Effect of purity on helium bubble formation in 9Cr martensitic steel during post-implantation annealing at 1105 K

Purification is essential to development of low-activation ferritic-martensitic steels for fusion reactor materials, not only from the practical viewpoint of reducing highly radioactive elements but also from the fundamental viewpoint of irradiation effects in ferritic steels and Fe-Cr alloys. A high-purity 9Cr-0.1C martensitic steel was prepared from an ultra-high-purity iron and high-purity alloying elements. In order to study the effect of large amount of helium on microstructure in the high-purity martensitic steel, 1000 appm of helium ions were implanted into the steel, and then TEM observation was performed after post-implantation annealing. It was observed that small cavities were distributed more densely in the high-purity steel than in commercial-grade ones. Moreover, the denuded zone near grain boundaries in the high-purity steel was much thinner than that in commercial-grade ones. The difference in cavity formation between the high-purity and commercial-grade steels is discussed. (orig.)