Effects of helium implantation rate on microstructure and helium embrittlement

Simulation studies of helium effects in future fusion reactor materials are done by fission reactor irradiations (helium generation via (n,α) nuclear reactions) and by helium implantations. The helium generation rates via (n,α) reactions and the helium implantation rates, respectively, are usually orders of magnitude apart, so that it is difficult to correlate the data without knowing the influence of the different rates. Therefore we have investigated rate effects by studying the helium bubble microstructure in different stainless steel samples after hot implantation (873-1073 K) to concentrations between 1 and 30 appm He using rates between 3.10-5 and 3.10-2 appmHe/s. Generally the resulting helium bubble microstructure is the coarser the lower the rate. This induces increased embrittlement effects in subsequent creep and tensile tests with Ttest≥950 K. At Ttest=873 K no pronounced embrittlement and therefore no effect of the implantation rate could be detected