Helium transport, fate and management in nanostructured ferritic alloys: In situ helium implanter studies

High helium levels produced in fusion neutron spectra may lead to severe increases in the brittle fast fracture temperature, enhanced void swelling and degradation of creep rupture properties at lower, intermediate and higher irradiation temperatures, respectively. Thus it is important to develop structural alloys with stable microstructures that can manage helium based on understanding of its transport, fate and consequences. We report on the initial results of a study of helium in a nanostructured ferritic alloy (NFA), MA957, that is dispersion strengthened by an ultra-high density of nm-scale Y–Ti–O nanofeatures (NFs). An in situ helium implanter technique uniformly deposited ≈380 appm helium to ≈6 μm in MA957 irradiated in the High Flux Isotope Reactor (HFIR) to ≈9 dpa at 500 °C. Through focus transmission electron microscopy (TEM) showed that helium is in extremely fine bubbles that often appear to coincide with bright field features taken as a NF.