Pronounced inhibitory mechanism of Cr on vacancy-like defects in Fe9Cr alloy irradiation with helium ions

Abstract

• First-principles calculations combined with positron annihilation spectroscopy. • The He atoms aggregate among (1 1 0) planes in the perfect Fe and Fe9Cr models. • Fe atoms around the He clusters are inclined to emit spontaneously. • The doping of Cr repels the He atoms and weakens the growth of He clusters. First-principles calculations and positron annihilation spectroscopy were used to investigate the effect of Cr atoms on He-induced vacancy-like defects in pure Fe and Fe9Cr alloys. The He atom is not energetically favorable at both the surface and the subsurface, but it becomes stable under the third atomic layer from the surface. The diffusion of He atoms to the vacuum and aggregation of He atoms into clusters coexist. We observed that the interstitial He n clusters prefer to stay along the (1 1 0) plane. Moreover, the small He n clusters induce strong lattice distortions, and the emission of a self-interstitial Fe atom located at the adjacent He n cluster is energetically favorable for n > 2 and vacancy formation energies of Fe9Cr are higher than that of Fe. This suggests that Cr atoms could effectively enhance the stability of Fe atoms around He n clusters but that the Fe atoms would still like to shift from the vicinity of He n to the surfaces of the models. In the experiment, Fe and Fe9Cr alloys were irradiated with He ions to a dose of 1 × 10 16 cm 2 and the positron annihilation results showed that the Fe9Cr alloy has a smaller relative S (ΔS) parameter value than pure Fe. The experimental results matched well with the theoretical analysis. These results would help provide additional insights into the behavior of He in irradiated Fe and Fe9Cr surfaces, necessary for the interpretation of the growth of the He bubble.