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Grain Morphology (grain + morphology)
Selected AbstractsFaceted Surface Grain Morphology of Rapidly Solidified Alumina: Characterization and Potential Applications,ADVANCED ENGINEERING MATERIALS, Issue 12 2009Sandip P. Harimkar Abstract The study reports on the formation of regular multi-faceted surface grains during laser surface modification of alumina ceramics. The formation of such faceted morphology of the grains can be strongly correlated with the formation of crystallographic texture. Such evolution of crystallographically-textured, multi-faceted grains opens up new avenues for laser surface modification, where regular three dimensional surface features are formed through solidification processing. [source] Distribution of Cerium Ions in Cerium-Doped ,,-SiAlON and Its Effect on Grain MorphologyJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2002Fang-Fang Xu Cerium-doped ,,-SiAlON material was prepared by spark plasma sintering at 1750°C under 30 MPa pressure for 10 min. Yttrium ,,-SiAlON seeds (1 wt%) were added to the starting powder mixture. Recent work showed that up to 45 wt% of ,,-SiAlON phases are formed in the present sintered ceramics. The material presented a microstructure, containing rodlike cerium-doped ,,-SiAlON crystals. In this paper, transmission electron microscopy and energy dispersive spectroscopy examinations of the ,,-SiAlON grains are reported. The structural analyses revealed a high density of domain boundaries, within which larger amounts of cerium ions were segregated than in the matrix. The density of the domain boundaries was proportional to the amount of incorporated cerium ions. These structural defects eventually dominated the growth habits of the ,,-SiAlON crystals, by modifying the structure of the interstices at the boundary sites. The role of yttrium ,,-SiAlON seeds also is discussed in this paper. [source] Effect of A-Site Ionic Radius on the Structure and Microwave Dielectric Characteristics of Sr1+xSm1,xAl1,xTixO4 CeramicsINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2010Min Min Mao SrSmAlO4 microwave dielectric ceramics were modified by Sr/Ti cosubstitution for Sm/Al. The effects of radius difference of A-site ions on the microwave dielectric characteristics were investigated together with the structure. Sr1+xSm1,xAl1,xTixO4 (x=0, 0.05, 0.10, and 0.15) ceramics were prepared by a solid-state reaction approach. X-ray diffraction studies revealed a single-phase K2NiF4 -type solid solution with corresponding peaks shifting to lower 2, as x increased. Minor inhomogeneous grain morphology for x=0.05 and a trace amount of second phases for x=0.10, 0.15 were detected by backscattered-electron microscopy and energy-dispersive X-ray analysis. With increasing Sr/Ti cosubstitution, the dielectric constant ,r increased from 18.4 to 20.4, and the temperature coefficient of resonant frequency ,f was adjusted from ,1.8 to 7.4 ppm/°C almost linearly. However, the Q×f value decreased from 74,500 GHz at x=0,53,000 GHz at x=0.15. The internal stresses caused by the decreased tolerance factor and the large ionic radii difference between Sr2+ and Sm3+ should be the predominant reasons for such a decrease in the Q×f value. The high-resolution transmission electron microscopic results revealed an increase in the lattice distortion with increasing Sr/Ti cosubstitution, and subsequently supported the above conclusion upon the increased internal stresses. [source] Fabrication of Silicon Nitride Nanoceramics and their Tribological PropertiesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2010Jae-Hee Kim Si2N2O secondary phase-free, fully dense nano-Si3N4 ceramics (,97% of theoretical density) were developed by combining carbothermal reduction treatment and spark plasma sintering (SPS), and their tribological properties were investigated by subjecting to self-mated sliding under unlubricated conditions. Commercially available Si3N4 nanopowder was used as a starting material and phenolic resin was used for carbothermal reduction, which was conducted at 1450°C for 10 h. Fully densified Si2N2O phase-free Si3N4 ceramics with a wide range of grain size from 90 nm to 1.5 ,m were fabricated by varying SPS temperature from 1550° to 1750°C. The microstructure of the developed Si3N4 ceramics was changed from nano equi-axed at 1550°C to large elongated bimodal grain morphology at 1700° or 1750°C. The frictional behavior was not dependent on the microstructure, but the wear rate was strongly influenced such that it decreased by an order of magnitude (from 9.7 × 10,5 to 0.88 × 10,5 mm3/N·m) with decreasing grain size. The dominant wear mechanism was changed from the delamination of tribochemical layer for the ceramics with nano equi-axed grain microstructure to the fracture and grain pull-out for the ceramics with duplex microstructure. [source] Microstructural Evolution of Calcium-Doped ,-AluminaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2003Arzu Altay Effect of different calcium doping levels on the microstructure of high-purity ,-alumina was studied as a function of sintering time and temperature using scanning electron microscopy (SEM). Microstructural evolution was related to hypothetical calcium excess at the grain boundaries (,Ca) that was calculated assuming zero solubility of calcium in bulk ,-alumina. Under all sintering conditions, grains were uniform in size and equiaxed for low calcium concentrations (<3 Ca atoms/nm2). The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of ,Ca= 3,3.5 Ca atoms/nm2 in all samples. The average grain sizes of undoped samples were ,10% larger than the average grain sizes of low-calcium-doped samples. This decrease is believed to be due to solute drag effect of segregated Ca impurities on the grain boundary mobility. For the samples that were sintered at 1500° and 1600°C, slablike abnormally grown grains appeared for critical calcium excess concentrations of ,Ca= 4.5,8 Ca atoms/nm2. With abnormally grown grains a dramatic increase in average grain size was observed. However, when the calcium concentration was increased further, above certain calcium excess concentration depending on sintering temperature, a significant decrease in grain size was observed. In contrast to samples sintered at 1500° and 1600°C, when the samples sintered at 1400°C, although the hypothetical calcium coverage exceeded ,Ca= 11 Ca atoms/nm2, only few grains grew abnormally without significantly affecting the average grain size. Observations clearly indicated that calcium impurities caused elongated (slablike) grain morphology when their excess concentrations reached a critical level at the grain boundaries. [source] High-Temperature Rheology of Calcium Aluminosilicate (Anorthite) Glass-Ceramics under Uniaxial and Triaxial LoadingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2001Balakrishnan G. Nair The high-temperature creep behavior of two fine-grained (,3 ,m) anorthite-rich glass-ceramics was characterized at ambient pressure and under a confining pressure of ,300 MPa. Experiments were done at differential stresses of 15,200 MPa and temperatures of 1200°,1320°C. Of the two materials, one had a tabular (lathlike) grain structure with finely dispersed second phase of mullite, mostly in the form of ,3,5 ,m grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (,400 nm) mullite precipitates concentrated at the anorthite grain boundaries. The results of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitude faster than the equiaxed material. Creep in the tabular-grained material at ambient pressure was accompanied by a significant extent of intergranular cavitation: pore-volume analysis before and after creep in this material suggested that >75% of the bulk strain was due to growth of these voids. The equiaxed material, in contrast, showed a smooth transition from Newtonian (n= 1) creep at low stresses to non-Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both materials underwent significant changes. Grain-boundary mullite precipitates in the undeformed, equiaxed-grain material were replaced by fine (,100 nm), intragranular precipitates of silliminate and corundum because of a pressure-induced chemical reaction. This was accompanied by a significant reduction in grain size in both materials. The substantial microstructural changes at high confining pressure resulted in substantially lower viscosities for both materials. The absence of mullite precipitates at the grain boundaries changed the behavior of the equiaxed material to non-Newtonian (n= 2) at a pressure of ,300 MPa, possibly because of a grain-boundary sliding mechanism; the tabular-grained material showed Newtonian diffusional creep under similar conditions. [source] Looking for a surface treatment to deposit a reactive element with potential industrial use in high temperature oxidation fieldMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 12 2005G. Bonnet Abstract In order to introduce a so-called reactive element simultaneously at all the surfaces of a NiCr (Ni-20wt.%Cr-1.5wt.%Si) metallic substrate alloy, an easy to apply surface treatment was used: an yttrium containing thin film (constituted of a hydrated yttrium hydroxide nitrate) was electrodeposited from a mixed water-ethanol solution. After a thermal conversion treatment at 600 °C under argon, leading to an Y2O3 coating, specimens were submitted to high-temperature oxidation testing, under artificial air at atmospheric pressure, in order to evaluate in these conditions the effect of the coatings on the behaviour of the studied alloy. Expected effects of the introduced reactive element (REE) were effectively shown: decrease of the coated sample weight gain, suppression of the formation of the base metal oxides and change of the oxide grains morphology. [source] | ||||||||||