Fig. 1. (a) Cross-section TEM image of helium-implanted α-Zr. The variation of the radiation damage (orange curve) and the helium concentration (blue curve) with depth is estimated by SRIM. The peak helium concentration locates between two white dotted lines in (a); (b, d) high density of helium bubbles with an average diameter of 2.2 nm in α-Zr irradiated at 200 °C. (c, e) enlarged images of dashed squares in (b) and (d), respectively. The inserts in (d) and (e) are selected diffraction and FFT patterns, which indicate ordered helium bubbles structure; (f) ordered helium bubbles with average size of 4.1 nm in α-Zr after annealing at 550 °C for 1 h; (g) enlarged TEM image of helium bubbles in (f); (h) ordered helium bubbles with an average diameter of 6.2 nm in α-Zr after annealing at 650 °C for 1 h; (i) HRTEM image of helium bubbles in (h). The FFT pattern inserted indicates the reserved bubble superlattice after annealing treatment; (j, k) faceted helium bubbles formed in α-Zr after helium implantation at 400 °C.

Fig. 2. Typical engineering stress-strain curves for FD-Zr (blue curve) and HB-Zr (orange curve) with different bubble size. The black arrow indicates the trend of yield stress of FD-Zr and HB-Zr: (a) samples are fabricated from A1 (with an oxygen content of 140 ppm) and the diameter of helium bubble is 6.2 nm. The inserted TEM images are the corresponding fracture morphology of the FD-Zr and HB-Zr curves; (b) The samples were cut from A2 (with an oxygen content of 950 ppm) and the diameter of helium bubble is 29.4 nm. The inserts are the dark field image of the fracture surface of FD-Zr and the bright field image of the fracture morphology of HB-Zr.

Fig. 3. Typical SEM images of fracture surface for FD-Zr and HB-Zr with loading axis of [11-20]: (a) SEM image of FD-Zr showing a cleavage fracture surface along prismatic plane; (b) SEM image of HB-Zr demonstrating evidence of bubble coalescence induced fracture. The inserts are the side view of the samples.

Fig. 4. Typical fracture surface of FD-Zr and HB-Zr (d = 22.3 nm) with loading axis of [0-335]: (a) schematic displaying loading axis and viewing direction; (b) random fracture surface formed in FD-Zr indicating by dashed line; (c) near-cleavage fracture along basal plane forming in HB-Zr.

Fig. 5. Variation of bubble spacing and Δτ for prismatic slip vs. bubble diameters. The insets are images of helium bubbles with different sizes and spacing. The solid blue line is the prediction by the Friedel equation. The red solid fitting line and dotted fitting line are the variations of the spacing of surface-to-surface and spacing of center-to-center, respectively.