Full Length ArticleSignificant change of local atomic configurations at surface of reduced activation Eurofer steels induced by hydrogenation treatments
Introduction
Special reduced-activation ferritic/martensitic steels such as Eurofer alloys [1], [2], [3], [4] are key candidates for ITER test blanket modules [5] and future DEMO applications [6], [7] in nuclear fusion technology, playing a crucial role as underlayers for plasma facing materials. Their main technical challenges consist of designing specific behaviors under temperature and irradiation constrains. In this respect, some studies investigated the effects induced by annealing in vacuum on the structural, mechanical and magnetic properties of Eurofer and ODS-Eurofer (oxide dispersion strengthened- Eurofer) steels. When subjected to annealing, both coercive field and hardness decrease in the ferritic regime (<850 °C) − due to microstructural changes related to static recovery − whereas oxide particles partially inhibit the recrystallization in ODS-Eurofer, contrary to the case of conventional Eurofer 97. In addition, the influence of the ageing processes was studied at temperatures of 500–600 °C up to 5 000 h [8] or even 12,000 h, indicating better ageing resistance for higher W content [9].
Serious concerns related to the brittleness of the steels in nuclear applications are related to radiation damage. In this respect, low temperature helium implantation in Eurofer was reported to affect the ductility, whereas post-annealing was efficient to improve the overall tensile properties [10]. The addition of oxide nanoparticles (ODS-Eurofer case) was also proposed [11], [12] to improve the mechanical properties of hydrogen implanted Eurofer steels. Although a microstructural stability was evidenced, the hydrogen absorption at room temperature increased considerably in ODS-Eurofer as compared to the conventional Eurofer 97 steel. It is worth mentioning that all the above discussed properties have to be related to local structure and atomic configuration which can be drastically modified by hydrogen insertion in the crystalline lattice of the steel.
The H transport and permeability properties of Eurofer 97 have been investigated in the temperature range of 103–451 °C [13] and at loading pressures of 0.5–1.5 bar by the gas evolution permeation technique. However, no studies were reported on local structure and atomic reconfigurations of Eurofer under hydrogenation treatments.
In some of our previous reports [14], [15], annealing treatments in hydrogen atmosphere at a pressure of 10 bar were applied (300 °C for 90 min) on thin films with composition approaching that of Eurofer alloys (Fe-Cr(-Al)), deposited on Si substrates. It was evidenced that the hydrogenation treatments influence drastically the structure of the deposited films with some positive effects consisting in the reduction of the Fe based layer and the removal of Si atoms penetrating from the Si substrate [16]. However, a fraction of the Cr atoms is also removed, inducing a modification of the initial composition of the film. Another approach considered low-pressure treatments in hydrogen plasma on a commercial stainless steel, along with heating with concentrated solar radiation, at high temperatures, up to approximately 1130 °C [17], inducing complete removal of the Cr oxides at the surface of the steels.
The aim of this study is to evidence the influence of multiple annealing treatments in hydrogen atmosphere on the local structure and atomic configuration in Eurofer slabs. In this respect, the main results are derived from CEMS and XPS, being also corroborated with SEM-EDS and XRD results.
Section snippets
Experimental details
The Eurofer slabs (notation: “EU”) of 1 mm thick have been prepared by arc melting and re-melting in vacuum (of mbar order), followed by annealing at 600 °C for 1 h, in the same vacuum conditions. The elemental composition of the alloy (as given by the provider) indicates an atomistic content of 89.3% Fe, 9.2% Cr, 1.2% W, and 0.3% other elements (C, V, Mn and Ta).
The hydrogenation treatments have been performed at 300 °C for 90 min, at a pressure of 10 bar, after 20 subsequent cycles of under vacuum
Results and discussions
The diffraction pattern of the as prepared Eurofer sample is shown in Fig. 1(a). A well crystallized structure with lattice parameters and intensity ratios very close to the typical values of body centered cubic structure is observed. The Si peak corresponds to the Si (100) wafer supporting the Eurofer slab in the diffractometer.
The morphology of the as prepared EU sample is observed from the SEM micrograph collected in secondary electron mode (see Fig. 1(b)), indicating a quite rough surface.
Conclusions
Structural and diffusional aspects induced by multiple annealing treatments in hydrogen atmosphere of EU samples have been investigated by means of conversion electron Mössbauer spectroscopy, X-ray photoemission spectroscopy, scanning electron microscopy and X-ray diffraction. The diffraction pattern of the as prepared sample indicates structural parameters close to the bcc phase, whereas the EDS data infer an average alloy composition specific to Fe0.9Cr0.1. After the hydrogenation treatments,
Acknowledgements
The Eurofer samples were provided in the frame of the EFDA program by Karlsruhe Institute for Technology. This work was supported by the Romanian National Program and the European Community under the EUROFUSION contract 1 EU-1 WPPFC-RO, as well as by the Romanian Ministry of Education and Research via projects PN-II-PT-PCCA-2013-4-0971 (contract 275/2014), by the Core Program 2016–2017 (PN16-480103), and POC P_37_697 REBMAT (contract 28/01.09.2016).
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Surface studies on the characterization of RAFM steel after deuterium plasma exposure
2019, Surfaces and InterfacesCitation Excerpt :Reduced-activation ferritic-martensitic (RAFM) steels are being developed as structural materials and also candidate materials for the first wall in fusion applications [1–6].