Mechanism of subsurface microstructural fatigue crack initiation during high and very-high cycle fatigue of advanced bainitic steels

Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.