Helium bubble formation in ultrafine and nanocrystalline tungsten under different extreme conditions

Bright-field TEM micrographs of UFG and NC tungsten irradiated with 2 keV helium ions (flux of 3.3 × 1016 ions m−2 s−1) at 1223 K demonstrating: (a) overview of sample with bubbles decorating grain boundaries at a fluence of 3.6 × 1019 ions m−2; (b) nanocrystalline grain with large facetted bubbles/voids on grain boundaries and few bubbles in the grain matrix at a fluence of 3.6 × 1019 ions m−2; and (c) grain boundary and (d) grain boundary triple-junction decorated by facetted bubbles with different sizes inside ultrafine grains at fluence of 4.0 × 1020 ions m−2.68