Zirconium δ-hydrides: Strain localisation, ratcheting, and fatigue crack propagation

• Digital image correlation based cyclic post-precipitation strain measurements are made of micrometre-scale zirconium hydride features. • An unexpected distinct strain directionality is measured and is shown to be linked with hydride stringer morphology using crystal plasticity finite element modelling. • Networks of geometrically necessary dislocations are shown to develop around an intragranular hydride stringer, which increase cyclically for up to five thermomechanical cycles (due to hydride strain ratcheting). • New insights into the mechanisms involved in microstructurally short fatigue crack propagation in hydrided zirconium alloys at room temperature are presented, the nature of which are shown to have a substantial effect on crack propagation rate. As many nations commit to achieving Net Zero, many low carbon scenarios indicate that civil nuclear power generation and the economics thereof are set to play a vital role. To maximise nuclear reactor operation lifetimes, it is essential to develop mechanistic understanding of failure and degradation mechanisms in safety-critical components for increasingly holistic reactor design codes and standards. In this paper, advanced micromechanical testing with in situ digital image correlation is used in combination with crystal plasticity modelling to study various aspects of damage associated with δ hydride precipitates in Zircaloy-4 for reactor fuel cladding applications. Measurements of static and cyclic hydride precipitation strains demonstrate a discernible strain field directionality (associated with intragranular precipitation) which was not previously reported, while cyclic thermomechanical loads are shown to promote the cyclic accumulation of strain due to repeated precipitation and dissolution of hydrides (hydride strain ratcheting) for up to five cycles, leading to the development of networks of geometrically necessary dislocations. Using crystal plasticity finite element modelling of the volumetric expansion associated with hydride precipitation, the strain directionality phenomenon is shown to be linked with hydride morphology. Comparisons with experimental strain fields also suggest that hydride plasticity is an important consideration for damage accumulation during precipitation. Experimental measurements of short fatigue crack propagation through Zircaloy-4 microstructures containing hydrides reveal new crack propagation mechanisms including decohesion, which on average, lead to accelerated rates of crack growth. Twins and hydride precipitation therein are also implicated in even more damaging fatigue behaviour as fatigue cracks are provided a seemingly brittle and direct path for fracture, which was not previously reported in the literature.