Investigation of the Orientation of δ Zirconium Hydrides Using Quantitative Phase Field Simulations Supported by Experiments

Light water reactor fuel cladding integrity is sensitive to the morphology of zirconium hydride precipitates,which can vary depending on the thermomechanical history of the cladding. This study provides insights on the orientation and stacking of nanoscale δ hydrides in a single grain of α zirconium using results from a newly developed quantitative phase field model and experimental observations. The model is informed by CALPHAD free energies and incorporates misfit strains due to the volume difference between the two phases, hydrogen in solid solution in the matrix, and thermal expansion. Simulation results on the growth, orientation, and stacking of nanoscale hydrides are supported by SEM observations of recrystallized zirconium.The model accurately predicts the elongated growth of δ hydrides along the basal plane of the α zirconium. Moreover, nanoscale hydrides are observed to stack circumferentially into long mesoscale hydrides. The effect of applied stress on the microstructure is investigated to probe the experimentally observed hydride reorientation phenomenon. This model predicts that hydride reorientation cannot be explained by radial growth of nanoscale hydrides, nor by a change of the most favorable stacking configuration in a single grain, since the applied stress needed to observe these phenomena is significantly greater than experimentally observed.