Hydrogen embrittlement and the micromechanics of crack initiation and propagation in polycrystals

Zirconium alloys are widely used in the core of CANada Deuterium Uranium (CANDU) nuclear reactors. Prolonged exposure to high temperatures, hot water coolant, and high mechanical loads lead to the formation of zirconium hydrides, which can affect the lifetime of reactor core components such as Zr-2.5Nb pressure tubes. This work investigates the micromechanics of crack initiation and propagation in zirconium alloys, with a primary focus on the fracture of hydride precipitates. Ex-situ and in-situ scanning electron microscope (SEM) tensile experiments were conducted to study deformation mechanisms and cracking of hydrides. Electron backscatter diffraction (EBSD) was used to measure the orientations of grains and map them into a crystal plasticity finite element (CPFE) model to study the evolution of localized deformation fields. It was shown that the transformation strain induced by hydride precipitation significantly influences microcrack nucleation and growth, highlighting the importance of considering local grain interactions and hydride morphologies in predicting microcracks within hydrides. Additionally, both slip-favorable and twinning-favorable textures were analyzed to assess interactions among hydrides, twins, microcracks, and slip bands. It was observed that {10-12} twins can nucleate either before or after microcrack formation within hydrides, whereas {11-21} twins nucleate at the hydride’s crack tips and grow within the zirconium matrix. Although the formation of twins may contribute to crack nucleation, slip activity within hydrides is consistently observed before cracking. Different damage initiation criteria were examined, and for the first time, a comparison of numerical results with observed microcrack locations showed that the local shear energy density within hydrides is the primary driving force for crack formation. To further validate this model, titanium hydrides were also investigated, and the obtained results confirmed the consistency in identified damage mechanism in both zirconium and titanium hydrides. Lastly, a dual-phase Zr-2.5Nb alloy is analyzed using high spatial resolution EBSD to characterize microstructure variations. Such variations can affect local stress fields and hydrogen accumulation. It is shown that the presence of β-phase affects crack propagation.