Investigating irradiation damage in tungsten for fusion power

Tungsten is the front-runner material for armour components in future fusion reactors. In-service, irradiation with fusion neutrons will generate displacement cascades, leaving behind lattice defects. Helium, injected from the plasma and produced by transmutation, aggravates damage by increasing defect retention. These effects can be mimicked using helium-ion-implantation. Helium-implantation-induced defects are probed here by measuring the lattice strains they cause, using energy- and depth-resolved synchrotron X-ray micro-diffraction. An increase in helium dose from 300 to 3000 appm increases volumetric strain ~ 2.4 times, indicating a ~ 3 times higher defect retention per injected helium at low helium doses. This suggests defect retention is not a simple function of implanted helium dose, but strongly depends on material composition and presence of impurities. The effect of helium-defects on deformation behaviour is examined by comparing spherical nano-indents in unimplanted and helium-implanted (W-3000He) regions of a tungsten grain. Helium-implantation increases hardness and causes large pile-ups. 3D-resolved X-ray micro-diffraction shows a more confined plastic zone under indents in W-3000He. Localised deformation and slip-channel formation is confirmed through high resolution electron backscatter diffraction and transmission electron microscopy on cross-section lift-outs from indents in W-3000He. Together, the observations suggest a large initial hardening due to helium-defects, followed by localised defect removal and subsequent strain softening. The hypothesis is examined by implementing it in a crystal plasticity finite element (CPFE) formulation, simulating nano-indentation in -W-3000He. With only one fitting parameter the simulation captures the localised large pile-up and predicts confined fields of lattice distortions beneath indents in quantitative agreement with experimental measurements. Indents in and grains of W-3000He show little pile-up. The same CPFE model captures this orientation dependence of pile-up, suggesting that the underlying strain localisation is orientation-independent and that changes in pile-up arise due to the relative orientations of slip systems, sample surface, and the indenter. These advances in the understanding of irradiation damage in tungsten, obtained through experimental and modelling techniques, inspire confidence in the feasibility of building predictive models from experimental observations, which can account for irradiation-induced changes when estimating armour component lifetime and in-service performance. This is key for the design of future fusion reactors.