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 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.