Analysis of VHCF damage in a duplex stainless steel using hard X-ray diffraction techniques

Effects of very high cycle fatigue (VHCF) damage were investigated in an austenitic–ferritic duplex stainless steel using the hard X-ray diffraction technique applying a beam diameter in the order of the mean grain size. Diffraction patterns were collected using a large 2D detector as function of the position along the load axis as well as perpendicular to the load axis of hourglass-shaped ultrasonic fatigue specimens. Intensities, angular positions and widths of Bragg reflections from individual grains were studied as a function of load cycles and stress amplitudes. Whereas rocking curves (RC) of ferrite grains behave nearly unaffected by the cyclic load, a splitting of RCs of austenite grains was observed and is taken as an indication for the VHCF damage. The frequency of split RCs of austenite grains increases with the number of load cycles and is found to be a function of the local stress amplitude. The latter one can be modeled by means of the finite element method (FEM). Taken from the 2Θ angles of Bragg peaks the internal compressive lattice strain of ferrite and austenite grains is found to be released for low but increases again for high numbers of load cycles. The evolution of lattice strain and the frequency of split RCs of austenite grains correlate with the appearance of slip bands at the sample surface seen by scanning electron microscopy (SEM) in combination with electron channeling contrast imaging (ECCI) and in the bulk verified by transmission electron microscopy (TEM). Microcrack formation in ferrite grains is assumed originated by the high density of slip bands in austenite grains generated by very high cycle fatigue