EELS analysis of He bubbles in ODS Steel and vanadium

Abstract

The understanding and the assessment of neutron irradiation in nuclear materials is critical in the design of the next‐generation nuclear fission reactors. Here, one of the most promising structural and fuel cladding materials, an oxide‐dispersion‐strengthened (ODS) steel was implanted with He and Fe ion in order to simulate the transmutant He and the damage (He/dpa) caused by neutron irradiation. The fine distribution of Y‐Ti‐O nanoparticles (NPs, 1‐20 nm) in the Fe‐Cr ferritic matrix is expected to improve thermal and mechanical properties. STEM‐EELS was used to investigate the structure and chemistry of these NPs and the He bubbles generated. The ODS material (Fe‐14Cr‐1W‐0.3TiH 2 ‐ 0.3Y 2 O 3 , wt.%) was prepared by mechanical alloying of Fe‐Cr‐W‐TiH2 and Y 2 O 3 powders followed by hot extrusion. Ion irradiation was carried out at 500°C, producing 5 dpa damage (Fe) with 1000 appm/dpa He implantation. Core loss spectrum‐images were denoised using principal component analysis [1]. Implanted He is shown to be trapped in some Y‐Ti‐O NPs (fig. 1) although bubbles also exist outside the NPs. The ion irradiation has also changed the Cr distribution, removing the Cr‐shell observed around the NPs in non‐irradiated ODS samples (fig. 1) [2,3] and rendering the Cr distribution in the metallic phase generally more heterogeneous. The He‐K line (21.218 eV for free atoms) shifts to higher energy in the bubbles (ΔE = 0.5 to 4 eV); this is shown to be correlated with the He density. He quantification has been carried out with three different methods: spatial difference, curve‐fitting and Multivariate analysis (MVA) methods (see below), as well as hybrid approaches combining the latter two methods which are proving promising for the elimination of the problems associated with each method on its own. The density and pressure values are found to reach 105 nm ‐3 and 8 GPa respectively, although the pressure measurement is only semi‐quantitative given that the error bars can reach 30%. The curve‐fitting method allows us to map the He‐K energy position and intensity, yielding the density, in individual bubbles (fig. 2). The spectral response of individual bubbles can be separated in an SI containing many bubbles at different densities using independent component analysis (ICA: example shown in figure 3) or vertex component analysis (VCA). Bubbles larger than 4 nm are shown to be under‐pressurized or at equilibrium with the Fe‐Cr matrix. Below 3.5 nm, the He pressure is shown to increase markedly, passing into the over‐pressurised regime. In the presentation we will also describe experiments and analysis on similar bubbles implanted in vanadium at 700 and 1000°C. Here the size/pressure relation is much less clear, but the MVA approach is similarly promising.