Microstructural analysis of mechanically tested reduced-activation ferritic/martensitic steels

To make the best use of a limited number of irradiated strength-test specimens, it is desirable to derive the microstructure information related to the mechanical properties from mechanically tested specimens. A focused ion beam micro-sampling system was utilized to make thin film samples for transmission electron microscope (TEM) observations from the crack or fracture surface of the strength-tested specimens. The microstructure of mechanically tested specimens of a reduced-activation ferritic/martensitic steel, F82H, was investigated focusing on the irradiation effects on fatigue fracture. On unirradiated F82H, SEM observations revealed that the fatigue crack initiated along a prior austenite grain boundary. The TEM micrograph around the crack showed the presence of polygonization and the formation of distinctive shaped subgrains along the prior austenite grain boundary ahead of the crack. The fatigue crack initiated from an intergranular crack, and the TEM micrograph indicated that the main crack developed in the same manner as in the unirradiated F82H.