The behavior of point defects and their impact on the formation of helium bubbles in refractory high-entropy alloys

The present study investigates the behavior of point defects and the formation mechanism of helium bubbles in a series of refractory high-entropy alloys (RHEAs), based on first-principles calculations. Compared with elemental V and Nb, all the RHEAs exhibit reduced formation energies for vacancies, substitutional He, and tetrahedral He. Moreover, although the average migration energies for vacancies and tetrahedral He in RHEAs are similar to those in elemental metals, the values exhibit wide and asymmetric distributions with a significant portion extending down to 0 eV, enabling numerous low-barrier pathways. In order to elucidate how defect energies depend on chemical environments and to identify the key elements that modulate the bubble formation, the charge density distributions are calculated, showing that the vacancy formation tends to be promoted by Ti, and suppressed by Nb. He ion irradiation experiments were also carried out to explore the influence of point defect energies on He bubble formation, and confirm that the bubbles in these RHEAs are considerably larger than those in the elemental metals. By comparing several conventional He bubble formation mechanisms, the vacancy and dissociation mechanisms can well correlate the calculated defect energies with the bubble sizes, demonstrating that the low vacancy formation energy and the weak binding energy of He-vacancy pairs could be the key factors that enhance the aggregation of He atoms in RHEAs. This work demonstrates that the understanding of the point defect behavior is necessary to evaluate the irradiation resistance of RHEAs.