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Reduced activation ferritic/martensitic steels are currently the most technologically mature option for the structural material of proposed fusion energy reactors. Advanced nextgeneration higher performance steels offer the opportunity for improvements in fusion reactor operational lifetime and reliability, superior neutron radiation damage resistance, higher thermodynamic efficiency, and reduced construction costs. The two main strategies for developing improved steels for fusion energy applications are based on (1) an evolutionary pathway using computational thermodynamics modelling and modified thermomechanical treatments (TMT) to produce higher performance reduced activation ferritic/martensitic (RAFM) steels and (2) a higher risk, potentially higher payoff approach based on powder metallurgy techniques to produce very high strength oxide dispersion strengthened (ODS) steels capable of operation to very high temperatures and with potentially very high resistance to fusion neutron-induced property degradation. The current development status of these nextgeneration high performance steels is summarized, and research and development challenges for the successful development of these materials are outlined. Material properties including temperature-dependent uniaxial yield strengths, tensile elongations, high-temperature thermal creep, Charpy impact ductile to brittle transient temperature (DBTT) and fracture toughness behaviour, and neutron irradiation-induced low-temperature hardening and embrittlement and intermediate-temperature volumetric void swelling (including effects associated with fusionrelevant helium and hydrogen generation) are described for research heats of the new steels.

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Some mechanically-alloyed (MA), oxide dispersion strengthened (ODS) ferritic alloys have exhibited dramatically improved high temperature creep properties compared to other ferritic alloys. These alloys are fabricated by ball milling powders of the prealloyed metal with an oxide powder. Atom probe tomography (APT) has revealed that some of these MA/ODS alloys [e.g., 12YWT] contain a high number density of Ti-, Y-, O-enriched particles that are stable to at least 1300°C (~85% T m ) [1]. In this paper, a commercial MA/ODS ferritic alloy MA957 is characterized by APT to determine if these ultrafine particles are a common feature of these MA/ODS ferritic alloys.The commercial MA957 alloy used in this study had a nominal composition of Fe-14 wt % Cr, 0.9% Ti, 0.3% Mo and 0.25% Y 2 O 3 [Fe-14.8 at. % Cr, 0.17% Mo, 1.0% Ti, 0.13% Y and 0.19% O] and contained trace levels of Al, Mn, Si, B and C. Specimens were cut from an extruded tube. This alloy was characterized in the as-received state and after annealing for 1 h at 1300 °C. TEM revealed partial recovery of the dislocation structure but no recrystallization had occurred during the high temperature anneal.The solute distribution in this MA957 alloy is shown in the atom maps in Fig. 1. A high number density (~2 x 10 24 m -3 ) of ultrafine Ti-, Y-and O-enriched particles are evident in the as-received condition, Fig. 1a. The number density of the particles decreased by an order of magnitude to ~2 x 10 23 m -3 after the 1 h at 1300°C anneal, Fig. 1b. The average Guinier radius of the particles, r G , was determined with the use of the maximum separation method [2] to be 1.2 ± 0.4 and 1.7 ± 0.7 nm, respectively for the as-received and 1 h at 1300 °C conditions. These results indicate that some coarsening of the particles had occurred during the anneal. A representative atom map of a 2-nmthick slice through the center of one of the particles in the annealed material is shown in Fig. 2. The Y atoms were predominantly found in the central region and surrounded with a Ti-and O-enriched shell, as apparent by the TiO 2+ ions. This differing solute distribution is also evident in the atom maps shown in Fig. 1. In both conditions, r G (Y) was approximately 90% of the overall r G value. The compositions of the individual particles were determined by the envelope method [2] with a grid spacing of 0.1 nm. The average compositions and the solute partitioning factors are given in Table 1. The oxygen content in the matrix was estimated to be <0.14 and <0.19 at. % O, respectively for the as-received and annealed conditions. These estimates are upper bounds due to the possible presence of Mo 3+ and Ti 3+ ions superimposing with the TiO 2+ and O + ions, respectively. These results are similar to previous APT studies of a 3 wt% W, Mo-free MA/ODS 12YWT ferritic alloy [1]. [3] [1] D.

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