Investigation of helium accumulation in implanted ODS steels

Our work was focused on the irradiation of the commercial ODS steels (MA 956, ODM 751 and MA 957) by helium ions and study of their microstructural changes in term of vacancy defects, helium presence and hardness up to the depth of 1.2 μm. The main purpose of our work was to detect helium in structure of implanted high chromium ODS steels. This paper studied the helium accumulation in microstructure of helium implanted ODS steels. During the research we applied new published methods of positron annihilation spectra treatment, but the data showed different results than was assumed. The steel MA 956 was in a good agreement with the data for Fe-9Cr model alloys, thus helium was probably accumulated in the depth of 1000 nm, which was confirmed also by a strong blistering effect in this layers and approx. 1 μm thick layer was removed just due to helium accumulation in vacancy defects and helium bubbles formation. On the other hand, the steels MA 957 and ODM 751 did not demonstrated visibly helium in positron data. However, from the hardness measurement we assume that helium could have been accumulated and tapped in layers closer to subsurface, which is not able to detect further by positron measurements due to overlapping of the radiation and helium peaks. (authors)