AbstractSub-surface precipitations of UH3 have been modelled with Finite Element methods. The model includes a pre-stressed oxide layer, metal anisotropy, metal plasticity and a hyper elastic coherent hydride. The model was used to investigate UH3 precipitating at a variety of depths from 0 to 2um (spherical shape), with a variety of sizes from 0.08 to 0.8 um diameter (spherical shape) and with a variety of shapes from horizontal ellipsoid to vertical ellipsoid (depth 0.5 um). UH3 precipitation close to the surface was found to be energetically favourable as observed in experiments. Simulations on the shape of the precipitate found that the vertical ellipsoidal precipitates were found to be energetically favoured in contrast to what has been experimentally observed. In all cases the system could not accommodate the phase change by elastic deformation alone but by a combination of elastic and plastic deformation. When anisotropy is introduced into the metal matrix, the precipitate is surrounded by compressive and tensile regions. Tensile regions are found in the x-y plane adjacent to the precipitate and it is suggested that these regions are more likely to transform into further UH3 (through increased hydrogen diffusion, solubility or ease of phase change). Such precipitate development in the lateral direction would result in the experimentally observed horizontal ellipsoids. Multiple sub-surface precipitates were simulated (0.5 um depth) and it is suggested that compressive regions that develop between the precipitates could act as a barrier to coalescence. The surrounding stress regions and energetic factors suggest that there is a barrier to amalgamation for a distance of 1.5 um. Whereas for precipitates closer than 1.5 um there is an energetic benefit to coalescence. The transformation from sub-surface UH3 precipitates into growth centres (exposed UH3 on the surface) was examined by monitoring the oxide stress. This work also suggests that the transformation of sub-surface UH3 to growth centres could be retarded by an increased oxide thickness and the presence of a work hardened layer.To aid confidence in the model nano-indentation experiments were carried out on constrained UH3 surface films in the absence of air. Collected data shows a bulk modulus of 180 +/- GPa which is more in line with DFT calculated results as compared to Diamond Anvil Cell experimental work. The nano-indentation work represents the first time this type of data has been derived for UH3 in this way.
|Date of Award||1 Aug 2015|
|Supervisor||Nicholas Stevens (Supervisor)|
- Uranium hydride