Blisters formed by D plasma exposure in an electron-transparent tungsten sample

By irradiating a tungsten sample for transmission electron microscopy with deuterium plasma at 230 K, we created a high density of blisters below the plasma-exposed surface. Blisters were even found in the electrontransparent region close to the central perforation. We determined the abundance of blisters depending on the local sample thickness and found that no blisters were formed for thicknesses below about 45 nm for these specific plasma exposure conditions. After removing the approximately 10 nm thick, defect-rich layer at the plasma-exposed surface, which corresponds roughly to the ion implantation zone, by electrochemical polishing, we were able to clearly identify material distortions and dislocation network halos corresponding to blisters by scanning transmission electron microscopy. Compared with unirradiated tungsten of the same grade, we found that in the blister zone the average dislocation density is about 2 orders of magnitude higher, which may, in addition to gas enclosed in blister cavities, explain enhanced deuterium retention in blistered tungsten samples. The proof-of-principle experiments described in this article have the potential to provide constraints for theoretical models for blister nucleation. They also pave the way for direct investigations of the spatial correlation of deuterium-plasma-induced blisters and intrinsic defects.