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Reduced activation ferritic steels are being explored as possible cladding tube materials for nuclear reactors because of their low activation and excellent irradiation resistance. In the current investigation, reduced activation ferritic steel (Fe–15Cr–2W) is processed by mechanical alloying of elemental powders followed by hot powder forging. Mechanical alloying is carried out in a Simoloyer attritor mill (Zoz GmbH), after which the powders are placed in a mild steel can and forged at 1200 °C in H2 atmosphere. X‐ray diffraction and transmission electron microscopy (TEM) investigation reveal that 10 h of mechanical alloying is required to achieve complete dissolution of Cr and W in the Fe matrix powder. The relative density and hardness distribution of the forged slab is evaluated in longitudinal as well as transverse direction to optimize the powder forging operation. Electron backscatter diffraction analysis showed dynamic recrystallization to take place during the course of hot powder forging. Tensile tests are performed at room temperature as well as at elevated temperatures (600 and 700 °C). The yield strength and ultimate tensile strength at room temperature as well as at elevated temperatures are found to be higher than those reported in literature for reduced activation ferritic steels consolidated by other techniques. [ABSTRACT FROM AUTHOR]

TEM studies were performed to examine the effect of holding of dispersion-strengthened heat-resistant reduced activation 12% chromium ferritic-martensitic steel EK-181 in static liquid lead for 3000 h at 600°C on the steel microstructure in comparison with the steel after conventional heat treatment by quenching and tempering at 720°C. It was found that the steel microstructure has good thermal stability under the specified experimental conditions. Microstructural deformation of EK-181 steel was studied in the neck region of tensile specimens tested at the temperatures 20, 680, 700, and 720°C with and without holding in liquid lead, and their fracture mechanisms were investigated. As a result of plastic deformation during tensile testing at room temperature, martensite plates and laths near the fracture surface are distorted and fragmented with the formation of new low-angle boundaries, and the dislocation density increases. At the deformation temperatures 680–720°C, nearly equiaxed ferrite grains are formed, the density and size of second-phase particles (M23C6 and MX) increases due to dynamic strain aging, and the dislocation density decreases locally. As the test temperature rises, the degree of martensite tempering increases. At T ≥ 700°C, some dynamic polygonization and dynamic recrystallization are observed. At elevated tension temperatures, ferrite coarsening is more significant in the specimens held in lead as compared to the conventionally treated material. The plastic deformation and fracture behavior of the steel are largely determined by the test temperature, rather than by the treatment mode. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) was successfully applied to fabricate strong and ductile reduced activation ferritic/martensitic (RAFM) steel by introducing heterogeneous multi-/bimodal microstructure. The SLM-built RAFM steel exhibited an excellent combination of strength and ductility (i.e. yield strength of 1053 ± 4.7 MPa and elongation of 16.9 ± 0.4%), well surpassing the previously reported counterparts. The superior strength was expected from the refined grains and lath martensites, and the high ductility was associated with the improved dislocation-controlled strain hardening capability. This work provides a basis for developing high-performance RAFM steel by microstructural design via SLM.

Nuclear fusion plasma-facing materials (PFM) will suffer from irradiation, leading to significant changes in the material properties. This study investigates the impact of displacement damage on the deuterium retention near room temperature.ITER grade tungsten, Eurofer-97, and HiperFer 17Cr5 steel samples are irradiated with a tandem accelerator with ~3 MeV protons at currents of 100–600 nA on 250–550 µm spots at 320±10 K. In total 33 spots from 0 to 0.9 displacements per atom (DPA) at 0–4 µm depth are irradiated on 5 samples. After irradiation, the samples are exposed to D2 plasmas with a peak ion-flux of 2.1 × 1021 D/m²s for 4 h at

Selective laser melting (SLM) was successfully applied to fabricate strong and ductile reduced activation ferritic/martensitic (RAFM) steel by introducing heterogeneous multi-/bimodal microstructure. The SLM-built RAFM steel exhibited an excellent combination of strength and ductility (i.e. yield strength of 1053 ± 4.7 MPa and elongation of 16.9 ± 0.4%), well surpassing the previously reported counterparts. The superior strength was expected from the refined grains and lath martensites, and the high ductility was associated with the improved dislocation-controlled strain hardening capability. This work provides a basis for developing high-performance RAFM steel by microstructural design via SLM. A new strategy, refers to heterogeneous multi-/bimodal microstructure, is proposed to develop strong and ductile RAFM steel via SLM and the achieved superior mechanical properties overcome the classic strength-ductility trade-off. [ABSTRACT FROM AUTHOR]

A Demonstration (DEMO) thermonuclear reactor is the next step after the International Thermonuclear Experimental Reactor (ITER). Designs for a DEMO divertor and the First Wall require the joining of tungsten to steel; this is a difficult task, however, because of the metals’ physical properties and necessary operating conditions. Brazing is a prospective technology that could be used to solve this problem. This work examines a state-of-the-art solution to the problem of joining tungsten to steel by brazing, in order to summarize best practices, identify shortcomings, and clarify mechanical property requirements. Here, we outline the ways in which brazing technology can be developed to join tungsten to steel for use in a DEMO application.

High temperature (550 °C) He ions irradiation was performed on nanostructured (NS) and coarsen-grained (CG) reduced activation steel to investigate the effects of GBs/interfaces on the formation of bubbles during irradiation. Experimental results showed that He bubbles were preferentially trapped at dislocations and/or grain boundaries (GBs) for both of the samples. Void denuded zones (VDZs) were observed in the CG samples, while VDZs near GBs were unobvious in NS sample. However, both the average bubble size and the bubble density in peak damage region of the CG sample were significantly larger than that observed in the NS sample, which indicated that GBs play an important role during the irradiation, and the NS steel had better irradiation resistance than its CG counterpart. [ABSTRACT FROM AUTHOR]

Severe plastic deformation, intense single-beam He-ion irradiation and post-irradiation annealing were performed on a nanostructured reduced activation ferritic/martensitic (RAFM) steel to investigate the effect of grain boundaries (GBs) on its microstructure evolution during these processes. A surface layer with a depth-dependent nanocrystalline (NC) microstructure was prepared in the RAFM steel using surface mechanical attrition treatment (SMAT). Microstructure evolution after helium (He) irradiation (24.8 dpa) at room temperature and after post-irradiation annealing was investigated using Transmission Electron Microscopy (TEM). Experimental observation shows that GBs play an important role during both the irradiation and the post-irradiation annealing process. He bubbles are preferentially trapped at GBs/interfaces during irradiation and cavities with large sizes are also preferentially trapped at GBs/interfaces during post-irradiation annealing, but void denuded zones (VDZs) near GBs could not be unambiguously observed. Compared with cavities at GBs and within larger grains, cavities with smaller size and higher density are found in smaller grains. The average size of cavities increases rapidly with the increase of time during post-irradiation annealing at 823 K. Cavities with a large size are observed just after annealing for 5 min, although many of the cavities with small sizes also exist after annealing for 240 min. The potential mechanism of cavity growth behavior during post-irradiation annealing is also discussed. [ABSTRACT FROM AUTHOR]

Advanced reduced activation alloy (ARAA) is a reduced activation ferritic/martensitic (RAFM) steel under development at the Korea Atomic Energy Research Institute. The transport of hydrogen and deuterium in ARAA was investigated in an elevated temperature range of 250–600 °C. A continuous-flow method, a time-dependent gas-phase technique, was used for the measurements. Complete sets of transport parameters (permeability, diffusivity, solubility, trap site density, and trapping energy) of hydrogen and deuterium in ARAA were successfully obtained. We show that appreciable trapping effects are observed only at low temperatures (250–350 °C) and that the isotope effect ratio for the diffusivity differs from the classical prediction. However, the measured values of permeability, effective diffusivity, and effective solubility of ARRA were within the range of results reported for other RAFM steels. [ABSTRACT FROM AUTHOR]

The temperature dependence of enthalpy increment ( H T − H298) of 9 mass% Cr–1 mass% W–0.23 mass% V–0.06 mass% Ta–0.09 mass% C reduced activation steel has been measured by inverse drop calorimetry in the temperature range 400 K to 1273 K. A critical comparison of present isothermal enthalpy measurements with the results of our previous dynamic calorimetry studies has been made to reveal clearly the occurrence of various diffusional phase transformations that occur at high temperature. These phase changes are marked by the presence of distinct inflections or cusps in an overall nonlinear variation of enthalpy values with temperature. The principal thermal relaxation step of the martensitic microstructure obtained through quenching from the high-temperature γ-austenite phase is observed around 793 K. The ferromagnetic-to-paramagnetic transition of the α-ferrite phase is found to occur at 1015 K. The equilibrium values of γ-austenite start (A e1) and finish (A e3) temperatures are found to be 1063 K and 1148 K, respectively. A value of 12 J · g−1 has been estimated for Δ° H α→ γ the latent heat associated with the α → γ transformation. The measured enthalpy increment variation of the α-ferrite phase with temperature has been fitted to a suitable empirical function to estimate the temperature-dependent values of the specific heat. A comparison of the drop calorimetry-based indirect estimate of the specific heat with the direct differential scanning calorimetry-based values revealed that the drop calorimetry estimates are systematically lower than its dynamic calorimetry counterpart. This difference is attributed to the fact that, under finite heating rate conditions that are typical of dynamic calorimetry, measurements are made under nonequilibrium conditions. Notwithstanding this limitation, there is a good overall agreement between the two C p values and also among the phase transformation temperatures so that a reliable assessment of thermal properties and phase transformation characteristics of reduced activation steel can be determined by a combined analysis of the results of drop and differential scanning calorimetry. [ABSTRACT FROM AUTHOR]

Abstract: Fusion plants will have a large scale and therefore need a high availability and reliability to generate electricity economically. The materials total life cycle controls the choices of crucial plant components such as blankets and divertors to a high degree. Materials applications so far rely on data obtained with fission reactors up to levels of 80dpa. Such fission simulations miss the effects of the He and H generated in a fusion plasma environment. The International Fusion Material Irradiation Facility, IFMIF, a 14MeV neutron source, will provide more realistic tests. Its design will rely on the databases obtained with fission reactors. IFMIF itself will produce materials databases that will be incorporated in design codes for fusion power plants. The first generation of blankets will be built with reduced activation steels. The race for higher thermal efficiencies leads further to the application of oxide dispersion strengthened steels and SiC ceramic composite. SiC structural utilisation requires breakthroughs. Tungsten alloys are attractive for divertors operating with high heat flux surfaces at high temperature. Their high temperature embrittlement is presently improved with promising thermo-mechanical treatments. Research results should be incorporated in “early bird” databases improving design and materials expert interaction. [Copyright &y& Elsevier]

Ferritic martensitic steels show limited swelling and susceptibility to helium effects and can be made with low activation chemical compositions. These properties make them the reference steel for the development of breeding blankets in fusion power plants. EUROFER97 is the European implementation of such a steel, where experience gained from an IEA co-operation with Japan and the US is also implemented. Results obtained so far show that EUROFER steel has attractive mechanical properties even after long ageing times. Compatibility tests in water and PbLi17 are in progress. Oxidised aluminium is the most effective protective layer in PbLi17. The displacement damage and helium formation strongly influence the hydrogen transport in the steel. Present experiments should be backed by tests in a more fusion relevant environment, e.g. IFMIF. The 2.5 dpa neutron irradiations at low temperatures result in a higher DBTT. High dose irradiations, up to 80 dpa, are underway. The early results of ODS grades with EUROFER steel composition show potential of these grades for increasing the operating temperature with 100–150 K. [Copyright &y& Elsevier]

A Demonstration (DEMO) thermonuclear reactor is the next step after the International Thermonuclear Experimental Reactor (ITER). Designs for a DEMO divertor and the First Wall require the joining of tungsten to steel; this is a difficult task, however, because of the metals' physical properties and necessary operating conditions. Brazing is a prospective technology that could be used to solve this problem. This work examines a state-of-the-art solution to the problem of joining tungsten to steel by brazing, in order to summarize best practices, identify shortcomings, and clarify mechanical property requirements. Here, we outline the ways in which brazing technology can be developed to join tungsten to steel for use in a DEMO application. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) was applied to fabricate high-strength and ductile reduced activation ferritic/martensitic (RAFM) steel via modification of heterogeneous microstructure that consists of domains of fine and coarse grains as well as lath martensites, but with different grain morphology in respond to the variation in the scanning strategy and resultant thermal history. The SLM-built RAFM steel exhibited an enhanced strength-ductility synergy with high yield strength of 967 MPa and high elongation of 15.1% under a meander scanning strategy, surpassing the previously reported counterparts, which was ascribed to the refined microstructure and improved strain hardening. In contrast, an island scanning strategy deteriorated the ductility down to 3.8% arising from the oriented alignment of columnar coarse grains around island, which initiated micro-voids and cracks during deformation and finally gave rise to a premature failure. This work proposed the microstructural design principles for additively manufacturing high-performance RAFM steels and is expected to facilitate their application in the fusion reactor. [ABSTRACT FROM AUTHOR]

Two types of scanning strategies were adopted to study the effect of scanning strategy, i.e., bidirectional scanning without (SS-X) and with 45° deviation from X/Y-axis and 90° rotation (SS-XY45) between adjacent layers, on microstructure and mechanical properties of reduced activation ferritic/martensitic (RAFM) steel fabricated by selective laser melting (SLM). The SLM-built RAFM steel exhibited a better combination of strength and ductility than the previously reported counterparts. The SS-X had higher yield strength of 911 MPa and higher elongation of 18.7% than the SS-XY with yield strength of 893 MPa and elongation of 15.0%. The results reveal that the variation in the yield strength was mainly ascribed to the different grain size and the improved ductility was associated with the modified grain structure that altered the tensile fracture mode with concurrent occurrence of intergranular brittle and transgranular ductile fracture. This finding provides an alternative to fabricate high-performance RAFM steel by tailoring the microstructure with different scanning strategies. Image 1 • The SLMed CLF-1 steel exhibited a superior combination of strength and ductility. • Scanning strategy modified the grain size and morphology of bimodal microstructure. • The variation in yield strength was mainly ascribed to different grain size. • The improved ductility was associated with altered tensile fracture mode. [ABSTRACT FROM AUTHOR]