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By uncoupling the manufacturability from the design process, additive manufacturing of RAFM steels like EUROFER97 can open significant design freedom for components in fusion reactors, e.g., breeding blankets and divertor applications. As mechanical properties of RAFM steels are based on their specific microstructure and high number density of precipitates, good control of the microstructure of additively manufactured parts has to be ensured. As additive manufactured components are known to possess unique microstructures compared to EUROFER97 from standard technologies, the aim of this paper is to investigate additive manufactured EUROFER97 components and the influence of post-processing steps on their microstructure and in particular on the formation of precipitates. This paper covers the technological fabrication process of EUROFER97 by selective laser melting (SLM), including the production and chemical analysis of pre-alloyed EUROFER97 powder. In the initial state after fabrication, SLM-EUROFER97 components exhibit a bimodal, anisotropic microstructure with large ferritic grains instead of the desired homogeneous, fully martensitic microstructure. Further, neither M23C6 nor MX precipitates were detected in the as-built condition. A heat treatment including hot isostatic pressing, austenitization, quenching and tempering, allows achieving a fully martensitic, uniform microstructure as well as the formation of M23C6 and MX precipitates with similar size and volume density as in conventionally produced EUROFER97. In combination with the chemical analysis of the raw material, impurities stemming from the atomization process forming oxides were detected. However, due to their small size, those are not expected to influence the performance of SLM-processed EUROFER97. The comparison of the as-built condition and the condition after hot isostatic pressing and heat treatments reveals the necessity of appropriate post-processing to tailor the microstructure to one comparable to conventional EUROFER97.

Recent analysis result in significant impact on the design of the Helium Cooled Pebble Bed (HCPB) Breeding Blanket (BB). Especially the main demanding sub-component of the BB in terms of fabrication, the First Wall (FW) is highly affected. As a matter of fact, the relevance of the developments made for the industrial fabrication of the FW used e.g. in the EU HCPB Test Blanket Module (TBM) for ITER is limited: On one hand in terms of the overall dimensions, but also in terms of geometry and cooling channel configuration. Thus, a fabrication strategy for the FW was proposed as an option in 2018 based on Additive Manufacturing (AM) offering a solution to cover the new aspects. The dedicated fabrication routine is based on Cold Spray (CS) in alternation with machining. In terms of manufacturing of non-plasma facing complex shaped thin- or double wall BB structures (e.g. Fuel Pins proposed for the HCPB BB) the use of AM also provides benefits compared to conventional technologies. Therefore, the AM process of Selective Laser Melting (SLM) is also investigated in terms of applicability for BBs. This paper gives an overview about the advances in different AM options applied for nuclear fusion structural low activation steels. Possibilities for spin-offs to other technological fields are discussed and conclusions are drawn reflecting licensing aspects and technological limits.

A major part in the EUROfusion materials research program is dedicated to characterize and quantify nuclear fusion specific neutron damage in structural materials. While the majority of irradiation data gives a relatively clear view on the displacement damage, the effect of transmutation – i.e. especially hydrogen and helium production in steels – is not yet explored very well. However, few available results indicate that EUROFER-type steels will reach their operating limit as soon as the formation of helium bubbles reaches a critical amount or size. At that point, the material would fail due to embrittlement at the considered load. This paper presents a strategy for the mitigation of the before-mentioned problem using the following facts: • the neutron dose and related transmutation rate decreases quickly inside the first wall, that is, only a plasma-near area is extremely loaded • nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium and hydrogen, which would suppress the formation of large helium bubbles • compared to conventional steels, ODS steels show improved irradiation tensile ductility and creep strength In summary, producing the plasma facing, highly neutron and heat loaded part of blankets by an ODS steel, while using EUROFER97 for everything else, would allow a higher heat flux as well as a longer operating period. Consequently, we (1) developed and produced 14 % Cr ferritic ODS steel plates. (2) We fabricated a mockup with 5 cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. And finally, (3) we performed high heat flux tests in the HELOKA facility (Helium Loop Karlsruhe at KIT) applying short and up to 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e., 550 °C) was finally exceeded by 100 K. Thereafter, microstructure and defect analyses did not reveal defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. This demonstrates that the use of ODS steel could make a decisive difference in the future design and performance of breeding blankets.

In future nuclear fusion reactors, plasma facing components have to sustain specific neutron damage. While the majority of irradiation data provides a relatively clear picture of the displacement damage, the effect of helium transmutation is not yet explored in detail. Nevertheless, available results from simulation experiments indicate that 9%-chromium steels will reach their operating limit as soon as the growing helium bubbles extent a critical size. At that point, the material would most probably fail due to grain boundary embrittlement. In this contribution, we present a strategy for the mitigation of the before-mentioned problem using the following facts. (1) The neutron dose and related transmutation rate decreases quickly inside the first wall of the breeding blankets, that is, only a plasma-near area is extremely loaded. (2) Nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium, which would suppress the formation of large helium bubbles for a much longer period. (3) Compared to conventional steels, ODS steels also provide improved irradiation tensile ductility and creep strength. Therefore, a design, based on the fabrication of the plasma facing and highly neutron and heat loaded parts of blankets by an ODS steel, while using EUROFER97 for everything else, would extend the operating time and enable a higher heat flux. Consequently, we (i) developed and produced 14%Cr ferritic ODS steel plates and (ii) optimized and demonstrated a scalable industrial production route. (iii) We fabricated a mock-up with five cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. (iv) Finally, we performed high heat flux tests in the Helium Loop Karlsruhe, applying a few hundred short and a few 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e. 550 ◦C) was finally exceeded by 100 K. (v) Thereafter, microstructure and defect analyses did not reveal critical defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. However, a solution to prohibit the formation of such heat affected zones is given. These research contributions demonstrate that the use of ODS steel is not only feasible and affordable but could make a decisive difference in the future design and performance of breeding blankets.

This paper is the second part of our series on ultrafine-grained (UFG) tungsten produced by cold rolling. The aim of this paper is to determine tensile properties, with a focus on uniform elongation and flow stress, and to identify the mechanism of plastic deformation in cold rolled tungsten plates and foils. For this purpose, five plates have been rolled out of a single sintered compact with increasing degrees of deformation, which enables the analysis of mechanical properties with changing microstructure without influence from the chemical composition. Uniaxial tensile tests have been performed in the range of room temperature to 800 °C. The tensile curves show a clear trend of increased strength for finer microstructures and simultaneously evolving room temperature ductility in the low temperature regime for cold rolled plates. The thinnest foil (100 μm) behaves uniquely by forming a plateau in the stress-strain curve, which hints at a change in the deformation mechanism. Analyzing the temperature and grain size dependence of the flow stress reveals that the bulk plasticity is still controlled by screw dislocations and further suggests a change in the dislocation-grain boundary interactions from blocking at low homologous temperatures to absorbing at elevated temperatures. A fracture analysis of cold rolled tungsten foils confirms a plastic deformation prior to fracture and reveals a steady transition from brittle towards lamellar fracture for the highly deformed tungsten foils.

Surface functionalization by topographic micro- and nano-structures in order to achieve unique properties, like super-hydrophobicity or ultrahigh light absorption, is a common strategy in nature. In this paper, direct laser interference patterning (DLIP) is presented as a promising tool allowing for the generation of such surface patterns on technical surfaces in order to mimic these biological surfaces and effects. Friction optimization and antibacterial effects by DLIP are exemplarily described. Topographic surface patterns on the micro- and nano-scale demonstrated a significant reduction in the coefficient of friction and bacterial adhesion. It was shown that in both cases, the control of the contact area between surfaces or between surface and bacteria is of utmost importance.

Conventionally produced tungsten (W) sheets are brittle at room temperature. In contrast to that, severe deformation by cold rolling transforms W into a material exhibiting room-temperature ductility with a brittle-to-ductile transition (BDT) temperature far below room temperature. For such ultrafine-grained (UFG) and dislocation-rich materials, the mechanism controlling the BDT is still the subject of ongoing debates. In order to identify the mechanism controlling the BDT in room-temperature ductile W sheets with UFG microstructure, we conducted campaigns of fracture toughness tests accompanied by a thermodynamic analysis deducing Arrhenius BDT activation energies. Here, we show that plastic deformation induced by rolling reduces the BDT temperature and also the BDT activation energy. A comparison of BDT activation energies with the trend of Gibbs energy of kink-pair formation revealed a strong correlation between both quantities. This demonstrates that out of the three basic processes, nucleation, glide, and annihilation, crack tip plasticity in UFG W is still controlled by the glide of dislocations. The glide is dictated by the mobility of the screw segments and therefore by the underlying process of kink-pair formation. Reflecting this result, a change of the rate-limiting mechanism for plasticity of UFG W seems unlikely, even at deformation temperatures well below room temperature. As a result, kink-pair formation controls the BDT in W over a wide range of microstructural length scales, from single crystals and coarse-grained specimens down to UFG microstructures.

Different manufacturing routes are investigated at the KIT INR for the realization of First Walls (FW) and internal components for the Helium Cooled Pebble Bed (HCPB) nuclear fusion reactor component Breeding Blanket (BB). As result of cost estimation studies in 2017 and 2018, Additive Manufacturing (AM) has been identified to provide promising options beyond conventional technologies. Selective Laser Melting (SLM) and Direct Energy Deposition (DED) applied as Cold Spray (CS) were addressed. SLM was followed with focus on the development of BB internal structures. CS turned out as a very goal oriented and effective fabrication process with regard to the First Wall geometrical requirements. Further, a large potential for cost reduction compared to other technologies was identified. Based on a variation of the DED process CS applied in alternation with machining and an option for temporary filling cavities, an extended process chain was developed in collaboration with industry. This process chain also enables Laser structuring of internal channel surfaces for heat transfer enhancement structures. This paper summarizes the developments in terms of application of AM for the HCPB BB concept where SLM is focused as an option for BB internal structures assembled as hybrid components and CS addressed for fabrication of the First Wall.

By uncoupling the manufacturability from the design process, additive manufacturing of the baseline material EUROFER97 can open significant design freedom for divertor and breeding blankets in fusion technology. As additive manufactured components are known to possess unique microstructures compared to EUROFER97 from standard technologies, the aim of this paper is to investigate additive manufactured EUROFER97 components and the influence of post processing steps on their microstructure and mechanical properties from a materials science point of view. This paper covers the technological fabrication process of EUROFER97 by selective laser melting (SLM), including the production of pre-alloyed EUROFER97 powder, an SLM-parameter study and the design and production of custom-build thin walled test components by SLM. In the initial state after fabrication, SLM-EUROFER97 components exhibit a bimodal, anisotropic microstructure with large ferritic grains. The fraction of ferritic grains increases with decreasing wall thickness. A heat treatment including austenitization, quenching and tempering, allows to achieve a fully martensitic, uniform microstructure for all wall thicknesses. Therefore, there is no influence of wall thickness on mechanical properties of EUROFER97 produced by SLM to be expected, as long as the SLM-part is submitted to an appropriate heat treatment. Further, the comparison of the initial state after fabrication and after post processing reveals the necessity of both hot isostatic pressing and heat treatment to improve the performance. While all material conditions lead to sufficient tensile properties, the Charpy impact properties of SLM-EUROFER97 are inferior in comparison to conventionally produced EUROFER97. A heat treatment alone only improves the ductile-to-brittle transition temperature, whereas hot isostatic pressing reduced the residual porosity of the SLM parts and a subsequent heat treatment improved the ductile-to-brittle transition temperature as well as the upper shelf energy.

This paper is the first part of our series on ultrafine-grained (UFG) tungsten produced by cold rolling. The aim of our project is to investigate the correlation between microstructure, mechanical properties and deformation mechanisms in UFG tungsten, a material interesting for future high-temperature applications. For this purpose, a batch of tungsten sheets with different thicknesses has been produced by subsequent cold rolling out of a single sintered compact. The aim of this paper is to characterise the microstructure of the as-received tungsten sheets. Quantitative grain size analysis by EBSD confirms a grain refinement by cold rolling well down into the UFG-regime, reaching 240 nm in the S-direction for the 100 μm foil. The grain refinement comes with an increase in the number of HAGBs, while LAGBs show no correlation with the degree of deformation introduced by cold rolling. All plates are distinctively textured, the main texture being the {001}〈110〉-orientation (rotated cube texture). Vickers microhardness measurements confirm a steady increase in hardness with advanced rolling up to 687 kg/mm 2 for the 100 μm foil. This indicates that the one-dimensional grain refinement to UFG by cold rolling is sufficient to achieve properties comparable to other UFG tungsten specimens. Therefore, it can be concluded that the batch of samples introduced in this paper will be fit to investigate the evolution of mechanical properties and deformation mechanisms in correlation with the microstructure.