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5 Materials and Microstructures
Pages 49-53

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From page 49...
... of fine-grained polycrystalline ceramics typically increases as the grain size decreases because of the dominate diffusive deformation mechanisms in these materials. In single-phase ceramics, the creep rate is controlled by the following relationship: ~Db/d ~ 1 ~ Db is the diffusivity, and n is an exponent between 1 and 3, depending on the dominant creep mechanism (e.g., n = 3 for grain boundary diffusion)
From page 50...
... One implication that has been adequately validated by expenment is that complex oxides with low Db, particularly mullite, have creep strengths supenor to single-phase oxides, such as alumina, yttna, and magnesia. However, the findings that solid solutions and nanoparticles profoundly affect creep rates in bulk polycrystalline oxides was not obvious from Temperature (C)
From page 51...
... Oxidation measurements indicate that a silica outer layer forms on A12O3 reinforced with SiC nanoparticles at a rate comparable to the rate it would form on SiC. However, a much thicker SiO2 layer forms below the surface of SiC nanoparticle-reinforced A12O3, wherein the SiC particles have a modified chemistry and morphology.
From page 52...
... This theory argues that grain boundary diffusion would be retarded along these boundaries because of their greater structural order, which would lead to slower creep kinetics. The committee recommends that research be continued on the effect of isovalent doping on the creep properties of oxide ceramics.
From page 53...
... clarification of the effect of solid-solution and secondphase additives (especially C and TiB2) on the creep behavior and grain growth of polycrystalline SiC fibers study of the effects of texture on the creep strength of fibers and the development of processing methods to induce the optimum orientation systematic studies of the effects of high-temperature heat treatment (aging)


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