The micromechanics of notched zirconium polycrystals
Abstract
Zirconium and its alloys are extensively used in the core of nuclear reactors, primarily chosen for their low neutron absorption cross-section. During service, components made of zirconium alloys, such as claddings and structural materials experience varied mechanical loads. Fretting flaws, induced by debris trapped between fuel bundles and pressure tubes, act as stress risers, accelerating degradation mechanisms. Hence, the lifetime of nuclear reactors depends greatly on the state of zirconium-alloy components. The development of localized stresses in these components is linked to the zirconium microstructure because of its high degree of anisotropy. This thesis employs three-dimensional synchrotron X-ray diffraction combined with crystal plasticity modelling to study the evolution of grain-scale stresses in double-edge-notched zirconium specimens subjected to low cyclic loading.
Initially, the electron backscatter diffraction technique is used to measure grain location, orientation, and morphology. These measurements are compared with the ones obtained from three-dimensional synchrotron X-ray diffraction that are reconstructed using the weighted Voronoi tessellation. More than 80% of grains were matched.
The effects of microstructure and notch geometries on stress distribution are investigated using two different textures and two different notch geometries. In the first texture, the c-axis of hexagonal close packed grains are distributed along and perpendicular to the loading direction. This distribution of crystallographic orientations introduces large stress heterogeneities. It is shown that the developed asymmetries in the stress distributions in the vicinity of notches at the early stages of plasticity remain persistent with further cyclic loading and progressing into the plastic zone. In the second texture, the c-axis of grains are oriented perpendicular to the loading direction. It is shown that grain-scale stress concentration factors vary significantly before the onset of plasticity, but they settle in the plastic zone and with the progression of cycles. For the second texture, although the c-axis of the grains are perpendicular to the macroscopic loading direction, a strong grain orientation effects on the grain-average stresses is observed.