Fatigue behavior and development of microcracks in F82H after helium implantation at 200 deg. C

The ferritic/martensitic steel F82H-mod has been push-pull fatigue tested at 200 deg. C before and after homogeneous implantation of 400 appm helium at about 0.5 dpa and 250 deg. C to assess the growth characteristics of short fatigue cracks and their impact on lifetime. Crack initiation and propagation on carefully polished specimen surfaces were monitored by a high resolution optical microscope during fatigue testing. Subsequent metallographic and microstructural investigations revealed crack initiation mainly along martensitic laths and transgranular crack propagation, partly affected by the orientation of martensitic bands in adjacent grains. A statistical analysis of the crack formation and propagation processes was performed. Irradiation has led to pronounced fatigue life reduction by a factor of ∼5 at higher strain ranges and to a moderate fatigue life enhancement at low strain ranges. This behavior can be understood by different types of microstructural barriers and their effect on crack formation and growth