Hareesh Tummala1 Laurent Capolungo1 Carlos Tomé1

1, Los Alamos National Laboratory, Los Alamos, New Mexico, United States

Hydride precipitates are the major cause for degradation of the mechanical properties (e.g. ductility, fracture toughness) of zirconium (Zr) alloy based clads for nuclear fuel in Light Water Reactors. Experimental studies highlighted the importance of the morphology and orientation of hydrides on hydrogen embrittlement in Zr alloys. In addition to showing a preferred alignment along the circumferential direction, hydrides were commonly observed to self-organize into stacks. The two-level organization of hydrides is connected to the mechanical fields around each hydride and the complex interaction with the surrounding hydrides and defects (e.g. dislocations).

In the present study, a Fast Fourier Transforms based Discrete Dislocation Dynamics (DDD-FFT) technique is used to solve the mechanical fields in and around a δ-hydride precipitate in Zr matrix. The largely anisotropic stress fields developed by an ellipsoidal-shaped δ-hydride precipitate is modified due to the presence of dislocations in the Zr matrix. In addition, the shielding distance, which is introduced as a measure for hydride separation distance in a stack, decreased four-fold in the direction of the largest misfit strain of the δ-hydride. The growth zones, which are the regions for preferred growth direction of a hydride, and the nucleation zones, which are the regions with highest probability for nucleating another hydride, were analyzed by plotting the driving force around the δ-hydride. Nucleation zones at a 45o angle with the long axis of the δ-hydride were identified as the reason behind the self-organization (stacks) of hydrides. Finally, we derive conclusions on the shielding effect of the δ-hydride due to dislocation nucleation near the hydride-matrix interface.