Significant change of local atomic configurations at surface of reduced activation Eurofer steels induced by hydrogenation treatments

Highlights

• Engineering of Eurofer slab properties by hydrogenation treatments.

• Hydrogenation modifies significantly the local atomic configurations at the surface.

• Hydrogenation increases the expulsion of the Cr atoms toward the very surface.

• Approaching binomial atomic distribution by hydrogenation in the next surface 100 nm.

Abstract

Reduced-activation steels such as Eurofer alloys are candidates for supporting plasma facing components in tokamak-like nuclear fusion reactors. In order to investigate the impact of hydrogen/deuterium insertion in their crystalline lattice, annealing treatments in hydrogen atmosphere have been applied on Eurofer slabs. The resulting samples have been analyzed with respect to local structure and atomic configuration both before and after successive annealing treatments, by X-ray diffractometry (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and conversion electron Mössbauer spectroscopy (CEMS). The corroborated data point out for a bcc type structure of the non-hydrogenated alloy, with an average alloy composition approaching Fe_0.9Cr_0.1 along a depth of about 100 nm. EDS elemental maps do not indicate surface inhomogeneities in concentration whereas the Mössbauer spectra prove significant deviations from a homogeneous alloying. The hydrogenation increases the expulsion of the Cr atoms toward the surface layer and decreases their oxidation, with considerable influence on the surface properties of the steel. The hydrogenation treatment is therefore proposed as a potential alternative for a convenient engineering of the surface of different Fe-Cr based alloys.

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.