Mechanical and structural transformations of tungsten implanted with helium ions

The near-surface structural and mechanical changes of tungsten upon exposure to 25 keV helium ions at doses ranging from 1 × 10 ² to 1 × 10 ⁴ ions/nm ² (1 × 10 ¹⁶ to 1 × 10 ¹⁸ ions/cm ² ) are investigated using site-specific implantation, imaging and probing techniques. Helium Ion Microscopy and Atomic Force Microscopy are used to investigate surface topography changes due to swelling and blistering, and nanoindentation is used to study changes in material hardness. An analytical model has been developed to qualitatively explain bubble formation, surface swelling and the variation in local hardness with depth observed experimentally. The model assumes that the implanted helium has a Gaussian depth distribution, which the helium nanobubbles also follow due to their fast formation. There are two competing processes proposed: (1) a reduction in hardness due to the decrease in material density from bubble formation, and (2) an increase in hardness due to bubbles acting as pinning points that impede dislocation motion.