Microstructure Studies of Polycrystalline Oxides

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Abstract
Dead load-time to failure studies on polycrystalline MgO exhibited a decreasing load bearing capacity with time which was interpreted by the Charles and Hillig stress corrosion model. The possibility of a mechanical model similar to that invoked for metals was considered and not ruled out. Chemical polishing of polycrystalline Al2O3 was accomplished, but mechanical tests failed to show a statistically valid strength improvement. Only a thermally etched surface from an earlier study exhibited a pronounced surface sensitivity to fracture strength, and dry testing conditions were required to unequivocally demonstrate this effect. Two new grades of high purity Al2O3 and MgO powders were fabricated, and mechanical testing of the Al2O3 showed equivalent brittle strengths and increased creep resistance when compared with less pure material with an equivalent microstructure. The explanation for the creep resistance may be either a slight increase in grain size which accompanied the test or decreased diffusivity due to the increased purity. Preliminary grain growth studies on high purity MgO indicate a normal grain growth behavior, but about an order of magnitude slower rate than 99.4% MgO. Press forging of polycrystalline Al2O3 was directed toward solving the engineering and process control problems of forging material with a high in-line optical transmission.

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