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A hot-pressing facility was commissioned and fabrication procedures established for the manufacture of high density, fine grained pure and alloyed alumina. Mechanical testing of the products, by four point bend tests, has commenced. A suitable chemical analysis procedure for determining Cr in alumina has been developed that is accurate to within plus or minus 0.5 micrograms Cr. Additionally, a theoretical consideration of the notch strength of alumina has been developed. This is based on a simple model for fracture and suggests that the mechanistic details of fracture nucleation close to the notch tip are of significant importance in determining the relationship between notch-strength and stress concentration factor at a blunt notch. (Author)

This work aims at increasing understanding of the mechanisms of brittle fracture in alumina and at application of this to its strengthening. In particular, the role of dislocation motion is investigated. Direct evidence of dislocation motion accompanying fracture has been obtained for alumina and magnesia and deformation twinning has been shown to occur during the fracture of alumina even at -196 C. Effective surface energy measurements up to 500 C have been obtained for alumina and a detailed study made of the fracture surfaces. The bend strength of pure and chromium doped single crystals of alumina has been determined from -268 to 800 C. The strength of alumina increases markedly on decreasing the temperature from -196 C to -268 C. Also, small additions of chromium give appreciable strengthening. Both these findings support the thesis that plastic deformation is involved in the 'brittle fracture' of alumina. Fracture velocities have been measured from Wallner lines on the surfaces of alumina and glass. Crack branching occurs not at a critical velocity, but when a critical stress intensity condition is fulfilled. A theory accounting for the detailed mechanism of crack branching is given. (Author)

Polycrystalline discs of pure alumina and of alumina containing 0.5 or 1.0 wt. % Cr or 1.0 wt. % Ti were prepared by vacuum hot-pressing. Band strengths were determined in vacuo at 70, 300 and 600C after outgassing in vacuo at 600C. The Cr alloys did not show any very definite evidence of an increase in strength compared to pure alumina. The Ti alloys showed weakening, but this could be due to the presence of a few large grains. (Author)