## 2 Properties of Fine Granular Area
### Influences Regarding the Formation Process of FGA
#### 2.1.2 Influence of the Material and Purity
They report that in the absence of non-metallic inclusions of sufficient size, crack initiation in ferritic/austenitic duplex steels is dominated by the formation of slip bands in the region with the highest resolved shear strain within the softer austenite phase on the surface of the specimen. The pile up of dislocations at the austenite/ferrite phase boundaries leads to plastification in the neighboring ferrite grains and eventually to crack initiation. However, if these cracks can propagate leading to failure depends on the barrier efficiency of the next grain or phase boundary [43]. A recent investigation by [PERSON] et al. [36] analyzing the effect of inclusion size and microstructure on the VHCF behaviors of high-strength bainite/martensite multiphase steels revealed that there is a competition between the subsurface defect-induced failure and the non-defect subsurface initiation. While specimens containing large inclusions showed fatigue crack initiation at inclusions, \"non-inclusion induced crack initiation\" (NIICI) was observed in specimens with coarse microstructure. In case of defect-induced crack initiation the crack propagation was perpendicular to the loading direction, while in case of NIICI the early crack propagation is along the plane of maximum shear stress. Similar findings are reported by [PERSON] et al. [26], who observed different VHCF failure mechanisms between a hot-rolled duplex stainless steel and a cold-rolled plate duplex stainless steel. For the former, crack initiation was observed due to inhomogeneous accumulation of plastic fatigue damage in the form of extrusions in the ferrite grains or at austenite-ferrite phase boundaries. The hot-rolled duplex stainless steel shows a crystallographic growth region (CGR), where the crack propagation is dominated by the microstructure following favorable crystallographic planes in each grain. Outside of the CGR the observed crack growth is almost perpendicular to the loading direction without any influence of grain or phase boundaries. Fatigue crack initiation of the cold-rolled strip duplex stainless steel occurred invariably because of surface defects resulting from the cold rolling process. An FGA was observed in the direct vicinity of the surface defect for all failed specimens [26]. According to [PERSON] [44], inclusions lead to localized increased strain due to their mechanical, physical, and chemical properties that deviate from the matrix. Consequently, the presence of such inclusions can adversely affect the fatigue properties. Similar observations are described by [PERSON] [45] and also [PERSON] and [PERSON] [46], who observed that the most influential parameters of inclusions for fatigue failure are their size, elastic properties, and adhesion to the matrix. The influence of the inclusion size was shown by the increase of the degree of purity, which led to smaller inclusions resulting in higher fatigue strength [47; 48]. The addition of rare earths, while hardly influencing the microstructure, strength, or hardness of the high-carbon-chromium steel was observed, also led to a significant increase in fatigue strength due to the reduced inclusion size [49]. Not only the size of the inclusions but especially the ratio of inclusion size to grain size is reported to be decisive for the fatigue properties [44; 50]. A recent publication by [PERSON] et al. [36] focusing on this issue has shown that the effect of inclusion size on fatigue strength is strongly dependent on the microstructure and vice versa. Inclusion-induced crack initiation was dominant for specimens containing large inclusions, while non-inclusion-induced failure was observed only for specimens with large grain sizes. [PERSON] et al. observed increasing FGA sizes with increasing hydrogen content [51] and concluded that the hydrogen trapped around non-metallic inclusions is responsible for the FGA-formation and postulated the model of \"hydrogen assisted crack growth of short cracks\" which is described in Section 3.1. [PERSON] et al.