Characterisation of MX Precipitate Density and Irradiation Hardening in Advanced Reduced-Activation Ferritic-Martensitic Fusion Steels
Reduced activation ferritic-martensitic (RAFM) steels are a recent class of radiation-resistant steels designed for the structural components of power-producing fusion reactors. In this work an advanced (A)RAFM steel has been developed with superior radiation hardening resistance with respect to the EUROFER-97 upon which it was based. 4D-STEM (scanning transmission electron microscopy) has been combined with a novel processing methodology to visualise all the fine MX precipitates that led to this outstanding radiation hardening resistance and determine a precipitate density of 5 × 1022 m-3. Self-ion irradiation campaigns up to 100 dpa at 350˚C show an increase in hardness of only 35% at 10 dpa where EUROFER-97 exhibits a near-doubling of its hardness. The initial work hardening response, as determined from spherical nanoindentation, is unchanged between the as-received state and irradiation to 100 dpa, implying that the alloy should retain reasonable ductility under these conditions. Proton irradiations at 250˚C, 350˚C, and 400˚C demonstrate that the low temperature hardening embrittlement threshold of the new steel is largely unaffected, increasing by only ~50˚C with respect to EUROFER-97. A refinement of alloy chemistry and a subsequent modification of the thermomechanical treatments to favour MX precipitates is therefore a very promising strategy for the further development of fusion steels.