A novel multi-scale microstructure to address the strength/ductility trade off in high strength steel for fusion reactors

As well as having suitable mechanical performance, fusion reactor materials for the first wall and blanket must be both radiation tolerant and low activation, which has resulted in the development of reduced activation ferritic/martensitic (RAFM) steels. The current steels suffer irradiation-induced hardening and embrittlement, such that they are not adequate for planned commercial fusion reactors. Producing high strength, ductility and toughness<jats:bold> </jats:bold>is difficult, because inhibiting deformation to produce strength also reduces the amount of work hardening available, and thereby ductility. Here we solve this dichotomy to introduce a high strength and high ductility RAFM steel, produced by a novel thermomechanical process route. A unique trimodal multiscale microstructure is developed, comprising nanoscale and microscale ferrite, and tempered martensite with low-angle nanograins. Processing induces a high dislocation density, which leads to an extremely high number of nanoscale precipitates and subgrain walls. High strength is attributed to the refinement of the ferrite grain size and the nanograins in the tempered martensite, while the high ductility results from a high mobile dislocation density in the ferrite, the higher proportion of MX carbides, and the trimodal microstructure, which improves ductility without impairing strength.</jats:p>