a b s t r a c tAs-received and recrystallized pure tungsten (W) were irradiated with 6.4 MeV Fe 3 + 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.

Ion-irradiation hardening of pure tungsten (W) single crystal was evaluated by nanoindentation (NI) technique considering material pileup effect. Pure W single crystals of (001) surface orientation were ion-irradiated with 6.4 MeV Fe 3+ to 0.1 dpa, 1 dpa or 2 dpa at 573 K. The irradiation hardening was evaluated by means of NI measurements with elastic-modulus-based correction (EMC) method [C. Heintze et al.: J. Nucl. Mater. 472 (2016) 196-205]. The effect of material pile-up in tungsten was so signi cant that the bulk equivalent hardness values by EMC method were about 70% and 85% of uncorrected results for irradiated and unirradiated W(001), respectively. The ion-irradiation hardening values by EMC based method were approximately 40%, 50% and 60% of uncorrected results for 0.1 dpa, 1 dpa and 2 dpa, respectively. The measured maximum pile-up height was higher for irradiated W(001) than for unirradiated W(001) at each indentation depth. An averaged pileup height that was associated with the actual area of contact of pile up obtained from EMC hardness showed different responses to ion-irradiation depending on the indentation depth.

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The local strain rate (LSR) dependence of nanoindentation (NI) hardness was investigated by using standard constant strain rate (CSR) test method and strain rate jump (SRJ) test method for W single crystal with the surface orientation of {001} before and after 6.4 MeV Fe 3+ irradiations (nominal damage level of 0.1, 1 and 2 dpa, 573 K). The effect of ion-irradiation on the LSR sensitivity of NI-hardness at room temperature (RT) was evaluated by changing LSR between 0.3 s −1 and 0.01 s −1 or 0.03 s −1 and 0.001 s −1. Under these experimental conditions, ion-irradiation increases NI-hardness and slightly decreases LSR sensitivity of NI-hardness for all damage levels. The effect is more pronounced with increasing damage level. The LSR sensitivity values are ranging between 0.015 and 0.04 in SRJ tests, and between 0.0425 and 0.06 in CSR tests, indicating that the deformation of bcc W{001} at RT is controlled by a high lattice friction stress. The decrease in LSR sensitivity by ion-irradiation could be attributed to the increase in the athermal stress caused by ion-irradiation induced defect structures, which is re ected to a decrease in the activation volume of dislocation motion in ion-irradiated W{001}.

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