Effect of inclusion and microstructure on the very high cycle fatigue behaviors of high strength bainite/martensite multiphase steels

We studied the mutual and relative effect of inclusion and microstructure on very high cycle fatigue (VHCF) behavior of bainite/martensite (B/M) multiphase steel by designing four combinations of various inclusion sizes and microstructures. Results show that the fatigue crack initiations are mostly induced by inclusions in the specimens with large inclusion size, whereas the "non-inclusion induced crack initiations" (NIICI) are observed in the specimens whose microstructure is coarse. Unlike the inclusion induced cracks that grow along the plane of maximum tensile stress, the early non-inclusion induced cracks within micro-facet propagate along the plane of maximum shear stress (Le., Stage I crack), which is related to the strain localization in oriented B/M laths during cyclic loading. Finally, with the assistance of scanning electron microscopy, electron backscatter diffraction, 3-Dimensional surface profile, focused ion beam and transmission electron microscopy, we proposed the NIICI mechanism of coarse B/M microstructure, and found the orientation and size of coarse B/M block and blocky retained austenite (RA) are crucial microstructural factors in facilitating the formation of NIICI within VHCF regime.