Evaluation of helium effect on ion-irradiation hardening in pure tungsten by nano-indentation method

Highlights: • The helium effect on the irradiation hardening in W was investigated at temperatures between 300 °C and 1000 °C. • A method is shown to evaluate the nano-hardness removing size effect. • The recrystallization effect on irradiation hardening is shown. - Abstract: As-received and recrystallized pure tungsten (W) were irradiated with 6.4 MeV Fe³⁺ up to 2 dpa with or without He⁺ at 300 °C, 500 °C, 700 °C and 1000 °C respectively. Irradiation hardening was measured by the nano-indentation method. An equation to evaluate the bulk equivalent hardness was derived on the assumption that the geometrically necessary dislocation (GND) densities at an indentation depth were the same before and after irradiation. Ion-irradiation always induces hardening in both as-received and recrystallized W at all the experiment temperatures. In the case of single-beam irradiation, the recrystallized W exhibited higher hardening than as-received one. The effect of helium on the irradiation hardening is dependent on the material condition: as-received W showed an additional hardening by helium at all the irradiation temperatures, while in recrystallized W the hardening was not affected by helium below 700 °C.