The early stage of fatigue crack initiation in a 12%Cr martensitic steel

•Two important morphological groups of extrusions were described: principal and secondary extrusions.•The heights of extrusions grow constantly during the fatigue life.•The extrusions were localized inside the laths or cells, while the intrusions are in the lath or cell boundaries.•The mechanism of extrusion-intrusion formation was proposed.

The objective of the paper is to understand the mechanism of early stage crack formation in the low cycle fatigue of a martensitic steel with a complex and hierarchic microstructure. As a result of interrupted test at a total strain range Δεt = 1.2% at room temperature, the evolution of deformation relief represented by extrusions and intrusions was characterized by Atomic Force Microscopy and Transmission Electron Microscopy on the surface and in the bulk of specimen. A mechanism of extrusion-intrusion creation based on the vacancy diffusion induced redistribution of matter and plane slipping was proposed. The extrusion-intrusion pair leads to the martensitic steel cracking in the interfaces of microstructure.