Elemental diversity in high entropy alloy MoNbTaVW: complex He diffusion paths and improved radiation resistance

The performance of plasma-facing components in nuclear fusion reactors is limited by radiation-induced degradation, particularly helium accumulation and migration. High-entropy alloys (HEAs) have emerged as promising candidates owing to their reported irradiation tolerance, yet the relationship between irradiation tolerance and sluggish diffusion remains debated. Here, we investigate helium diffusion pathways in body-centered cubic refractory HEAs (MoNbTaVW), medium-entropy alloys MEAs (MoNbTa), and elemental Ta using ion implantation, annealing, transmission electron microscopy, elastic recoil detection, and advanced molecular dynamics simulations. We find that increasing elemental diversity progressively suppresses long-range helium migration while promoting localized trapping by elements (V, Nb, Ta) with lower local valence-electron density, where lattice distortion creates potential-well volumes. This confinement yields complex diffusion pathways, resulting in limited net displacement but persistent local mobility that drives cavity formation in the damage-peak region. Our results clarify the origin of sluggish diffusion in refractory HEAs and provide design principles for radiation-tolerant structural materials for nuclear applications.