Effect of helium on tensile properties and microstructure in 9%Cr–WVTa–steel after neutron irradiation up to 15dpa between 250 and 450°C

Helium formation in reduced-activation martensitic-ferritic (RAFM) 8―10%Cr―WTaV steels and their impact on mechanical properties are primary subjects of fusion materials research. In this connection, three heats were melted with different nat B- and 10 B-contents on the basis of the chemical composition of EUROFER97. EUROFER97 had the lowest content of nat B < 10 ppm. Two heats were alloyed with boron, one with ∼82 ppm nat B and the other with ∼83 ppm 10 B, and a third heat with a very high concentration of ∼1160 ppm 10 B. Tensile specimens were irradiated up to ∼15 dpa at 250-450 °C. The post-irradiation tensile tests showed the typical strengthening between 250 and 350 °C. The effect of the dpa and irradiation temperature was explained by microstructural changes, such as dislocation loops, α'-precipitates, and helium bubbles. The lower 10 B containing specimens exhibited a ductile fracture appearance, but with a loss in elongation. The specimens with the highest concentration of 10 B exhibited always a brittle fracture appearance. The phenomena observed in the microstructures and fractures were correlated with the mechanical properties.