Effects of helium on radiation-induced defect microstructure in austenitic stainless steel

In the construction materials surrounding the spallation neutron source (SNS) mercury target, considerable quantities of transmutation products, particularly hydrogen and helium, will be generated due to the exposure to a high flux of 1 GeV protons and associated neutrons. In an effort to investigate the effects of high helium, therefore, bubble formation and defect clustering processes in AISI 316 LN austenitic steel were studied as a function of helium concentration and displacement damage dose with 360 keV He^- and 3500 keV Fe^- ion beams at 200 °C. Helium irradiation was less effective in producing defects such as black dots and dislocation loops than Fe^- ion irradiation at equivalent displacement dose. On the other hand, the formation of helium bubbles produced a strong depressive effect on the growth of loops and the evolution of line dislocations. The results indicated that the effect of helium bubbles was augmented as the bubble number density and size increased with increasing helium beyond 1 atomic percent (at.%). In such a case, the effect of helium bubbles can be more important than that of radiation-induced defects on the evolution of microstructure and the change in mechanical properties.