• Energy Research

Small-angle neutron scattering (SANS) and neutron diffraction have been utilized for micro-structural characterization of Eurofer97/2 heats submitted to thermo-mechanical treatments, with the aim to investigate macroscopic material volumes and to complete the local information provided by scanning electron microscopy. The measurements were carried out at MLZ in Garching, utilizing for SANS also a polarized neutron beam. The investigated samples had been submitted to austenitization and tempering at different temperatures, as well as to double austenitization and to “ausforming”, that is austenitization followed by hot rolling. For most of the examined treatments, nearly identical SANS cross-sections were measured, close to the one of Eurofer97/1. In the ausformed sample the nuclear SANS cross-section is also nearly identical to the other samples but the magnetic one is one order of magnitude higher. Furthermore, over a wide experimental interval its nuclear-magnetic interference, measured by polarized SANS, is of opposite sign with respect to the non-ausformed samples. Neutron diffraction measurements provided strong evidence that the origin of such effects is due to the presence of the non-magnetic austenite phase in the ausformed sample, with an estimated volume fraction of 0.17 ± 0.02; within the experimental resolution, it disappears after subsequent tempering.

The contribution of ENEA together with Rina-CSM to the Eurofusion programme “WPMAT-Advanced Seels” deals with the development of innovative RAFM steels able to withstand the critical temperatures typical of the different operational environments foreseen for the blanket of the first DEMO reactor. The optimization of the chemical composition and the Thermo Mechanical Treatment for these materials should be done according to the blanket operating temperatures that are related to two possible working conditions: the WCLL-BB (Water Cooled Lead Lithium Breeding Blanket) or the H(D)CLL-BB (Helium (Dual) Cooled Lead Lithium Breeding Blanket). On the one hand the “water-cooling” option implies a minimum irradiation temperature for the blanket material in the range of 280–350 °C. On the other hand, the “helium-cooled” and the “dual-coolant” solutions imply an operating temperature for the blanket material in the range of 650 °C. Therefore in the first case the target is the improvement of the toughness of the martensitic alloys; whilst concerning the second scenario the target is the development of more creep resistant martensitic steels, suitable to tolerate such a high operating temperature. In both the cases the Tungsten content plays a key role, both in terms of solid solution hardening and influence on the DBTT. Two alloys aimed at fulfilling the specifications for the two DEMO operating conditions, both with increased Tungsten content respect to Eurofer, have been produced and characterized. The mechanical properties of these two alloys are hereby reported and discussed.

One of the options currently taken into account for the realization of the first DEMO reactor is the “water-cooled lanket”. This option implies an irradiation temperature for the blanket material in the range of 280–350 °C. Therefore, in light of the under irradiation behaviour of EUROFER, namely of the DBTT shift toward high temperature due to the low irradiation temperature embrittlement, the target of the hereby reported activities is the development of much tougher alloys, to try to tolerate the embrittlement due to the low irradiation temperature. We report in this paper the work done to optimize the toughness of Eurofer 97, increasing the normalizing temperature and maintaining a small grain size using multiple normalizing treatments. We report also the mechanical behaviour of two 9Cr1WTa type alloys, produced and tested with the same aim to find alloys more resistant to embrittlement at low irradiation temperature. Keywords: EUROFER 97, RAFM steels, Microstructure, Heat treatment, Tensile, Charpy, KLST

Pyrolectric materials are able to harvest energy both from naturally and artificially occurring temperature changes. These materials could be the right way to recover some of the enormous amount of energy wasted as heat and help to develop new devices for harvesting thermal ambient energy. In this work it was investigated the development of bulk, dense pyroelectric ceramics, ideally with a highly developed texture and small grain size, using a micron-sized powder of the ZnS wurtzite phase as precursor material. The Two-Step Sintering (TSS) process is a useful method to obtain high sintered density and to limit the grain growth associated with the final stage of the sintering process. One of the main advantages of this method is the lowering of the sintering temperature. The microstructural, morphological and electrical properties of TSS-ZnS were determined and compared to ZnS produced by the conventional sintering process, performed at 1250°C. TSS-ZnS showed comparable density and a finer microstructure than conventional ZnS (five times lower grain size). It was demonstrated that the TSS process is a pressureless, simple and cost‐effective sintering method to obtain high density materials with controlled grain growth, without using a dopant or binder. The TSS produced ZnS ceramic was tested for pyroelectric energy harvesting. It is expected that the efficiency of the ceramic in harvested energy could be further improved by decreasing the grain size down to the nanoscale. Acknowledgement: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 797951.

Abstract To address the issue of the breakdown into fine powders that occurs in the practical use of metal hydrides, the possibility of using a polymeric material as a matrix that contains the active metal particles was experimentally assessed. A ball milling approach in the tumbling mode was used to develop a metal hydride–polymer composite with a high metal to polymer weight ratio. The alloy powder was blended with the polymer and a coating of the metal particles was obtained. The composite was consolidated by hot pressing and the pellets were characterized in terms of their hydriding–dehydriding properties. The materials did not show significant losses in either loading capacity or kinetic properties. The polymeric matrix resulted as being stable under hydrogen cycling. Further, from SEM observation it was confirmed that the metal powders remained embedded in the polymeric matrix even after a number of cycles and that the overall dimensional integrity was retained.

One of the options currently taken into account for the realization of the first DEMO reactor is the “water-cooled blanket”. This option implies a minimum irradiation temperature for the blanket material in the range of 280–350 °C. In addition to the DBTT (Ductile to Brittle Transition Temperature) shift due to the DPA (displacement per atom) damage under irradiation, also the issue of the increased embrittlement due to He production must be taken into account. This issue appears even more detrimental and less manageable because the DBBT shift due to the Helium production does not saturate with the dose, as it results from previous works reported in literature. The experimental results and the difference in behaviour between ODS (Oxide Dispersion Strengthened Steels) RAFM (Reduced Activation Ferritic Martensitic) and other FM (Ferritic Martensitic) alloys (EM10, P91) showed that it is possible to improve the resistance to He embrittlement by both intra-granular precipitation of Y-Ti oxides and by decreasing the grain size at the same time. Nevertheless, anyway, the multiplication of the grain boundaries increases the dilution of He on grain surface, delaying the formation of He bubbles on grain boundaries and, therefore, the susceptibility to the He embrittlement. Several grain size reduction strategies have then been investigated on EUROFER both at the austenitization stage, on the PAGS (Prior Austenite Grain Size), and at the tempering stage, on the tempered martensite. The microstructural observations have been carried out by means of SEM (Scanning Electron Microscopy). Also the effect of grain size reduction on the toughness of the material will be taken into account; The DBTTs resulting from impact tests on KLST specimens will be shown. The outcomes of the microstructural observations, as well as the preliminary mechanical characterization (impact tests) will be discussed in this paper. Keywords: EUROFER 97, RAFM steels, Microstructure, Multiple normalization, Asymmetric rolling, Recrystallization, KLST

Among the options currently taken into account for the realization of the first DEMO reactor there are the “helium-cooled” and the “dual coolant” breeding blanket. Therefore the high temperature (650 °C) behavior of the proposed innovative martensitic alloys should be improved, namely the frame of the hereby reported activities is the development of martensitic alloys more resistant to creep, suitable to tolerate such a high operating temperature. In order to improve the high temperature mechanical properties, concerning the alloy design strategies, two alternative routes are proposed; the effect of Nitrogen and Tungsten increase are taken into account as well as the addition of carbo-nitride forming elements, like Vanadium, combined with the “ausforming” thermo-mechanical treatments. Two alloys have been designed and a special thermo-mechanical treatment on Eurofer 97-2 is proposed. The “ausforming” treatment, consisting in a sort of hot-working at a lower temperature with respect to the austenitization one after the austenitization stage, is aimed at the achievement of a beneficial dislocation “pinning” at high temperature due to carbide precipitation. Generally the improvement of tensile properties is associated to the hardening of the steel due to dislocation network and precipitation effects. This hardening is accompanied by a DBTT increase to markedly higher values with respect to Standard Eurofer. The proposed materials should be, in any case, at least room temperature ductile in order to undergo safe manufacturing and assembling processes. Therefore the issue of the DBTT increase has been taken into account by tuning the tempering temperature adequately. The two variations from chemical composition of Eurofer 97 have been casted and the thermo-mechanical treatments have been selected by means of SEM and hardness measurements to tune grain size and precipitation of carbides. The outcomes of the preliminary mechanical characterization (tensile, creep and impact tests) will be discussed in this paper. Keywords: EUROFER 97, RAFM steels, Thermo-mechanical treatment, Ausforming, Tensile, Creep, Charpy