Peculiarities of helium porosity evolution in the ferritic-martensitic steels produced by spark plasma sintering

Oxide dispersion strengthened (ODS) ferritic-martensitic steels are considered as promising structural materials for fusion reactors, as well as for active zone of new generations fast reactors. In this connection, peculiarities of helium porosity formation and gaseous swelling have been investigated in the dispersion-strengthened EP-450 ODS steel with 0.3 and 1 wt.% Y2O3 dispersant produced by spark plasma sintering (SPS) as compared with the matrix EP-450 steel, EP-450 ODS steel produced using a hot extrusion (HE) as well as reactor austenitic ChS-68 steel. The samples were irradiated by 40-keV Не+ ions at 923 K up to fluence of 5 × 1020 ion/m2. Microstructural investigations of irradiated samples were performed using a transmission electron microscope. It is found that plurality of zones with a very different type of helium porosity and different character of their distribution is developed in steel with 1 wt.% Y2O3. Such zones are less in steel with 0.3 wt.% Y2O3 as opposed to matrix EP-450 steel, EP-450 ODS steel obtained by HE, and austenitic ChS-68 reactor steel. It is found in comparing the character of helium porosity formation in the matrix steel EP-450, steel EP-450 ODS (HE) and EP-450 ODS (SPS) that bubbles are developed with a smaller average sizes and, therefore, helium swelling is lower in all ODS steels than that in steel EP-450, but for ODS steel made by SPS, swelling is significantly higher than in ODS steel produced by hot extrusion. At the same time, austenitic steel ChS-68 shows a minimum gaseous swelling for the used conditions of helium ion irradiation. An assumption is made that the extremely nonuniform distribution of helium bubbles (gas filled pores) both in volume and size in SPS steel is associated with the initially highly defect structure, including the residual porosity in 1-3% as well as a result of strong redistribution of chromium between ferritic grains and grains of tempered martensite during manufacturing of samples.