Helium blistering at high irradiation doses

The helium blistering in a Cr-Ni alloy and an austenite steel was investigated at 40 keV ion energy at doses from 3.1018 cm−2 to 1020 cm−2 at room temperature. At doses D ⩽ 3.1018 cm−2 most of the blisters are unruptured. At the dose of 6.1018 cm−2 one exfoliated skin layer and the formation of the second and third generations of blisters are observed. The diameter of blisters decreases from generation to generation, and after the last generation of blisters further increase of dose results in sputtering of the surface and in formation of a porous surface structure. At D ⩾ (6.1019 to 1020) equilibrium sponge-like structures are formed. A finite number of generations of blisters, depending on ion energy, is observed. Samples prebombarded with 40 keV He ions (D = 1020 cm−2) were irradiated by 100 keV He ions to a dose of 2.1019 cm−2. After such irradiation the surface had the same sponge-like structure as before 100 keV irradiation. Theoretical analysis shows: (1) blisters of subsequent generations can be formed only where blister covers of previous generations are opened; (2) the blister diameter decreases from one generation to the next one. Hence, only a finite number of blister generations can be formed on a given surface. The number of possible blister generations is estimated as a function of ion energy.