Discrete dislocation modelling of δ hydrides in Zr: towards an understanding of the importance of interfacial stresses for crack initiation

We carried out discrete dislocation plasticity (DDP) simulations in the infinite-matrix approximation of δ Zr micro-hydrides – including their elastic field - first precipitating in Zr and then subjected to an external stress. We calculate and present the dislocation density, maximum principal shear and stresses, configurational energy density and plastic displacement. It was found that even hydrides as thin as 20 nm have a large impact on dislocation density, plastic deformation and stress build-up, and that this effect increases for increasing hydride thickness. It also appears that the critical location for crack nucleation is at the Zr/hydride interface. In fact, slip bands that develop on one side of the interface, and are stopped by it, are accompanied by the formation of high stresses. The observation that the interfacial stress is the only way in which hydrides are represented in the simulations, with no point obstacles or change in the slip systems at the interface, combined with the observed strong influence of the hydrides on the results, suggests a possible mechanistic explanation for the embrittlement caused by the precipitation of intragranular hydrides. The mechanism is rooted in the volumetric expansion upon precipitation and naturally includes a thickness dependence that is also experimentally observed.