Irradiation-enhanced superficial modification and evolution of mechanical behavior in TaTiNbZr refractory high entropy alloy films exposed to low energy helium plasma

In this work, TaTiNbZr refractory high entropy alloy (RHEA) films constructed with nano-crystalline and amorphous composite structures were irradiated by helium (He) plasma with a fixed flux of 5 × 1022 m−2·s−1 under bias-voltage driven plasma energy (0–70 eV) to systematically explore the evolution of morphological modification and mechanical properties. The results indicated that the RHEA exhibited composite structures after He plasma irradiation, and the irradiation-enhanced surface blisters were strongly dependent on He plasma energy. After exposure to He plasma, the size and density of blisters on the surface of RHEA films gradually increased with the enhancement of plasma energy, these blisters originated from the formation of bulges underneath the surface which were induced by the large-sized bubbles. Some blister bursts were initiated at a critical energy of 50 eV (irradiation temperature was 600 K). Non-obvious superficial sputtering can be observed for the RHEA films irradiated at energy less than 50 eV. At the He plasma energy of 70 eV, cracks and abundant blister exfoliation were produced on the RHEA surface due to the synergistic effect of enhanced energy and irradiation temperature (650 K). Surface sputtering and the thermal diffusion-driven mass flow were induced by He plasma due to the sputtering threshold energies in Ti and Zr elements than that of Ta and Nb elements, which were responsible for the growth of fiber-like structures on the surface of RHEA film as the energy was fixed at 70 eV. Moreover, the maximum hardening ratio of 21.4% was revealed from the TaTiNbZr RHEA nano-film irradiated at plasma energy of 50 eV, the role of plasma energy on the evolution of mechanical properties, and the corresponding mechanisms in response to irradiation damage were also discussed in detail.