A multi-perspective study on the evolution behavior of helium bubbles in FeCrVTix medium-entropy alloys

The presence of helium (He) bubbles, a typical irradiation defect, leads to swelling, hardeningand embrittlement in structural materials, resulting in a shortened service life. Nevertheless, certain high/medium-entropy alloys (HEAs/MEAs) are recognized for their significant potential in advanced nuclear systems due to their exceptional resistance to He ion irradiation. This work investigates the evolution behavior and mechanisms of He bubbles in BCC-phase HEAs/MEAs with irradiation tolerance from multiple perspectives (irradiation fluence, irradiation temperature and Ti addition) using FeCrVTix x MEAs as model materials. We propose a new indicator to determine the He bubble evolution mechanism based on the ratio of the diffusion coefficients of He atoms to those of vacancies, which can more clearly and vividly represent the evolution characteristics of He bubbles with temperature in HEAs/MEAs. The excellent tolerance of FeCrVTix x MEAs to He ion irradiation-induced swelling is consistent with the predictions obtained by He bubbles are near equilibrium pressures with slight under- pressurized and the percentage of He atoms inside bubbles does not exceed 10%. Furthermore, Ti addition can suppress the nucleation, growth and coarsening of He bubbles in the BCC-phase matrix by increasing local lattice distortion and chemical composition complexity as well as forming Laves-phase precipitation, thereby further enhancing the tolerance to swelling induced by He bubbles. These findings provide valuable theoretical insights for the development of HEAs/MEAs with outstanding irradiation resistance.