Correlation between embrittlement and bubble microstructure in helium-implanted materials

In order to obtain further knowledge on the helium embrittlement phenomenon, bubble size distributions in helium-implanted and creep-ruptured Fe–Ni–Cr materials have been analyzed from the standpoint of bubble stability. The observed mean distance of intercrystalline helium bubbles exceeding the minimum critical size was compatible with the pore spacing on intergranularly fractured surfaces. This fact strongly suggests that unstable expansion of grain boundary bubbles with super-critical sizes acted as a working mechanism of helium embrittlement. It was found from calculations estimating the helium atom number in individual bubbles that the increase of helium retention in the inside of grains would reduce helium embrittlement to some extent.