The mechanical properties and microstructure of the OPTIMAX series of low activation ferritic-martensitic steels

Polycrystalline specimens of the ferritic-martensitic OPTIMAX A steel have been irradiated on the one hand with neutrons at 523 K to a dose of 2.5 dpa and with 590 MeV protons at ambient temperature and 523 K to doses of about 0.3 and 1 dpa on the other. Charpy tests reveal a shift of the ductile-to-brittle transition temperature from about 190 to 268 K in the neutron-irradiated steel. Proton irradiations at ambient temperature lead to hardening and reduction of tensile ductility of the material. Both phenomena are strongly and positively dependent on dose. After irradiation at 523 K, they appear negligible (at least for the dose of 0.75 dpa). Transmission electron microscopy observations reveal that neutron irradiation leads to the formation of only a few Visible black dots, together with a few faceted cavities, while proton irradiations also produce few visible defect clusters with sizes of about 1-2 nm, with no clear difference in size and density with dose and temperature. As a particular result, from proton irradiations performed at ambient temperature, the embedded carbides become amorphous, while at 523 K they remain crystalline. (C) 2000 Elsevier Science B.V. All rights reserved.