Investigation of mechanical property changes in He2+ ion irradiated MA957 through nanoindentation and in situ micro-tensile testing

Abstract A sample of oxide dispersion strengthened (ODS) MA957 steel was irradiated with 5 MeV He2+ ions in the STAR accelerator and its mechanical property changes were tested by nanoindentation and also in tension using an in situ micro-mechanical testing device inside a scanning electron microscope. The ion irradiation process was accomplished using an Al degrader wheel to obtain a relatively uniform damage profile compared to single energy irradiation, with an average dose of ~1.5 displacements per atom (dpa) up to a depth of ~ 10 μm. The small grain size (0.25 – 1.7 μm) of the material enabled the micro-tensile samples (~ 5 × 5 × 17 μm) to be treated essentially as polycrystalline specimens. The results of the tensile tests before and after the radiation are analysed with the aid of strain measurement through a digital correlation method. These results are then compared with nanoindentation hardness measurements. Transmission electron microscopy images reveal a high density of fine (1 nm diameter) bubbles, along with a relatively low density of dislocation loops. An estimation of strengthening due to these sources using the dispersed barrier hardness model shows that a root sum square superposition law predicts the increase in strength to a good degree of accuracy. It is apparent that a major fraction of the increase in strength due to ion irradiation could be attributed to He bubbles, and only a relatively small proportion of the hardening to dislocation loops and black dot damage.