MICROSTRUCTURE STUDIES OF POLYCRYSTALLINE REFRACTORY OXIDES.
Summary rept., 25 Mar 65-24 Mar 66,
AVCO CORP WILMINGTON MASS SPACE SYSTEMS DIV
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The effect of surface structure on the transverse bend strength of polycrystalline Al2O3 and MgO was studied. A consistent relationship between strength and improved surface structure was attained for MgO tested at 77K while such a relationship did not exist for testing at 296K. In Al2O3, the strength-surface structure relationships were not consistent although several thermal cycles strengthened this material at both 77K and 296K by approximately 40. Hot forging achieved crystallographic andor mechanical texturing in polycrystalline MgO and Al2O3. Specimens were obtained which in the case of Al2O3 showed strengths at 23C and 1300C between 25 and 200 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. SiO2 additions caused a marked weakening of Al2O3, 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 50C. The strongest MgO specimen reported to date, 55,000 psi, was achieved by what appears to be a compressive surface layer of Mg2SiO4. Author
- Ceramics, Refractories and Glass