Significance and Mechanism of the Crack Initiation Process during Very High Cycle Fatigue of Duplex Stainless Steel

In the very high cycle fatigue (VHCF) regime, fatigue damage starts by the formation of slip bands within the softer austenite phase. By high energy X-ray diffraction it was shown that local austenite plasticity due to elastic/plastic anisotropy results in the gradual formation of residual stresses within the ferrite grains. Consequently, cracks are initiated either transgranularly or intergranularly at austenite/ferrite phase boundaries or ferrite/ferrite grain boundaries in the vicinity of the austenite slip bands. By means of ultrasonic fatigue testing of miniature specimens in combination with (i) in-situ observation, (ii) electron back- scatter diffraction, (iii) synchrotron computer tomography, and (vi) high-energy X-ray diffraction, the fatigue crack initiation process and the interactions of the cracks with the first microstructural barriers were quantified. By implementing the results in a numerical modeling approach, the experimental observation that the first barrier seems to limit the VHCF life was supported.