a b s t r a c tCracking is observed when a UO 2 single crystal is oxidised in air. Previous studies led to the hypothesis that cracking occurs once a critical depth of U 3 O 7 oxidised layer is reached. We present some l-Laue Xray diffraction results, which evidence that the U 3 O 7 layer, grown by topotaxy on UO 2 , is made of domains with different crystalline orientations. This observation was used to perform a modelling of oxidation coupling chemical and mechanical parameters, which showed that the domain patterning induces stress localisation. This result is discussed in comparison with stress localisation observed in thin layer deposited on a substrate and used to propose an interpretation of UO 2 oxidation and cracking.
Abstract. The present study deals with the oxidation behavior under residual stress of shot-peened plates of "commercially pure" Zirconium exposed for 30 min at 650°C. The influence of the shot-peening on a preoxidizedmaterial is presented. The results have been used to determine the influences of these chemical (preoxidation) and mechanical (shot-peening) treatments on the high temperature oxidation of Zirconium. The oxygen profile was revealed using micro-hardness techniques and confirmed by SEM-EDS techniques. After pre-oxidation the samples were first resurfaced then shot-peened in order to introduce residual stress. A significant increase of the hardness of about +400 HV was observed on pre-oxidized shot-peened samples. The thermogravimetric analysis for 30 min at 650°C under 200mbar O 2 shows a significantly slower oxidation rate for shot-peened samples.A comparison with our computations points out the role of the residual stresses on the diffusion and, consequently, on the oxidation.
Abstract. The growth of a U 3 O 7 oxide layer during the anionic oxidation of UO 2 pellets induced very important mechanical stresses due to the crystallographic lattice parameters differences between UO 2 and its oxide. These stresses, combined with the chemical processes of oxidation, can lead to the cracking of the system, called chemical fragmentation. We study the crystallographic orientation of the oxide lattice growing at the surface of UO 2 , pointing the fact that epitaxy relations at interface govern the coexistence of UO 2 and U 3 O 7 . In this work, several results are given:-Determination of the epitaxy relations between the substrate and its oxide thanks to the Bollmann's method; epitaxy strains are deduced.-Study of the coexistence of different domains in the U 3 O 7 (crystallographic compatibility conditions at the interface between two phases: Hadamard conditions).-FEM simulations of a multi-domain U 3 O 7 connected to a UO 2 substrate explain the existence of a critical thickness of U 3 O 7 layer.
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