Mechanical Behavior of UO2 at Sub-grain Length Scales: Quantification of Elastic Plastic and Creep Properties via Microscale Testing

Research project

Description

Summary: thermo-mechanical behavior of oxide fuels is extremely important, since elasticity, plasticity and creep are key to understand Pellet-Cladding Mechanical Interactions (PCMI) that can lead to fuel fracture, which in turn can affect fuel performance significantly. In addition, subgrain scale mechanical behavior, e.g., anisotropy of elastic properties as well as dislocation driven plasticity and creep, can also play a significant role on microstructure evolution of fuels. Therefore, careful experimental measurements of mechanical properties are key for the validation of robust fuel performance codes able to predict this behavior from inputs at the micro-scale, e.g., MARMOT, and to quantify its effects on other aspects of fuel behavior. In UO2, the thermo-mechanical response at the meso-scale depends strongly on crystallography of individual grains. The available datasets are far from complete in terms of temperature, stress and stoichiometry in parameter space. Therefore, a thorough understanding of the mechanical response at the sub-grain level will be key to validate advanced fuel performance codes with multiscale predictive capabilities. To this end, experiments in large single crystals are ideal, but testing of bulk monocrystalline samples can be complicated and costly, particularly after irradiation. This also makes it quite difficult to use enough samples to study reproducibility and intrinsic material scatter, which are key to calibrate and validate predictive codes. It is proposed to develop techniques to measure properties at sub-grain scales using depleted Uranium Oxide (d-UO2) samples heat-treated to obtain different grain sizes and oxygen stoichiometries, through three main tasks: 1) sample processing and characterization, 2) microscale and conventional testing and 3) modeling. Grain size and crystallography will be characterized using Scanning Electron Microscopy and Electron Backscattering Diffraction. Grains will then be carefully selected based on their crystallographic orientations to perform insitu micromechanical tests with samples machined via Focused Ion Beam (FIB), with emphasis on micro-pillar compression and micro-cantilever bending. These experiments will be performed under controlled atmosphere, to insure stoichiometry control, at temperatures up to 700 C and potentially higher and will allow measurement of properties involving elastic (effective Youngs modulus), plastic (critical resolved shear stresses) and creep (creep strain rates) behavior. Conventional compression experiments will be performed simultaneously to validate the in-situ measurements and study potential size effects. Modeling will be implemented using finite elements with anisotropic elasticity and inelastic constitutive relations for plasticity and creep based on kinematics and kinetics of diffusion assisted dislocation glide and climb that account for the effects of crystal orientation, stress, temperature and stoichiometry. The models will be calibrated and validated using the experimental data. This project will result in correlations between stoichiometry, crystallography and mechanical behavior in advanced oxide fuels, provide valuable experimental data to validate and calibrate meso-scale fuel performance codes and also a framework to measure sub-grain scale mechanical properties that should be suitable for use with irradiated samples due to small volumes required. The team at ASU will perform sample characterization, select grains and orientations for micromechanical testing, conventional compression tests, and develop finite element models to interpret experiments. The team at UCB will fabricate both micro-pillars and micro-cantilever beams, perform experiments to measure the onset of plastic deformation and creep rates as a function of stress and temperature under controlled atmospheres and assist on data analysis and model formulation. Collaborators at LANL will provide d-UO2 samples. Cost structure: 3 years for $800k; ASU: $136.7 k/Y; UCB $130k/Y
StatusFinished
Effective start/end date12/17/1312/16/17

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Creep
Plastics
Testing
Stoichiometry
Experiments
Temperature
Crystallography
Plasticity
Oxides
Elasticity
Protective atmospheres
Mechanical properties
Compression testing
Focused ion beams
Cantilever beams
Backscattering
Crystal orientation
Uranium
Data structures
Shear stress