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Session Chairperson: S.X. Mao, Department of Mechanical Engineering, University of Calgary, Calgary, Alberta, Canada, T2N 1N4
2:00 pm INVITED
CREEP DEFORMATION AND RUPTURE OF Al2O3 UNDER STATIC AND CYCLIC LOADING: Darrell Socie, Department of Mechanical Engineering, University of Illinois, Urbana, IL 61801
Results of the tension and torsion tests on a commercial grade of vitreous bonded, 99.8% Al2O3, show that creep deformation is enhanced significantly under shear stress. Microstructural observations show that extensive grain boundary sliding occurs in the torsion specimens and gives rise to a large initial creep deformation. Evidence of cavity formation is found throughout the specimen. Little evidence of grain boundary sliding or cavitation was found in the tensile specimen even though the tensile stress was four times larger. The combined action of one tensile stress 1 and one compressive stress, -3, doubles the grain boundary shear stresses to 2. This results in more grain boundary sliding and early cavity formation under shear loading. In addition the compressive stress will cause a wedging action on individual grains that will result in a grain facet stress of n. This stress will be added to the tensile stress, 1 , to enhance cavity coalescence and microcrack growth.
AN INVESTIGATION OF FIBER/MATRIX INTERFACE OF A NICALON FIBER REINFORCED SILICON CARBIDE COMPOSITE WITH A SILICON CARBIDE INTERFACIAL COATING: Wei Zhao , Peter K. Liaw, David C. Joy, Dept. of Materials Sciences and Engineering, The University of Tennessee, Knoxville, TN 37996-2200; Elizabeth R. Kupp, David P. Stinton, The Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
The mechanical properties of ceramic matrix composites (CMCs) can be controlled by tailoring the fiber/matrix interface, especially the type of interfacial materials and their thickness. A new Nicalon/SiC composite with SiC as an interfacial coating processed by a forced-flow chemical vapor infiltration (FCVI) method at the Oak Ridge National Laboratory demonstrates improved high-temperature oxidation resistance than that with a carbon interfacial coating. However, it is difficult to differentiate the SiC interface from the matrix using scanning electron micrographs, because of nearly identical microstructures and similar chemical compositions of the fiber, the matrix, and the interfacial coating. In this paper, the line-scan chemical analysis technique in scanning electron microscopy (SEM) is used to investigate the SiC interfacial coating. As-received and oxidized specimens are studied. The existence of elements, such as Si, C, O, and Cl, and their distributions along the fiber/matrix interface, are detected. The profile of interface chemical composition can be used to identify interface structure as well as thickness. The effect of interface composition on mechanical properties is also studied.
CRACK NUCLEATION ON ELASTIC POLYCRYSTAL SURFACE IN CORROSIVE ENVIRONMENT: Z. Suo, H. Yu, Mechanical and Environmental Engineering Department, Materials Department, University of California, Santa Barbara, CA 93106
This paper analyzes a process of crack nucleation on the surface of a ceramic subject to a stress parallel to the surface, in an environment where the ceramic evaporates slowly and deforms elastically. Both grain boundary tension and elastic stress concentration cause the surface to groove along its intersections with the grain boundaries. Two behaviors are identified. If the applied stress is small, the grooves approach an invariant shape, and the ceramic erodes by gross mass loss. If the applied stress is large, the grooves sharpen into crack fronts, and the ceramic breaks by decohesion. The models relate crack nucleation threshold and time to the applied stress, surface and grain boundary tensions, chemical free energy, grain size, and kinetic parameters. Surface self-diffusion is also included in the analysis.
3:30 pm BREAK
ROLE OF GRAIN BOUNDARY PHASE DURING HIGH-TEMPERATURE FATIGUE-CRACK GROWTH IN CERAMICS: J. K. Shang, D. Yao, C. Huang, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801
Role of grain boundary phase on high-temperature fatigue-crack growth were examined in a TiB2/SiC composite and polycrystalline aluminas of different purities at 800-1100°C. The low-purity alumina and the composite contained a continuous film of glassy phase on the grain boundary while the high-purity aluminas contained either little or no continuous grain boundary phase. In the low-purity alumina and the composite, fatigue crack growth rate increased drastically as the cyclic frequency was decreased. The cyclic crack growth rate was slower that the creep crack growth rate at the same maximum stress intensity. In contrast, fatigue crack growth in the high-purity alumina was relatively insensitive to cyclic frequency and the fatigue crack growth rate was faster than the creep crack growth rate for the same maximum stress intensity. The difference in the fatigue crack growth behavior is explained in terms of the different fatigue crack growth mechanisms in these ceramics.
THE MECHANICAL BEHAVIOR OF CONTINUOUS FIBER REINFORCED CERAMIC COMPOSITES (CFCCs): N. Miriyala, P.K. Liaw, C.J. McHargue, The University of Tennessee, Knoxville, TN 37996; L.L. Snead, Oak Ridge National Laboratory, Oak Ridge, TN 37831
The mechanical behavior of two Nicalon fabric reinforced ceramic matrix composites, with alumina and silicon carbide as the matrix materials, respectively, were investigated. Four point-bend tests were conducted at ambient and elevated temperatures to study the monotonic and cyclic fatigue behavior. The stress-strain curves were non-linear for both composites, at ambient as well as elevated temperatures. During cyclic loading, the modulus of the composite was monitored to serve as a measure of the loss in load bearing capacity of the composites due to cyclic fatigue loading. Progressive damage in the composites was monitored by optical and electron microscopy techniques. The differences in the composite monotonic and cyclic fatigue behavior, depending on the orientation of the fabric plies to the loading direction, and the micromechanisms responsible for the differences, will be the focus of the paper.
MECHANICAL PROPERTIES OF SILICON NITRIDE CERAMICS WITH ANISOTROPIC MICROSTRUCTURE: Hisayaki Imamura, Pine Ceramics Research Association, Synergy Ceramics Laboratory, Nagoya, Japan; Kiyoshi Hirao, Manual E. Brito, Motochiro Toriyama and Shuzo Karzaki, National Industrial Research Institute of Nagoya, Nagoya, Japan
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