The present study documents that at loading amplitudes close to the fatigue limit, cyclic irreversible plastic deformation in form of slip band generation in the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318 LN) mainly takes place in few austenite grains without any microcrack initiation in these grains. This was shown by means of focused ion beam (FIB) cutting in combination with high resolution scanning electron microscopy (SEM) at pronounced extrusion-intrusion-pairs in several austenite grains. Investigations by means of confocal laser scanning microscopy (CLSM) revealed that the slip band density in these grains increases with the number of loading cycles and remains constant in the very high cycle fatigue (VHCF) regime. Under such loading conditions, fatigue cracks frequently initiate in the ferrite phase due to anisotropy stresses which are strongly superimposed by stress intensifications at the tip of austenite slip bands. TEM investigations revealed that austenite slip bands, which are piling up against phase boundaries, cause localized dislocation generation and motion in neighboring ferrite grains. The cyclic irreversible motion of these dislocations on several parallel slip planes is correlated with the stage of fatigue crack initiation. A crystal plasticity model based on a finite element program, which considers anisotropic elasticity, allows for the determination of crack initiation sites in real microstructures according to the above mentioned mechanisms. Crystallographic orientations, measured by means of the electron back scatter diffraction (EBSD) technique, serve as input parameters for the calculations regarding microcrack initiation as well as for the analysis of the subsequent short fatigue crack propagation, which is strongly affected by microstructural barriers such as grain and phase boundaries.
Very high cycle fatigue crack initiation mechanism according to a 3D model of persistent slip bands formation in α-ferrite
