Evolution of surface deformation during fatigue of PH 13-8 Mo stainless steel using atomic force microscopy

The relationship between microstructure and nucleation of fatigue cracks in PH 13‐8 Mo stainless steel was explored with the use of atomic force microscopy (AFM) that allowed an accurate quantitative characterization of the surface features. Fully reversed strain‐ controlled fatigue tests were performed at 0.4 and 0.6% strain amplitudes, and the evolution of the surface deformation was observed at various fractions of life. At 0.4% strain amplitude, fatigue surface damage occurred first in the shape of streaks about 4 nm deep that formed at the interface between martensite laths and at prior austenite grain boundaries, and eventually coalesced to form crack nuclei. The increase in strain amplitude to 0.6% led to the formation of large extrusions, on average between 2 and 5 μm long with heights between 10 and 200 nm, which were the preferred crack nucleation sites.