Grain-interior and grain-boundary strength of tungsten under helium-ion and neutron irradiation revealed by micro-cantilever bending

Tungsten (W), widely to be used as a fusion divertor material, suffers from irradiation-induced embrittlement caused by displacement damage and transmutation helium (He). In this study, micro-cantilever specimens were fabricated in recrystallized pure W by focused ion beam milling and bent by nanoindentation to separately quantify grain-interior and grain-boundary strengths under three conditions: unirradiated, neutron-irradiated (0.74 dpa at 830 °C), and He-implanted W (200 appm, 50 MeV He 2+ ) followed by post-implantation annealing (PIA; 1500 °C for 1 h). Grain-interior micro-cantilever specimens exhibited ductile bending without fracture, accompanied by slip-band formation. The mean maximum grain-interior bending stresses for unirradiated, neutron-irradiated, and He-implanted W plus PIA were 920 MPa, 1092 MPa, and 1454 MPa, respectively. In contrast, all grain-boundary micro-cantilever specimens exhibited brittle fracture along grain boundaries. The mean maximum grain-boundary bending stresses were 872 MPa for unirradiated W and 839 MPa for neutron-irradiated W, indicating no pronounced grain-boundary weakening under the present neutron-irradiation conditions. For He-implanted W plus PIA, grain-boundary stress exhibited large scatter, with high-strength boundaries (mean 1167 MPa) and weak boundaries (mean 417 MPa). Some grain-boundary cantilever specimens fractured at boundaries not aligned with the loading direction, suggesting preferential failure along weak sub-grain boundaries. These results support a Ludwig–Davidenkov (L–D) interpretation in which neutron irradiation is dominated by hardening embrittlement, whereas He promotes non-hardening, grain-boundary embrittlement at specific boundaries.