Helium bubble microstructure in SS 316L, 20% c.w. and its correlation to the in-beam mechanical properties

The helium bubble microstructure of in-beam creep ruptured specimens of AISI 316 L, 20% c.w. was investigated in detail in order to identify the reasons for the catastrophic embrittlement of this material at 873 K compared to other materials. The main reason seems to be the inhomogeneous precipitate structure in the grain boundaries leading to a bimodal bubble size distribution. Using the modal size of the larger bubbles instead of the mean radius in comparison with the critical radius explains the higher susceptibility of this alloy for grain boundary embrittlement. In contrast to higher temperatures for which gas driven growth of the grain boundary bubbles up to the critical radius was the mechanism controlling the rupture time, it is shown from microstructural data that at 873 K this mechanism does not apply. Instead the microstructural data are in agreement with the assumption that the creep constant growth of the grain boundary bubbles determines the rupture time. This is also deduced from the stress dependence of the in-beam rupture times compared with that of the unimplanted controls. (orig.)