Reduced-Activation Ferritic/Martensitic Steels for Fusion Reactors: A Critical Review of Microstructure, Mechanical Behavior, and Irradiation Response

Reduced-activation ferritic/martensitic (RAFM) steels are the reference structural materials for the first wall and breeding blanket of ITER, DEMO, and the fusion power plants that will follow. They were obtained in the early 1980s by reengineering the chemistry of the qualified creep-resistant power plant–grade Mod-9Cr–1Mo (Grade 91/P91): molybdenum was replaced by tungsten and niobium by tantalum, and residual nickel, copper, and nitrogen were suppressed, so that the isotopes generated under a 14-MeV fusion neutron spectrum decay to hands-on activity within roughly a century.The attraction of the family for fusion is a balance of properties, high thermal conductivity, low thermal expansion, strong resistance to void swelling relative to austenitic steels, a comparatively benign response to transmutation helium, and a mature, inexpensive melt-forge-roll production route, rather than any single figure of merit.This review synthesizes more than 3 decades of international work on the principal national grades, F82H and JLF-1 (Japan), EUROFER97 and the developmental OPTIFER/MANET alloys (Europe), CLAM (China low activation martensitic steel) (China), INRAFM (Indian reduced-activation ferritic/martensitic steel) (India), ARAA (advanced reduced-activation alloy) (Korea), and the oxide dispersion–strengthened (ODS) variants, and organizes it around a single recurring theme: across nearly every property of consequence to fusion, the behavior of these steels is governed by the prior austenite grain structure and by the type, size, and spatial distribution of the two carbide populations that decorate the tempered martensite matrix, namely coarse Cr-rich M23C6 on boundaries and fine MX (M = V, Ta, Nb and X = C, N) carbonitrides within the laths.We examine, in turn, the alloy family and its metallurgy, phase transformations and thermal stability, thermophysical properties, and the full mechanical response, including tensile behavior, creep (analyzed through the Larson-Miller and Monkman-Grant frameworks), low- and high-cycle fatigue and fatigue crack growth, and fracture toughness and the ductile-to-brittle transition described by the master curve, before turning to the irradiation degradation that fixes the operating window at roughly 325°C to 550°C, to welding, and to the ODS, castable-nanostructured, and architectured alloys now being developed to widen that window.Throughout, we distinguish the established results from the contested interpretations, identify where the database is thin, and highlight the qualification gaps, such as the synergy of helium and displacement damage, irradiation creep, and the behavior of welds and large heats under a fusion-relevant spectrum, that still stand between these steels and a demonstration power plant.Four themes are developed here in more depth than earlier surveys of this material class have attempted: (1) the irradiation-induced defect microstructure, which is now described quantitatively in terms of the measured densities and sizes of the dislocation loops, cavities, and irradiation-modified precipitates as functions of dose and irradiation temperature, is connected to the macroscopic hardening through a dispersed barrier framework and to the transition temperature shift through the change in the strain-hardened flow stress; (2) the techniques used to simulate fusion damage, including fission reactor irradiation, isotopic doping, spallation sources, and heavy ion beams, are compared explicitly, together with what each can and cannot reproduce; (3) the behavior of helium is treated mechanistically through its migration, trapping, and detrapping and the resulting bubble-to-void transition; and (4) the position of RAFM steel relative to the competing candidate structural materials, ODS-RAFM and nanostructured ferritic alloys, SiCf/SiC composites, tungsten alloys, and vanadium alloys, is set out so that the boundary of applicability of the RAFM family can be stated rather than implied.