Recent studies to the impact of a ceramic breeder environment on the mechanical properties of EUROFER97 under operating conditions

The reduced activation ferritic-martensitic steel EUROFER is a prime candidate for fusion structural applications. In the advanced helium-cooled pebble bed blanket concept of DEMO, this steel is in direct contact with Li-ceramics pebbles and experiences cyclic thermomechanical loading under operating conditions. In this work, the effect of ceramic breeder environment on both the microstructure and particularly the fatigue properties of the RAFM steel was investigated. For this purpose, EUROFER samples were embedded in ceramic pebbles and annealed at 550 degrees C under a purge gas atmosphere for durations ranging 8-128 days. Microstructural studies show that annealing at the given conditions leads to the formation of a complex corrosion multilayer consisting of ferrite and iron oxide in an outer layer, as well as chromium oxide in both an intermediate and an inner layers. For annealing durations up to 32 days, the thickness of the oxide layer is continuously increasing. However, for more prolonged annealing, the thickness of the corrosion layer reaches saturation at about 20 mu m. Similar to the growth behavior of the oxide layer, the strain-controlled LCF tests showed a remarkable reduction of the LCF lifetime with increasing in the annealing duration up to 32 days. However, beyond an annealing duration of 64 days, an additional decrease of lifetime is not observed. When performing additional experiments (annealing with ceramic pebbles in vacuum and annealing in purge gas without pebbles) it is found that corrosion is mainly caused by purge gas impurities, namely oxygen and water. Furthermore, the decrease in fatigue lifetime is related to the degradation of surface quality, causing an accelerated progress of damage. Finally, Charpy impact tests combined with carrier gas hot extraction investigations revealed that hydrogen embrittlement of the samples, annealed in a breeder environment containing 0.1 vol.% H-2, can be excluded. (c) 2022 Elsevier B.V. All rights reserved.