties of commercial nanocrystalline magnesium hydride (Tego Magnan® from Degussa-Goldschmidt) processed by controlled mechanical milling (CMM) are investigated.
Abstract: The hydrogen desorption properties of commercial nanocrystalline magnesium hydride (Tego Magnan® from Degussa–Goldschmidt) processed by controlled mechanical milling (CMM) are investigated. A profound effect of the powder particle size on the hydrogen desorption characteristics has been observed. The onset (TON) and peak hydrogen desorption temperatures measured by differential scanning calorimetry (DSC) decrease initially slowly with decreasing mean particle size of hydride, and when the particle size reaches a certain critical threshold value, the desorption temperatures start decreasing more rapidly with further decrease of particle size. The total drop of desorption temperature from its initial value for the as-received MgH2 to the value attained for the milled MgH2 having a particle size of ~500–600 nm is within the range 40–60 °C. The metastable γ-MgH2 hydride coexists with the stable nanocrystalline β-MgH2 in the microstructure of the MgH2 powders ball milled for 10 h and longer. Quantitative evidence shows that two factors, namely the refined powder particle size and the γ-MgH2 phase residing within the powder particles, acting additively, are responsible for a substantial reduction of the hydrogen desorption temperature of MgH2 hydride.
TL;DR: In this paper, a sintered sample of the nanopowder fabricated by hydrothermal synthesis has a high piezoelectric constant d33 due to fabrication by microwave sintering.
Abstract: Hydrothermally synthesized BaTiO3 powders with nanoscale-sized particles were densified by microwave sintering. A sintered sample of the nanopowder fabricated by hydrothermal synthesis has a high piezoelectric constant d33 due to fabrication by microwave sintering. The maximum value of the piezoelectric constant d33 of a specimen fabricated by microwave sintering was approximately 350 pC/N for a small grain size of 2.1 µm. Detailed microstructures of the samples were observed by transmission electron microscopy (TEM) and scanning electron microscopy/electron backscattered diffraction analysis/orientation imaging microscopy (SEM/EBSD/OIM). The size of ferroelectric domains in the samples showing superior piezoelectric properties was less than 50 nm. SEM/EBSD/OIM observations revealed that the fraction of random boundaries was higher by approximately 10% in microwave sintered samples than in conventionally sintered ones. It is suggested that the small size of domain and the higher fraction of random boundaries might be responsible for the excellent piezoelectric properties of small grains, which can partially be attributed to domain size.
TL;DR: The grain coarsening in dense nanocrystalline gadolinia-doped ceria (CGO) and CeO2 was studied for the grain size regime from 10 to 250 nm as discussed by the authors.
TL;DR: In this paper, four different ultrafine-grained (ufg) Al-7.5% Mg alloys were synthesized by consolidation of a mixture of as-received and cryomilled Al-Mg powders with a ratio of 1:9, yielding a bimodal microstructure consisting of coarse grains (grain sizes, d cg, typically of several micrometers).
TL;DR: In this paper, the spread in misorientation from the central grain orientation is measured and a parameter called the modified crystal deformation is determined from the spread of the misorientations.
TL;DR: In this article, the electrical properties of nanostructured, heavily yttria- or samaria-doped ceria ceramics are studied as a function of grain size using electrochemical impedance spectroscopy (EIS).
Abstract: The electrical properties of nanostructured, heavily yttria- or samaria-doped ceria ceramics are studied as a function of grain size using electrochemical impedance spectroscopy (EIS). A remarkable enhancement in the total ionic conductivity of about one order of magnitude is found in nanostructured samples, compared with the intrinsic bulk conductivity of conventional microcrystalline ceramics. This effect is attributed to the predominance of grain-boundary conduction in the nanostructured materials, coupled with an increase in the grain-boundary ionic diffusivity with decreasing grain size.
15 Oct 2006-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, FSP was applied to cast A356 Al to modify the as-cast microstructure, which significantly influenced microstructural development in the processed zone of cast aluminum matrix.
Abstract: Friction stir processing (FSP) was applied to cast A356 Al to modify the as-cast microstructure. FSP homogenizes and. refines the cast microstruclure, completely eliminates porosity, and creates a microstructure with fine Si particles (0.25-0.42 mu m) distributed in a fine grain aluminum matrix (3-4 mu m). Further, FSP parameters significantly influence microstructural development in the processed zone of cast A356 Al. Generally, higher tool rotation rate creates a more homogeneous microstructure. At lower tool rotation rates, a macroscopically visible banded structure characterized by a low density of coarse particles, was detected in the nugget zone. However, FSP parameters did not significantly influence the Al matrix grain size. The varied distribution pattern, size, and volume fraction of Si particles at different locations within the FSP zone indicates inhomogeneous material flow. A post-FSP T6 heat treatment did not alter the Si particle distribution, but did significantly coarsen the Si particles. (c) 2006 Elsevier B.V. All rights reserved.
TL;DR: In this article, the authors propose a kinetic description for the growth and evaporation of dirty dust particles, which consist of numerous small islands of different solid materials like Mg 2 SiO 4, SiO 2, Al 2 O 3, Fe and TiO 2.
Abstract: In this paper, we propose a kinetic description for the growth and evaporation of oxygen-rich, dirty dust particles, which consist of numerous small islands of different solid materials like Mg 2 SiO 4 , SiO 2 , Al 2 O 3, Fe and TiO 2 . We assume that the total surface of such a grain collects condensible molecules from the gas phase and that these molecules are rapidly transported by diffusive hopping on the surface to the respective solid islands, where finally the constructive surface chemical reactions take place which increase the size of the grain. Applied to a typical dust forming region in a brown dwarf atmosphere, turbulent temperature fluctuations enable the creation of first seed particles (nucleation) at high supersaturation ratios. These seeds are then quickly covered by different solid materials in a simultaneous way, which results in dirty grains. Our treatment by moment equations allows for the calculation of the time-dependent material composition of the dust grains and the elemental composition of the gas phase. We argue that the depletion of condensible elements from the gas phase by dust formation may be incomplete and occurs in a patchy, non-uniform way which possibly makes metallicity measurements highly uncertain.
TL;DR: In this article, the authors examined the dielectric properties of CaCu3Ti4O12 (CCTO) ceramics under various sintering temperatures from 1000 to 1120°C at an interval of 10°C.
Abstract: CaCu3Ti4O12 (CCTO) ceramics are prepared by the conventional solid-state reaction method under various sintering temperatures from 1000to1120°C at an interval of 10°C. Microstructures and crystalline structures are examined by scanning electronic microscopy and x-ray diffraction, respectively. Dielectric properties and complex impedances are investigated within the frequency range of 40Hz–110MHz over the temperature region from room temperature to 350°C. It has been disclosed that the microstructures can be categorized into three different types: type A (with the small but uniform grain sizes), type B (with the bimodal distribution of grain sizes) and type C (with the large and uniform grain sizes), respectively. The largeness of low-frequency dielectric permittivity at room temperature is closely related to the microstructure. Ceramics with different types of microstructures show the diverse temperature-dependent behaviors of electrical properties. However, the existence of some common characteristics is also found among them. For all of the ceramics, a Debye-type relaxation emerges in the frequency range of 100Hz–100kHz at high measuring temperatures, which has the larger dielectric dispersion strength than the one known in the frequency range above 100kHz. Thus, the high-temperature dielectric dispersion exhibits a large low-frequency response and two Debye-type relaxations. Furthermore, all of the ceramics show three semicircles in the complex impedance plane. These semicircles are considered to represent individually different electrical mechanisms, among which the one in the low-frequency range arises most probably from the contribution of the domain boundaries, and the other two are ascribed to the contributions of the domains and the grain boundaries, respectively.
TL;DR: In this article, the effects of grain size reduction on the electrochemical corrosion behavior of nanocrystalline Ni produced by pulse electrodeposition were characterized using potentiodynamic polarization testing and electrochemical impedance spectroscopy.
TL;DR: In this article, the authors investigated deformation-induced grain growth in electrodeposited nc Ni during high pressure torsion (HPT) and showed that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters.
Abstract: Deformation-induced grain growth has been reported in nanocrystalline (nc) materials under indentation and severe cyclic loading, but not under any other deformation mode. This raises an issue on critical conditions for grain growth in nc materials. This study investigates deformation-induced grain growth in electrodeposited nc Ni during high-pressure torsion (HPT). Our results indicate that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters. Also, texture evolution suggests that grain-boundary-mediated mechanisms played a significant role in accommodating HPT strain.
TL;DR: In this paper, the authors carried out an integrated experimental and simulation study of thermal transport in ultrananocrystalline diamond (UNCD) films and showed that UNCD films with a grain size of 3-5nm have thermal conductivities as high as 12W∕mK at room temperature.
Abstract: Although diamond has the highest known room temperature thermal conductivity, k∼2200W∕mK, highly sp3 amorphous carbon films have k<15W∕mK. We carry out an integrated experimental and simulation study of thermal transport in ultrananocrystalline diamond (UNCD) films. The experiments show that UNCD films with a grain size of 3–5nm have thermal conductivities as high as k=12W∕mK at room temperature, comparable with that of the most conductive amorphous diamond films. This value corresponds to a grain boundary (Kapitza) conductance greater than 3000MW∕m2K, which is ten times larger than that previously seen in any material. Our simulations of both UNCD and individual diamond grain boundaries yield values for the grain boundary conductance consistent with the experimentally obtained value, leading us to conclude that thermal transport in UNCD is controlled by the intrinsic properties of the grain boundaries.
TL;DR: In this paper, the properties of dense nanocrystalline BaTiO 3 ceramics with grain size (GS) down to 50nm were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), impedance spectroscopy and Raman Spectroscopy.
Abstract: Dense nanocrystalline BaTiO 3 ceramics with grain size (GS) down to 50 nm were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), impedance spectroscopy and Raman spectroscopy. A continuous reduction of the tetragonal distortion towards the pseudo-cubic state was obtained when the GS was reduced. Therefore, even the finest structure (ceramic with average GS of 50 nm) is still non-centrosymmetric. The dielectric constant ( K ) shows relative thermal stability in a large range of temperatures and is strongly depressed in the nanocrystalline ceramics, in comparison with the micrometric ones ( K being below 1000 for the ceramic with 50 nm GS). The losses are smaller than 5% in the frequency range of 10 2 –10 6 Hz and temperatures below 200 °C. As the GS decreases, the structural phase transitions assume a more diffuse character. A decrease of the Curie temperature with reducing the GS was confirmed by X-ray, calorimetric and permittivity data. The Raman spectra collected for the range 80–800 K provided evidence for the presence of all the crystalline phases of BaTiO 3 , as in single-crystal and micrometric ceramics; a few differences can be attributed to GS effects and to the high density of the non-ferroelectric grain boundaries. Evidence for the different phase transitions were provided by the disappearance of some bands and by anomalies in positions and intensities of selected Raman modes. The overall properties of the nanocrystalline BaTiO 3 ceramics can be explained as a combination of intrinsic effects, associated with the decrease of tetragonality and heat of transition with reducing GS, and extrinsic contributions due to the non-ferroelectric grain boundaries causing a “dilution” of the ferroelectric properties.