The effect of surface structure on the transverse tend strength of polycrystalline Al2O3 and MgO was studied. A consistent relation¬ship between strength and improved surface structure was attained for MgO tested at 77°K while such a relationship did not exist for testing at 296°K. A maximum mean strength Increase of 6156 was attained for air annealed MgO. In Al2O3, the strength-surface structure relation¬ships were not consistent although several thermal cycles strengthened this material at both 77°K and 296°K by approximately 40%. A theory -elating the incidence of tranagranular fracture to crack velocity rather than solely to intergranular bonding is presented.nHot forging achieved crystallographic end/or mechanical texturing in polycrystalline MgO and Al2O3. Specimens were obtained which in the case of AI2O3 showed strengths at 3°C and 1300°C between 25% and 20056 higher than would be predicted from existing grain size - porosity -strength relations. The bend strengths for forged MgO were equivalent to hot pressed material having a similar microstructure.nThe mechanical properties of AI2O3 doped with MgO- and SiO2 were investigated. SiO2 additions caused a marked weakening of AI2O3, while MgO additions gave specimens which were similar to or perhaps slightly stronger than unalloyed specimens. The magnesia addition increased the high temperature ductility and lowered the brittle-ductile transition temperature by approximately 50°C. The strongest MgO specimen reported to date, 55,000 psi, was achieved by what appears to be a compressive surface layer of Mg2SiO4.
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