The role of microstructure in fatigue crack initiation of 9–12%Cr reduced activation ferritic–martensitic steel

• As-received: small-highly misoriented areas and large-slightly misoriented areas.
• After cycling: extrusions over large areas of slightly misoriented lath structures.
• We observed pile-ups of dislocation stress concentration.
• The observed pile-ups speak of the irreversibility of slip during cycling straining.
• Dislocation arrangement is the origin of microcrack nucleation at lath boundaries.

Electron back-scattered diffraction together with scanning and transmission electron microscopy were used to study nucleation of microstructural fatigue cracks in reduced-activation ferritic–martensitic steel, EUROFER 97. Cylindrical specimens were cycled over different plastic-strain ranges in order to evaluate the evolution of the dislocation structure. Surface-damage evolution was studied in smooth notched specimens by an optical in situ system equipped with a high-resolution camera. In order to understand the crack-initiation mechanism, the dislocation microstructure that develops in the near-surface regions of the notch was compared with that of the bulk. The results demonstrate a strong influence of lath-martensite boundaries on fatigue-crack nucleation.