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Detailed knowledge of the microstructural evolution of reduce activation ferritic-martensitic steel EUROFER97 after exposure at high temperatures is essential for determining its applications potential. For this proposal, EUROFER97 was annealed in the temperature range between 450 °C and 650 °C for up to 122,000 h (≈14 years) and subsequently analyzed using transmission electron microscopy (TEM) including high resolution TEM and two-dimensional energy dispersive X-ray (EDX) mapping. The study demonstrates the effects of thermal treatment on the size and composition of the precipitates and allows conclusions about their stability. Application of the extraction replication technique was used to analyze composition and morphology of four particle types present in the untreated EUROFER97: M23C6, VN, TaC and TiN with sufficient statistics. The rapid coarsening of the M23C6 precipitates was observed at 650 °C, while the MX particles were found to be more stable upon thermal treatment. It has been proved that new Laves (WFe2) and modified Z-phases (Cr(V,Ta)N) precipitates are formed in the temperature range from 500 °C to 600 °C. The detailed analysis allows the drawing a time–temperature formation diagram for these two phases, which could be valid for alloys with composition similar to EUROFER97.

The paper presents a detailed analysis of helium (He) bubble development in ODS-EUROFER steel caused by helium ion implantation in different regimes, with a particular attention to the role of the oxide nanoparticles in promoting the growth of He bubbles, helium accumulation and gas-driven swelling. The Transmission Electron Microscopy (TEM) characterization of steel samples implanted applying systematic variation of experimental parameters has allowed clarifying the trends of the bubble microstructure evolution depending on the implantation dose, flux, and sample temperature. It was found that in all investigated implantation regimes He bubbles formed both in the grain bulk and on various structural defects (dislocations, grain boundaries, oxide particles and carbide precipitates), but the sizes and densities of bubbles in different bubble populations were sensitive to particular irradiation conditions. In the majority of cases the main traps for implanted helium and the main contributors to the estimated swelling were bubbles associated with grain boundaries, though in some cases (high implantation dose or lower temperature) the bubbles in the grain bulk were competitive with the grain boundary bubble population. Oxide particles in ODS-EUROFER were found to be excellent nucleation sites for He bubbles and practically each observed particle hosted a single relatively large bubble, sometimes as large as the particle itself. However, the contribution of oxide-associated bubbles to the estimated swelling and He inventory was found to be minor as compared to other bubble populations because of a relatively low number density of nano-oxides. Comparison of ODS-EUROFER and EUROFER 97 samples implanted with He ions in identical regimes has demonstrated lower efficiency of ODS-EUROFER for accumulating implanted helium in bubbles and noticeably higher share of helium atoms trapped in the vacancy defects invisible by TEM.

The micro-structural effects of different neutron irradiation temperatures and helium contents, for 16 dpa dose, have been investigated by means of small-angle neutron scattering (SANS) in B-alloyed ferritic/martensitic steel Eurofer97-1 (0.12 C, 9 Cr, 0.2 V, 1.08 W wt%, B concentrations up to 1000 ppm); due to B transmutations, fusion relevant He/dpa values are expected to be produced under neutron irradiation. SANS measurements have been carried out on a sample irradiated at 350 °C, with estimated helium content of 5600 appm, and compared to previous SANS results, obtained on two other irradiated samples of this same B-alloyed steel. These new measurements confirm that for such high helium contents the SANS cross-section increases in order of magnitude and the magnetic SANS component is strongly reduced, compared to lower helium content (400 appm). Such effects are attributed to increase in helium bubbles density and to the presence of micro-cavities, produced after dissolution of large B-carbides. The SANS data analysis procedure has been improved, also thanks to the additional information provided by the new measurements, and more accurate helium bubble size distributions have been obtained for all the investigated samples. For 5600 appm helium content, bubble volume fractions are found of 0.025 for the sample irradiated at 350 °C and of 0.041 for the previously investigated sample irradiated at 400 °C, significantly increasing with the irradiation temperature. These values are approximately one order of magnitude larger than the value of 0.003 previously found for the sample with 400 appm helium. The size distributions are compared with electron microscopy observations of these same samples. It appears that the occurrence of complex micro-structural changes in irradiated Eurofer97-1 steel should be taken in due account when considering its application under high He/dpa ratio values. Keywords: Helium effects, Neutron irradiation, Small angle neutron scattering, Electron microscopy

The 9Cr steels EUROFER and F82H-mod are the candidate materials for future fusion reactors. The extension of the operation limits including temperature, strength and toughness are still the scope of ongoing research. In a pulsed reactor operation, fatigue lifetime is one of the major properties for the steels. While the oxide dispersion strengthened EUROFER-ODS variant showed significant improvements in this area, the production costs and availability of large quantities of materials drastically limits its applications. In the present study, different surface nitriding treatments of EUROFER972 have been performed and the impact on microstructure, dynamic fracture toughness and high temperature fatigue has been analysed. Four different states of EUROFER including different heat treatments, nitriding of the surface and the ODS variant are tested and compared in this work. Low cycle fatigue tests show the improvements after certain treatments. Charpy impact tests and microstructural investigation by scanning electron microscopy and analytical transmission electron microscopy are also performed to compare the materials against the reference (EUROFER97). While conventional gas nitriding showed no beneficial effect on the material, the Hard-Inox-P treatment showed a significant improvement in the cycles to failure while retaining an acceptable toughness. Microstructural investigations showed the presence of very small chromium- and nitrogen-rich precipitates in the area close to the surface.

This contribution presents a comparative microstructural investigation, carried out by Small-Angle Neutron Scattering (SANS), of ferritic/martensitic steel Eurofer97 (0.12 C, 9 Cr, 0.2V, 1.08Wwt%) neutron irradiated at two different neutron sources, the HFR-Petten (SPICE experiment) and the BOR60 reactor (ARBOR experiment). The investigated “SPICE” sample had been irradiated to 16dpa at 250°C, the investigated “ARBOR” one had been irradiated to 32dpa at 330°C. The SANS measurements were carried under a 1 T magnetic field to separate nuclear and magnetic SANS components; a reference, un-irradiated Eurofer sample was also measured to evaluate as accurately as possible the genuine effect of the irradiation on the microstructure. The detected increase in the respective SANS cross-sections of these two samples under irradiation is attributed primarily to the presence of micro-voids, for neutron contrast reasons; it is quite similar in the two samples, despite the higher irradiation dose and temperature of the “ARBOR” sample with respect to the “SPICE” one. This is tentatively correlated with the higher helium content produced under HFR irradiation, playing an important role to stabilize the micro-voids under irradiation. In fact, the size distributions obtained by transformation of the SANS data yield a micro-void volume fraction of 1.3% for the “SPICE” sample and of 0.6% for the “ARBOR” one.

During the Engineering Validation and Engineering Design Activities (EVEDA) phase (2007-2014) of the International Fusion Materials Irradiation Facility (IFMIF), an advanced engineering design of the High Flux Test Module (HFTM) has been developed with the objective to facilitate the controlled irradiation of steel samples in the high flux area directly behind the IFMIF neutron source. The development process addressed included manufacturing techniques, CAD, neutronic, thermal-hydraulic and mechanical analyses complemented by a series of validation activities. Validation included manufacturing of 1:1 parts and mockups, test of prototypes in the FLEX and HELOKA-LP helium loops of KIT for verification of the thermal and mechanical properties, and irradiation of specimen filled capsule prototypes in the BR2 test reactor. The prototyping activities were backed by several R&D studies addressing focused issues like handling of liquid NaK (as filling medium) and insertion of Small Specimen Test Technique (SSTT) specimens into the irradiation capsules. This paper provides an up-todate design description of the HFTM irradiation device, and reports on the achieved performance criteria related to the requirements. Results of the validation activities are accounted for and the most important issues for further development are identified.

Excellent mechanical properties of ODS steels are directly related to the high density of homogeneously distributed, well-formed oxide particles (such as Y2O3, or Y-Ti-O). However, atom probe tomography study of ODS steels revealed that in addition they contain almost a hundred times more nanoclusters enriched in Y, O and V/Ti (if present in the alloy composition) than larger oxide particles. In this work, we carried out atom probe tomography (APT) and transmission electron microscopy (TEM) studies of three different ODS steels produced by mechanical alloying: ODS Eurofer, 13.5Cr ODS and 13.5Cr-0.3Ti ODS. These materials were investigated after irradiation with Fe (5.6MeV) or Ti (4.8MeV) ions up to 1015ion/cm2 and part of them up to 3×1015ion/cm2. In all cases, areas for TEM investigation were cut at a depth of ∼ 1.3µm from the irradiated surface corresponding to the peak of the radiation damage dose. It was shown that after irradiation at RT and at 300°С the number density of oxide particles in all the samples grew up. Meanwhile, the fraction of small particles in the size distribution has increased. APT revealed an essential increase in nanoclusters number and a change of their chemical composition at the same depth. The nanostructure was the most stable in 13.5Cr-0.3Ti ODS irradiated at 300°С: the increase of the fraction of small oxides was minimal and no change of nanocluster chemical composition was detected.

High He/dpa microstructural effects have been investigated, by means of small-angle neutron scattering (SANS) and transmission electron microscopy (TEM), in B-alloyed ferritic/martensitic steel Eurofer97-1 (0.12 C, 9 Cr, 0.2V, 1.08W wt%, B contents variable between 10 and 1000ppm), neutron irradiated at the High Flux Reactor of the JRC-Petten at temperatures between 250 °C and 450 °C, up do a dose level of 16 dpa. Under these irradiation parameters, B activation is expected to produce corresponding helium contents variable between 80 and 5600appm, with helium bubble distributions relevant for the technological applications. The SANS measurements were carried out under magnetic field to separate nuclear and magnetic SANS components; a reference, un-irradiated sample was also measured to evaluate as accurately as possible the genuine effect of the irradiation on the microstructure. Increasing the estimated helium content from 400 to 5600appm, the analysis of the SANS cross-sections yields an increase in the volume fraction, attributed to helium bubbles, of almost one order of magnitude (from 0.007 to 0.038); furthermore, the difference between nuclear and magnetic SANS components is strongly reduced. These results are discussed in correlation with TEM observations of the same samples and are tentatively attributed to the effect of drastic microstructural changes in Eurofer97-1 for high He/dpa ratio values, possibly relating to the dissolution of large B-carbides due to transmutation reactions.