Void evolution in tungsten and tungsten-5wt.% tantalum under in-situ proton irradiation at 800 and 1000 °C

Highlights: • Addition of Ta to W reduces vacancy mobility and delays void formation at 800 °C. • Ta atoms induce a significant increase in void density at 1000 °C as compared to W. • At 1000 °C and ≥0.2 dpa void size saturates in W–5Ta, but it still increases in W. • Voids in W and W–5Ta transit from spherical to faceted shape at ≥ 0.3 dpa. • At 1000 °C and 0.4 dpa fraction of faceted voids is 30% in W and 12% in W–5Ta. - Abstract: We have probed void evolution in polycrystalline W and W-5wt.%Ta material at 800 and 1000 °C, by transmission electron microscopy during in-situ irradiation with a 40 keV proton beam. The presence of radiation-induced dislocation loops was not observed prior to void formation at those elevated temperatures. The damaged W microstructure was characterised by the presence of a population of randomly distributed voids, whose number density reduces when the irradiation temperature increases. Soft impingement of voids becomes noticeable at damage levels ≥0.2 dpa. In contrast, the excess of free vacancies in the W-5wt.%Ta material irradiated at 800 °C only leads to the formation of visible voids in this TEM study (≥2 nm) after post-irradiation annealing of the sample at 1000 °C. Solute Ta atoms also cause a significant increase in the number density of voids when comparing the microstructure of both materials irradiated at 1000 °C, and a gradual progression towards saturation in average void size at ≥0.2 dpa. Moreover, we have detected a progressive transition from a spherical to a faceted shape in a number of voids present in both materials at damage levels ≥0.3 dpa.