Mechanisms of helium irradiation blistering and surface deformation in tungsten

Helium irradiation blistering in tungsten is of intense interest for plasma-facing materials under fusion-relevant conditions. Previous studies have revealed that the surface blistering is closely related to the evolution of subsurface microstructures (e.g., bubbles and cracks), but its dependence on the surface orientation and deformation remains unclear. The present work reports the helium irradiation blistering on the surface planes of tungsten {100}, {110} and {111}. A systematic study of helium irradiation blistering was conducted at room temperature by utilizing a Helium Ion Microscope. The cross-sectional microstructural analyses at different ion fluences confirmed that the blistering deformation starts with the nanocrack nucleation, initiated at the peak in the bubble depth distribution via interbubble fracture, and is followed by the gas buildup in a primary cavity. In addition, the comparison of blistering behaviors between 1-µm-diameter and 2-µm-diameter circular areas revealed that only the blisters formed on tungsten {100} can grow to a larger size, whereas those formed on tungsten {110} or {111} tend to become wrinkled or burst when the irradiated area increases. Combined with the micropillar compressions in a Scanning Electron Microscope, it was also found that the surface deformation is orientation-dependent and associated with the number, symmetry and distribution of effective slip systems in body-centered-cubic transition metals. Our findings provide new evidence and insights into the mechanisms of helium irradiation blistering and surface deformation in tungsten.