Formation and migration of helium bubbles in Fe and Fe–9Cr ferritic alloy

Formation and migration of helium bubbles in high purity Fe and an Fe–9Cr ferritic alloy have been studied by in situ electron microscopy and scanning transmission electron microscope–electron energy loss spectrometry (STEM–EELS) analysis. The swelling under irradiation with 10 keV He+ ions in the ferritic alloy was retarded in the low fluence region at 400 °C and was reduced at 600 °C even under heavy irradiation. These results suggest trapping effects of vacancies by Cr and its modulation with helium atoms in the bubble formation. Furthermore, it is found that the mean square of the bubble migration distance in both materials is proportional to time, which is a quantitative evidence of Brownian type motion and yields lower diffusivities of the bubbles in Fe–9Cr than in Fe. The retardation of the bubble mobility in Fe–9Cr should be caused by Cr segregation on the bubble surface which was revealed by STEM–EELS analysis.