| Summary: | 博士 === 國立中山大學 === 材料科學研究所 === 94 === The success in fabrication of various nano-sized powders, wires or tubes has arisen the new possibility in modifying the existing commercial materials in terms of their functional or structural characteristics. In this study, the AZ61 Mg alloy is adopted as the matrix, and nano-sized SiO2 particulates are introduced into the alloy by means of casting, powder metallurgy, or spray forming processes to fabricate a high performance Mg matrix composite.
The strengthening mechanisms, fracture toughness and bending toughness of the AZ61 Mg based composites are examined. The composites were prepared either by spray forming, ingot metallurgy, or powder metallurgy, followed by severe hot extrusion. The spray formed composites exhibit the best nano particle distribution and toughness, but the volume fraction of the nano particles that can be inserted is limited. The nano composites fabricated through the powder metallurgy method possess the highest strength due to the extra strengthening effect from the MgO phase. Strengthening analysis based on the Orowan strengthening mechanism can predict well the composite strength provided that the nano particles are in reasonably uniform dispersion. For composites containing higher nano particle volume fractions greater than 3%, the experimental strength data fall well below the theoretical predictions, suggesting poor dispersion of the reinforcement.
The creep properties of the composites are also explored. The specimens are subjected to tensile loading at temperatures 200 to 400oC and strain rates 1x10-3 to 1x10-1. The creep mechanism is identified as dislocation creep controlled with the rate controlling diffusion step being the magnesium lattice diffusion at low strain rates and grain boundary diffusion at high strain rates.
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