Fig. 1. Depth profiles of the irradiation damage and injected helium concentration in pure Pd irradiated by 30 keV He+ with a fluence of 1.1 × 1016 He+/cm2 calculated by SRIM program in quick Kinchin-Pease mode.

Fig. 2. In-situ bright-field TEM images showing the evolution of dislocation loops and helium bubbles at the monitored region with 80 nm thickness during in-situ 30 keV He+ irradiation at 573 K, the irradiation fluence was: (a) Non-irradiation; (b) 5 × 1014 He+/cm2; (c) 1.2 × 1015 He+/cm2; (d) 2.5 × 1015 He+/cm2; (e) and (f) 5.0 × 1015 He+/cm2. (a-e) TEM images taken using g = 200 and (f) TEM image taken using g=-200 two-beam conditions near [011] zone axis. The scale bar in (a) applied to all micrographs.

Fig. 3. (a) The average size of dislocation loops and helium bubbles as a function of irradiation fluence; (b) The relationship of the volume number density of dislocation loops and irradiation fluence, as well as for helium bubbles. All data were obtained at the monitored region with 80 nm thickness.

Fig. 4. In-situ TEM images showing the evolution of several typical loops marked in Fig. 2(b) at the monitored region with 80 nm thickness, the irradiation fluence was: (a) 5.0 × 1014 He+/cm2; (b) 9.9 × 1014 He+/cm2; (c) 1.2 × 1015 He+/cm2; (d) 2.5 × 1015 He+/cm2; (e) 3.5 × 1015 He+/cm2, and (f) 5.0 × 1015 He+/cm2.

Fig. 5. The schematic illustration of the evolution and corresponding average size of (a) loop k; (b) loop l; and (c) loop m that were marked in Fig. 2 as a function of irradiation fluence, as well as for helium bubbles inside them. The green circles represented the dislocation loops, and the blue solid dots represented helium bubbles. (d) Ratio of the total area of bubbles inside the loop to the loop area as a function of irradiation fluence.

Fig. 6. In-situ TEM images of dislocation loops at the monitored region with 80 nm thickness taken at (a) g = 200, (b) g = $11 \bar{1}$, (c) g = $1 \bar{1} 1$, and (d) g = 0 $\bar{2} 2$ near [011] zone axis at the fluence of 2.5 × 1015 He+/cm2. Solid lines represented the visible dislocation loops. Dotted lines represented the invisible loops. The red triangle, green circle, and orange rectangle were the loop family ‘A1’, family ‘B1’, and family ‘B2’, respectively. (e) and (f) showed the sketch of projected loops at g = 200 and g = 0 $\bar{2} 2$ diffraction conditions, respectively.

Fig. 7. In-situ bright-field TEM images showing the characteristics of dislocation loops at the monitored regions with different thicknesses: (a) and (c) 50 nm thickness; (b) and (d) 80 nm thickness. (a) and (b) Irradiation with the fluence of 9.9 × 1014 He+/cm2; (c) and (d) Irradiation with the fluence of 3.5 × 1015 He+/cm2.

Fig. 8. The average size and volume number density of dislocation loops at the monitored regions with 50 nm and 80 nm thickness as a function of irradiation fluence.

Fig. 9. The size of the loops marked letter ‘n’, ‘o’, ‘p’ and ‘q’ at the monitored region with 50 nm thickness in Fig. 7(c) as a function of irradiation fluence.

Fig. 10. In-situ observation of the interaction among dislocation loops at the monitored region with 85 nm thickness at the fluence of (a) 5.0 × 1015 He+/cm2; (b) 6.9 × 1015 He+/cm2; (c) 8.9 × 1015 He+/cm2; and (d) 1.1 × 1016 He+/cm2. The diffraction conditions: (a), (b1), (c) and (d) g = 200; (b2) g = $11 \bar{1}$ ; and (b3) g= $\bar{1}$ $1 \bar{1}$ near the [011] zone axis.

Fig. 11. Bubbles and dislocation loops at the monitored region with 85 nm thickness in the Pd irradiated with the fluence of 1.1 × 1016 He+/cm2.