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Microstructural Development (microstructural + development)
Selected AbstractsSintering of AlN Using CaO-Al2O3 as a Sintering Additive: Chemistry and Microstructural DevelopmentJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2002Eirik Hagen The densification of aluminum nitride using Ca12Al14O33 as a sintering aid has been studied with emphasis on the effect of using coarse or fine powder, the amount of sintering aid, the sintering temperature, and embedding. Both crystalline and amorphous grain boundary phases were observed. Significant weight losses were observed for coarse-grained samples, and if suitable embedding was not used. Porous and coarse-grained ceramics with high contiguity and minor amounts of secondary phases were obtained by enhanced evaporation while dense ceramics were obtained limiting the evaporation. High weight losses in the graphite environment resulted in formation of a dense AlN surface layer. [source] Influence of shear flow on polymorphic behavior and microstructural development during palm oil crystallizationEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 3 2009Veerle De Graef Abstract The influence of shear on the crystallization of palm oil was studied at four different crystallization temperatures (18, 20, 22 and 25,°C). Time-resolved X-ray analyses were carried out to study the effect of continuous shear on the crystallization kinetics of the fat. Rheological measurements were used to assess the effect of a shear step on crystallization, and finally polarized light microscopy was used to follow changes in microstructure due to the applied initial shear step. It was shown that shear enhanced the primary crystallization, even when low shear rates were applied for a short period. Furthermore, a shear step prior to crystallization without shear has a marked influence on the microstructural development. [source] Evaluation of Transient Liquid Phase Bonding Between Titanium and SteelADVANCED ENGINEERING MATERIALS, Issue 7 2009Ahmed Elrefaey An investigation of microstructural development in transient liquid phase (TLP) bonds between commercially pure titanium and low carbon steel is presented in this study. Changes in the joint region and surrounding substrates were examined as a function of holding time at temperatures of 870 and 910 °C. Both interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate joint quality. [source] Shear band evolution and accumulated microstructural development in Cosserat mediaINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2004A. Tordesillas Abstract This paper prepares the ground for the continuum analysis of shear band evolution using a Cosserat/micropolar constitutive equation derived from micromechanical considerations. The nature of the constitutive response offers two key advantages over other existing models. Firstly, its non-local character obviates the mathematical difficulties of traditional analyses, and facilitates an investigation of the shear band evolution (i.e. the regime beyond the onset of localization). Secondly, the constitutive model parameters are physical properties of particles and their interactions (e.g. particle stiffness coefficients, coefficients of inter-particle rolling friction and sliding friction), as opposed to poorly understood fitting parameters. In this regard, the model is based on the same material properties used as model inputs to a discrete element (DEM) analysis, therefore, the micromechanics approach provides the vehicle for incorporating results not only from physical experiments but also from DEM simulations. Although the capabilities of such constitutive models are still limited, much can be discerned from their general rate form. In this paper, an attempt is made to distinguish between those aspects of the continuum theory of localization that are independent of the constitutive model, and those that require significant advances in the understanding of micromechanics. Copyright © 2004 John Wiley & Sons, Ltd. [source] Regional variability in secondary remodeling within long bone cortices of catarrhine primates: the influence of bone growth historyJOURNAL OF ANATOMY, Issue 3 2008Shannon C. McFarlin Abstract Secondary intracortical remodeling of bone varies considerably among and within vertebrate skeletons. Although prior research has shed important light on its biomechanical significance, factors accounting for this variability remain poorly understood. We examined regional patterning of secondary osteonal bone in an ontogenetic series of wild-collected primates, at the midshaft femur and humerus of Chlorocebus (Cercopithecus) aethiops (n = 32) and Hylobates lar (n = 28), and the midshaft femur of Pan troglodytes (n = 12). Our major objectives were: 1) to determine whether secondary osteonal bone exhibits significant regional patterning across inner, mid-cortical and outer circumferential cortical rings within cross-sections; and if so, 2) to consider the manner in which this regional patterning may reflect the influence of relative tissue age and other circumstances of bone growth. Using same field-of-view images of 100-µm-thick cross-sections acquired in brightfield and circularly polarized light microscopy, we quantified the percent area of secondary osteonal bone (%HAV) for whole cross-sections and across the three circumferential rings within cross-sections. We expected bone areas with inner and middle rings to exhibit higher %HAV than the outer cortical ring within cross-sections, the latter comprising tissues of more recent depositional history. Observations of primary bone microstructural development provided an additional context in which to evaluate regional patterning of intracortical remodeling. Results demonstrated significant regional variability in %HAV within all skeletal sites. As predicted,%HAV was usually lowest in the outer cortical ring within cross-sections. However, regional patterning across inner vs. mid-cortical rings showed a more variable pattern across taxa, age classes, and skeletal sites examined. Observations of primary bone microstructure revealed that the distribution of endosteally deposited bone had an important influence on the patterning of secondary osteonal bone across rings. Further, when present, endosteal compacted coarse cancellous bone always exhibited some evidence of intracortical remodeling, even in those skeletal sites exhibiting comparatively low %HAV overall. These results suggest that future studies should consider the local developmental origin of bone regions undergoing secondary remodeling later in life, for an improved understanding of the manner in which developmental and mechanical factors may interact to produce the taxonomic and intraskeletal patterning of secondary bone remodelling in adults. [source] The integration of experimental in-situ EBSD observations and numerical simulations: a novel technique of microstructural process analysisJOURNAL OF MICROSCOPY, Issue 3 2004S. Piazolo Summary The combination of subgrain- and grain-scale microstructural data collected during in-situ heating experiments and numerical simulations of equivalent microstructural development offers an innovative and powerful tool in the advancement of the understanding of microstructural processes. We present a system that fully integrates subgrain- to grain-scale crystallographic data obtained during in-situ observations during heating experiments in a scanning electron microscope and the two-dimensional hybrid numerical modelling system Elle. Such a system offers the unique opportunity to test and verify theories for microstructural development, as predictions made by numerical simulations can be directly coupled to appropriate physical experiments and, conversely, theoretical explanations of experimental observations should be testable with numerical simulations. Discrepancies between data obtained with both techniques suggest the need for an in-depth investigation and thus open up new avenues of theory development, modification and verification. In addition, because in numerical models it is possible to select the processes modelled, the effect of individual processes on the microstructural development of a specific material can be quantified. To illustrate the potential and methodology of the so-called EBSD2Elle system, two in-situ experiments and their equivalent numerical experiments are presented. These are static heating experiments of (a) an annealed Ni-foil coupled with a front tracking model for grain growth and (b) a cold deformed rock salt with kinetic Monte Carlo simulations for subgrain growth. [source] Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline AluminaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004Guo-Dong Zhan The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of ,-Al2O3 nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiCw/,-Al2O3 nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (,118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of ,-Al2O3 to ,-Al2O3 of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiCw/HEBM,-Al2O3 nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m1/2. [source] Effects of High Water-Vapor Pressure on Oxidation of Silicon Carbide at 1200°CJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2003Peter F. Tortorelli The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air + 15 vol% H2O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water-vapor pressure on oxidation rates and microstructural development. At a water-vapor pressure of 1.5 atm (150 kPa), distinct SiO2 scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO2 layer, which remained constant in thickness with time as the crystalline SiO2 continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water-vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO2 volatility is rate controlling at high gas-flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO2 to porous, nonprotective SiO2. [source] Grain Growth of ,-SiAlON in the Calcium-Doped SystemJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2002Ya-Wen Li Two calcium-doped ,-SiAlON compositions (Ca0.6Si10.2Al1.8,O0.6N15.4 and Ca1.8Si6.6Al5.4O1.8N14.2) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from ,-Si3N4 to ,-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of ,-SiAlON. Elongated ,-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the ,-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the ,-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development. [source] Microstructure in Silicon Nitride Containing ,-Phase Seeding: III, Grain Growth and CoalescenceJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2001Horng-Hwa Lu The mechanical properties of Si3N4 materials depend mainly on the microstructure, which originates during the densification process. The microscopic evidence indicates that ,-Si3N4 seeds incorporated in the starting powders play an important role in microstructural development, especially in the heterogeneous grain growth of ,-Si3N4 grains during sintering. The growth of ,-grains is initiated from the ,-seeds, resulting in a core/shell microstructure. The presence of Moiré fringes and dislocations is attributed to misfit strain and compositional differences between the core and the shell. Coalescence can occur at the final stage of sintering. [source] Effect of chorioamnionitis on brain development and injury in premature newborns,ANNALS OF NEUROLOGY, Issue 2 2009Vann Chau MD Objective The association of chorioamnionitis and noncystic white matter injury, a common brain injury in premature newborns, remains controversial. Our objectives were to determine the association of chorioamnionitis and postnatal risk factors with white matter injury, and the effects of chorioamnionitis on early brain development, using advanced magnetic resonance imaging. Methods Ninety-two preterm newborns (24,32 weeks gestation) were studied at a median age of 31.9 weeks and again at 40.3 weeks gestation. Histopathological chorioamnionitis and white matter injury were scored using validated systems. Measures of brain metabolism (N-acetylaspartate/choline and lactate/choline) on magnetic resonance spectroscopy, and microstructure (average diffusivity and fractional anisotropy) on diffusion tensor imaging were calculated from predefined brain regions. Results Thirty-one (34%) newborns were exposed to histopathological chorioamnionitis, and 26 (28%) had white matter injury. Histopathological chorioamnionitis was not associated with an increased risk of white matter injury (relative risk: 1.2; p = 0.6). Newborns with postnatal infections and hypotension requiring therapy were at higher risk of white matter injury (p < 0.03). Adjusting for gestational age at scan and regions of interest, histopathological chorioamnionitis did not significantly affect brain metabolic and microstructural development (p > 0.1). In contrast, white matter injury was associated with lower N-acetylaspartate/choline (,8.9%; p = 0.009) and lower white matter fractional anisotropy (,11.9%; p = 0.01). Interpretation Histopathological chorioamnionitis does not appear to be associated with an increased risk of white matter injury on magnetic resonance imaging or with abnormalities of brain development. In contrast, postnatal infections and hypotension are associated with an increased risk of white matter injury in the premature newborn. Ann Neurol 2009;66:155,164 [source] | ||||||||||||||||