Influence of strain conditions on the grain sub-structuration in crept uranium dioxide pellets

Abstract Stoichiometric uranium dioxide (UO2) pellets were deformed by uniaxial compression creep tests at 1500 °C. Strains comprised between 3% and 11% were applied at strain rates ranging from ∼ 5 × 10−6 s−1 to 70 × 10−6 s−1 to deform pellets in the dislocational creep regime. An optimized protocol for Electron BackScattered Diffraction (EBSD) data acquisition and processing was applied to detect and quantify low and very low angle sub-boundaries disoriented down to 0.25° inside the prior UO2 grains. Three EBSD based parameters were calculated and are efficient to quantify the evolution of the deformed microstructure with the deformation conditions: the linear fraction of the sub-boundaries, the corresponding Geometrically Necessary Dislocations (GNDs) density and the resulting mean sub-grain size. The linear fraction of sub-boundaries (and their transcription in GNDs) increased significantly with the strain level and rate. New very low angle boundaries disoriented by less than 1° were continuously created whereas already existing sub-boundaries increased continuously their disorientation. This microstructural evolution can be described as a dynamic recovery mechanism by progressive increase of sub-boundaries disorientation angle.