Characterization of helium-vacancy complexes in He-ions implanted Fe9Cr by using positron annihilation spectroscopy.

The formation of helium bubble precursors, i.e., helium-vacancy complexes, was investigated for Fe9Cr alloy, which was uniformly irradiated by using 100 keV helium ions with fluences up to 5 × 10 16 ions/cm 2 at RT, 523, 623, 723, and 873 K. Helium-irradiation-induced microstructures in the alloy were probed by positron annihilation technique. The results show that the ratio of helium atom to vacancy (m/n) in the irradiation induced He m V n clusters is affected by the irradiation temperature. Irradiated at room temperature, there is a coexistence of large amounts of He m V 1 and mono-vacancies in the sample. However, the overpressured He m V n (m > n) clusters or helium bubbles are easily formed by the helium-filled vacancy clusters (He m V 1 and He m V n (m ≈ n)) absorbing helium atoms when irradiated at 523 K and 823 K. The results also show that void swelling of the alloy is the largest under 723 K irradiation. [ABSTRACT FROM AUTHOR]