Fig. 1. (a) Predictions of He concentration and damage as a function of depth. The region for further investigation is highlighted. Fresnel-contrast TEM imaging of He bubbles in the FeNiCoCr HEA at (b) under-focus (-300 nm) and (c) over-focus (+300 nm) conditions. (d) Size distribution of He bubbles in the FeNiCoCr HEA irradiated at 873 K.

Fig. 2. (a) The variations of atomic density highlighted by 90 atoms nm-3 iso-surfaces at the He implantation peak. The λ-type density variation at a bubble location is determined by (b) a two-dimensional density contour plot using a 1 × 20 × 20 nm3 cuboidal region-of-interest and (c) a one-dimensional local density profile using a 1 × 1 × 20 nm3 cuboidal region-of-interest.

Fig. 3. Atom maps of Fe, Ni, Co, and Cr in the samples from (a) the He implantation peak and (b) the un-irradiated region, i.e., ～2 μm beneath the surface of the irradiated sample. The thickness of the slices is 2 nm. (c) One-dimensional concentration profiles across the He bubble determined in Fig. 2(b) and (c), obtained by calculating element concentrations in 0.5 nm-wide bins from a cylindrical region-of-interest.

Fig. 4. Comparison in the magnitude of RIS, i.e., change in composition relative to that in the un-irradiated matrix, at He bubbles (this work), voids [17], and dislocation loops [18,19] in FeNiCoCr HEAs. The data of RIS at dislocation loops [18] and grain boundaries [28] in FeNiCoCrMn HEAs are also included.

Fig. 5. (a) HAADF STEM image of He bubbles in the FeNiCoCr HEA. The inset is the corresponding He chemical map of the red rectangular area. (b) EELS spectra collected from the matrix (position 1) and the He bubble (position 2) marked in (a).

Fig. 6. (a) The estimated He density as a function of the inverse radii. (b) The pressure inside He bubbles in the FeNiCoCr HEA at 873 K as a function of the inverse radii. Also shown is the line P = 2γ/r with the γ of 2.3 J m-2 for the FeNiCoCr HEA. Data of He density and pressure obtained from Ni by Granberg et al. [32] and PE16 alloys by Walsh et al. [31] and McGibbon [33] are also displayed.

Fig. 7. Relationships between formation energies of a single He atom in the FeNiCoCr HEA at substitutional, octahedral, and tetrahedral positions and the number of Ni/Co neighboring atoms surrounding the He atom. The data are plotted according to the re-analysis of our previous first-principles results [14].

Fig. 8. Stacking fault energy of the matrix and near He bubbles at room temperature and irradiation temperature. The error bars indicate the standard deviation of the calculated SFE near 10 different He bubbles.