Cracking and Void Formation in Bulk W Components Manufactured for JET ITER-Like Wall

Abstract

When manufacturing W for fusion applications, it is crucial to consider microstructural features, such as grain shape and size, dislocation densities, distribution of defects, and impurities. These factors have a significant impact on a component’s lifetime in the fusion environment. This work provides valuable insights into the potential failure mechanisms that impact bulk W components. A bulk W lamella, which was manufactured for the JET divertor as part of the ITER-like wall (ILW) campaigns, was studied. A crack network had formed on the plasma-facing surface of the sample due to electrical discharge machining (EDM). The cracks predominantly propagated around larger grains due to grain misorientation and impurities. Notably, P and Fe were observed via atom probe tomography (APT), which appears to promote the diffusion of O. The results suggest that the initiation of crack propagation is linked to the nucleation of voids which exhibit behavior akin to KSiAl-doped W wires, involving diffusion-driven processes. O signal found inside some of the voids, confirmed by analytical transmission electron microscopy (TEM), suggests W oxide had formed on the internal surface, supporting the theory of oxidation-assisted void formation. It is, therefore, extremely important to consider the distribution of impurities and preexisting surface damage in W plasma-facing components (PFCs) before they are placed in the vessel.