Abstract text (incl. figure legends and references)
Introduction

Reduced-activation materials that exhibit high radiation resistance are required for the design, construction, and reliable operation of environmentally friendly fusion power plants. One of the promising candidates for such an application, the reduced activation ferritic-martensitic steel (RAFM) EUROFER97 (EF97), has been systematically developed and extensively characterized in Europe for this purpose. Liquid metal corrosion is a significant problem for structural materials that needs to be avoided using a corrosion protection layer. In this case Al2O3 is used to separate the Pb-Li liquid metal from the EUROFER.

Objectives

A 5 µm thick Al2O3 layer was deposited by chemical vapor deposition on EF97 to evaluate the corrosion resistance of this composite in a Pb-Li liquid metal. The aim of this study is the comprehensive characterization of the oxide layer as well as the EF97/Al2O3 interface. The particular focus is on the analysis of the microstructural changes in both the Al2O3 layer and the EF97 substrate after liquid metal corrosion.

Materials & methods

The Al2O3 layer and its interface to the EF97 substrate have been analyzed in cross section by scanning electron microscopy (Crossbeam Auriga, ZEISS) and transmission electron microscopy (TEM) in combination with energy-dispersive X-ray analysis (STEM-EDX in a Thermo Fisher Scientific - Talos F200X). For TEM experiments a lamella was prepared using the Crossbeam FIB/SEM Auriga, ZEISS.

Results

Figure 1 shows a SEM cross-section of the sample structure with the 5 µm Al2O3 layer on EF97 substrate. The EF97/Al2O3 interface appears to be smooth and free of visible structural defects. Channeling contrast using a FIB scan reveals that the EF97 grain size is reduced in a 1-2 µm wide region close to interface. Furthermore, the figure shows that M23C6 and MX precipitates are located along grain or package boundaries.

Figure 1: SEM Cross-Section showing the Al2O3 layer on EF97.

TEM analysis shows that the Al2O3 layer consists of 50-300 nm grains, which are randomly oriented. The crystalline structure of the layer corresponds to the Al2O3 modification.

Figure 2 shows a STEM-EDX elemental mapping of the EF97/Al2O3 interface. In the EF97 several M23C6 and MX-type precipitates (TaC and VN) can be identified, which are comparable to bulk EF97 measurements indicating that the EF97 structure is still intact. At the EF97/Al2O3 interface a 10-15 nm thick V enrichment exists with localized Al and N rich particles having a size up to 200 nm. The presumable case of the V enrichment at the interface is related to the manufacturing of the layer or the treatment process. Close to the interface, diffusion of W, Cr and C along EF97 grain or lath boundaries was detected. This behavior is also observed in bulk EF97. The STEM bright- and dark-field images show that Al2O3 layer is not grown homogeneously, i.e., voids and Al platelets are observed.

Figure 2: STEM-EDX elemental maps after applying corrosion tests.

Conclusions

The microstructure of EF97 protected by a 5 µm Al2O3 layer has been examined in detail. As a result, significant changes in the EF97 substrate could not been observed. In addition, there is no evidence of oxidation in the EF97 matrix.