Comparative analysis of low cycle fatigue behavior of pre-corroded standard and sub-sized EUROFER97 specimens exposed to ceramic breeder environment

In the helium-cooled pebble bed blanket concept of DEMO, the reduced activation ferritic-martensitic steel EUROFER97 will be in direct contact with lithium ceramic pebbles and undergo cyclic thermomechanical loading during operation. This study compares the impact of pre-corrosion for 64 days, simulating working conditions of EUROFER97, on the microstructure and low cycle fatigue properties of full-sized standard and sub-sized specimens. In both cases, the pre-corrosion process leads to the formation of three layers with a combined thickness of approximately 17 mu m. Standard specimens (08.8 mm) exhibit a notable decrease in fatigue life after pre-corrosion, though not as severe as the sub-sized specimens (02 mm). At a 0.6 % strain range, pre-corroded standard and sub-sized specimens experience a reduction in fatigue life by 35 % and 60 %, respectively. Microstructural studies reveal, that surface micro-cracks, commonly observed after exposure at contact zones with lithium ceramic pebbles, accelerate the evolution of surface damage during cycling and often act as initi-ation sites for fatigue crack. Potential factors contributing to the observed difference in fatigue life based on specimen size are discussed. The estimated maximum allowable deformation range of EUROFER97 components, without considering creep and neutron irradiation effects, is determined to 0.25 %.