Microstructure Studies of Refractory Polycrystalline Oxides

Several facets of the effect of microstructure and chemistry on the mechanical properties of magnesia and alumina were studied. Comparison of the deformation properties of high purity Al2O3 with a standard research grade of Al2O3 14 MgO demonstrated high purity material possessed a normalized creep rate approximately a factor of five 5 lower and a higher strain rate sensitivity suggesting an increased importance of diffusional creep. At strains from 3-12 evidence was obtained for strain hardening, a Bauschinger effect and polygonization which suggested considerable dislocation and grain boundary sliding activity. Also, high temperature stress-strain curves showed evidence for blunting a potentially catastrophic failure event. Al2O3 press forging experiments resulted in a better understanding of deformation and primary recrystallization processes. Bend tests of a uniform 8 micron grain size textured body demonstrated higher elevated temperature strength than 1-2 micron hot pressed Al2O3 suggesting a real benefit from the textured structure resulting from one of a number of potential models. Dead load stress corrosion studies on polycrystalline MgO demonstrated that the shape of the stress corrosion curve was dependent on the starting chemistry andor minor grain boundary phases. Also, the resistance to dead load was markedly influenced by the introduction of 1 molar dimethyl formamide which had been shown to affect dislocation mobility in MgO.