Effects of very high cycle fatigue (VHCF) damage in an austenitic-ferritic duplex stainless steel have been investigated by in-situ hard X-ray diffraction technique applying up to 109 cycles and nominal load amplitude of 360MPa. Due to the coarse grain structure of specimen distinct Bragg peaks appear allowing for single grain analysis of ferritic and austenitic grains. We investigated the evolution of peak widths and angular positions of these individual Bragg spots as a function of fatigue cycles and stress amplitude, i.e. the distance from the minimum cross section of the hourglass-type specimens. During in-situ observation an increasing splitting of Bragg peaks of austenite reflections was monitored as function of load cycles which can be taken as an indication for the VHCF damage, whereas ferrite reflection remain nearly unchanged. In addition we observed a rapid strain release resulting from internal stress relaxation for both types of reflections for low number of cycles, while internal strain increases again at very high number of cycles. Using X-ray nanobeam inspection by white x-ray µLaue diffraction we could interpret these findings as the formation of stress-induced planar dislocation arrangements and dislocation pile-ups in austenite grains acting as nuclei for crack formation in adjacent ferrite grains.
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