Fracture morphology and mechanical properties of blisters in poly- and single crystal tungsten irradiated by MeV protons

The mechanical properties and the fracture morphology of the inner surface of blisters formed in polycrystalline and single crystal tungsten under MeV proton irradiation, were studied. The tungsten samples were irradiated at Soreq Applied Research Accelerator Facility (SARAF) by 2.2 MeV protons, at various samples temperatures and proton doses. The morphology of the irradiated samples was observed by examining cross sections of blisters obtained by focused ion beam (FIB). The thickness of the blister cap was found to be in good agreement with the irradiating protons stopping range, regardless of sample structure, temperature or total dose. Increased curvature of the surfaces was observed with increased temperature for polycrystalline and single crystal blisters and was interpreted as increased ductility of the blister growth process. The ductility is more substantial in single crystal blisters, pointing to the role of grain boundaries in blisters growth mechanism. The cap of one blister was removed, exposing the inner fracture surface of a blister and supporting the validity of the cross sectional observations. The mechanical properties in the vicinity of a blister formed in a single crystal sample were characterized by nano-indentation. The hardness near the irradiated surface was found to increase both in the vicinity of the blister and beyond. This observation suggests that the elevation of the blister cap does not cause significant strain hardening. For the blister cap, the hardness was found to increase with increasing depth from the irradiated surface, correlating with increased irradiation damage.