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Grain-boundary Sliding (grain-boundary + sliding)
Selected AbstractsExperimental Evidence for Grain-Boundary Sliding in Ultrafine-Grained Aluminum Processed by Severe Plastic Deformation,ADVANCED MATERIALS, Issue 1 2006Q. Chinh Evidence for grain boundary sliding in ultrafine-grained aluminum after processing with equal channel angular pressing (ECAP) is presented (see Figure). Pure aluminum is used as a model material; depth sensing indentation testing and atomic force microscopy are used to measure the nature of the displacements around indentations for samples in an annealed and work-hardened condition, and after processing using ECAP. [source] Superplastic Behavior of Fine-Grained ,-Silicon Nitride Material under CompressionJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2000Guo-Dong Zhan The deformation behavior of a hot-pressed, fine-grained ,-Si3N4 ceramic was investigated in the temperature range 1450°,1650°C, under compression, and the results for strain rate and temperature dependence of the flow stress are presented here. The present results show that the material is capable of high rates of deformation (,10,4,10,3 s,1) within a wide range of deformation temperatures and under a pressure of 5,100 MPa; no strain hardening occurs in the material, even at slow deformation rates, because of its stable microstructure; Newtonian flow occurs, with a stress exponent of approximately unity; and the material has activation energy values for flow in the range 344,410 kJ·mol,1. Grain-boundary sliding and grain rotation, accommodated by viscous flow, might be the mechanisms of superplasticity for the present material. [source] Deformation-enhanced metamorphic reactions and the rheology of high-pressure shear zones, Western Gneiss Region, NorwayJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2006M. P. TERRY Abstract Microstructural and petrological analysis of samples with increasing strain in high-pressure (HP) shear zones from the Haram garnet corona gabbro give insights into the deformation mechanisms of minerals, rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions. Scanning electron microscopy with electron backscattering diffraction (SEM,EBSD), compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization. Plagioclase with a shape-preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the (0 0 1)[1 0 0] slip system. In omphacite, crystallographic preferred orientations indicate slip on (1 0 0)[0 0 1] and (1 1 0)[0 0 1] during deformation. Fine-grained garnet deformed by diffusion creep and grain-boundary sliding. Ilmenite deformed by dislocation glide on the basal and, at higher strains, prism planes in the a direction. Relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500,650 °C and 0.5,1.4 GPa and continued during prograde metamorphism to ultra-high-pressure (UHP) conditions. Both products and reactants show evidence of syn- and post-kinematic growth indicating that prograde reactions continued after strain was partitioned away. The restriction of post-kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn-kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away, demonstrating that deformation played an important role in enhancing metamorphic reactions. Two important consequences of deformation observed in these shear zones are: (i) the homogenization of chemical composition gradients occurred by mixing and grain-boundary migration and (ii) composition changes in zoned metamorphic garnet by lengthening diffusion distances. The application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100,150 MPa and lower limits for strain rates of 10,12 to 10,13 s,1 as deformation conditions for the shear zones in the Haram gabbro that were produced during subduction of the Baltica craton and resulted in the production of HP and UHP metamorphic rocks. [source] Correlation Between Microstructure and Creep Behavior in Liquid-Phase-Sintered ,-Silicon CarbideJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2006M. Castillo-Rodríguez The influence of increasing the sintering time from 1 to 7 h on the microstructure evolution and the mechanical properties at high temperature was studied in ,-silicon carbide (,-SiC) sintered in argon atmosphere with Y2O3,Al2O3 (10% weight) as liquid phase (LPS-,-SiC). The density decreased from 98.8% to 94.9% of the theoretical value, the grain size increased from 0.64 to 1.61 ,m, and some of the grains became elongated. The compression tests were performed in argon atmosphere, between 1450°C and 1625°C and stresses between 25 and 450 MPa, with the strain rate being between 4.2 × 10,8 and 1.5 × 10,6 s,1. The stress exponent n and the activation energy Q were determined, finding values of n between 2.4±0.1 and 4.5±0.2 and Q=680±35 kJ/mol for samples sintered for 1 h, and n between 1.2±0.1 and 2.4±0.1 and Q=710±90 kJ/mol for samples sintered for 7 h. The correlation between these results and the microstructure indicates that grain-boundary sliding and the glide and climb of dislocations, both accommodated by bulk diffusion, may be two independent deformation mechanisms operating. At the temperatures of the tests, the existence of solid-state reactions between SiC and the sintering additives is responsible of the microstructural changes observed. These effects are not a consequence of the process of deformation, but rather they are because of the thermal treatment of the material during the creep. [source] Grain-Boundary Viscosity of Preoxidized and Nitrogen-Annealed Silicon CarbidesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2001Giuseppe Pezzotti Internal friction experiments were conducted on a model SiC polycrystal prepared from preoxidized (high-purity) SiC powder. This material contained high-purity SiO2 glass at grain boundaries in addition to a free-carbon phase, which was completely removed upon powder preoxidation. Comparative tests were conducted on a SiC polycrystal, obtained from the as-received SiC powder with the addition of 2.5 vol% of high-purity SiO2. This latter SiC material was also investigated after annealing at 1900°C for 3 h in a nitrogen atmosphere. Electron microscopy observations revealed a glass-wetted interface structure in SiC polycrystals prepared from both as-received and preoxidized powders. However, the former material also showed a large fraction of interfaces coated by turbostratic graphite. Upon high-temperature annealing in nitrogen, partial glass dewetting occurred, and voids were systematically observed at multigrain junctions. The actual presence of nitrogen could only be detected in a limited number of wetted interfaces. A common feature in the internal friction behavior of the preoxidized, SiO2 -added and nitrogen-annealed SiC was a relaxation peak that resulted from grain-boundary sliding. Frequency-shift analysis revealed markedly different characteristics for this peak: both the magnitude of the intergranular glass viscosity and the activation energy for grain-boundary viscous flow were much higher in the nitrogen-annealed material. Results of torsional creep tests were consistent with these findings, with nitrogen-annealed SiC being the most creep resistant among the tested materials. [source] Effect of Microstructure on High-Temperature Compressive Creep of Self-Reinforced Hot-Pressed Silicon NitrideJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2000Martha A. Boling-Risser An experimental self-reinforced hot-pressed silicon nitride was used to examine the effects of microstructure on high-temperature deformation mechanisms during compression testing. At 1575,1625°C, the as-received material exhibited a stress exponent of 1 and appeared to deform by steady-state grain-boundary sliding accommodated by solution-reprecipitation of silicon nitride through the grain-boundary phase. The activation energy was 610 ± 110 kJ/mol. At 1450,1525°C for the as-received material, and at 1525,1600°C for the larger-grained heat-treated samples, the stress exponent was >1. Damage, primarily in the form of pockets of intergranular material at two-grain junctions, was observed in these samples. [source] Texture Development in Silicon Nitride,Silicon Oxynitride In Situ Composites via Superplastic DeformationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2000Rong-Jun Xie Silicon nitride,silicon oxynitride (Si3N4,Si2N2O) in situ composites have been fabricated via either the annealing or the superplastic deformation of sintered Si3N4 that has been doped with a silica-containing additive. In this study, quantitative texture measurements, including pole figures and X-ray diffraction patterns, are used in conjunction with scanning electron microscopy and transmission electron microscopy techniques to examine the degree of preferred orientation and texture-development mechanisms in these materials. The results indicate that (i) only superplastic deformation can produce strong textures in the ,-Si3N4 matrix, as well as Si2N2O grains that are formed in situ; (ii) texture development in the ,-Si3N4 matrix mainly results from grain rotation via grain-boundary sliding; and (iii) for Si2N2O, a very strong strain-dependent texture occurs in two stages, namely, preferred nucleation and anisotropic grain growth. [source] | ||||||||||