Bubble formation in VTaTi refractory high entropy alloy and elemental V under He ion irradiation at various temperatures

Refractory high entropy alloys (RHEAs) have recently gained much attention as potential structural materials in advanced reactors, where they may suffer from the synergistic threatens from both displacement damage and transmutation-induced He, calling for the necessities of understanding the He bubble behavior in these novel alloys. In this work, VTaTi is selected as a model RHEA, and the formation of He bubbles under 2.5 MeV He ion irradiation at various temperatures (i.e., room temperature, 400, and 700℃) is compared with elemental V, which shares the same body-centered cubic (BCC) structure. Unlike the cases for the widely studied 3d-transition-metal HEAs, in which increasing compositional complexity generally assists in suppressing the bubble growth, here the sizes and the total volume fractions of He bubbles in VTaTi are larger than those in V at all the three irradiation temperatures. First-principles calculations are performed to unveil the underlying mechanisms through the comparison of typical defect energies associated with classical He diffusion models. The lower vacancy formation energy, reduced He migration energy, and lower binding energy of He-vacancy complexes may contribute to the promoted He aggregation in VTaTi.