Ductile Creep Cracking in Uranium Dioxide

Abstract

Compression creep tests were performed on sintered uranium dioxide of mean grain sizes between 2μm and 10μm at temperatures within the range 1273K to 1873K. The creep strains which were typically obtained for specimens with coarse grains before fracture occurred were less than 5.5% of original length. The failure mode consists of creep cavitation at grain boundaries and triple-point junctions of grains, followed by rapid propagation of cracks through shear (mode II loading) on maximum shear stress planes. Clearly, fracture occurred when the stress intensities at the tips of microscopic shear cracks increased with creep (as a result of crack growth through grain boundary sliding and/or void growth and coalescence) until they exceeded the mode II fracture toughness of the material. The brittle nature of the ceramic offers no effective blunting of the crack tips to reduce the stress concentrations at these locations.

By contrast, the 2μm grain size material exhibited various amounts of ductility when similarly tested in compression. Samples have been tested without failure by fracture after reductions in height of 68.6%. These samples are characterised by a fine, uniform grain structure and by the presence of very fine pores at triple-point junctions in the as-sintered state. It is postulated that these fine pores contribute to the ductility of the material by acting as crack stoppers that stop the development of microscopic cracks into macroscopic cracks. This crack-stopping action allows the grains to reorientate and re-sinter through a modified Ashby-Verral1 grain flow mechanism. The resultant microstructure shows large numbers of well-rounded pores at grain boundaries and triple-point junctions, but few extended cracks.