## 2 Properties of Fine Granular Area
### Characteristics of the FGA
#### 2.2.4 Characteristics Observed by Electron Backscatter Diffraction and Inverse Pole Figure  
[PERSON] et al. [88] used electron backscatter diffraction (EBSD) imaging to investigate the grain refinement of a crack with FGA-formation of a martensitic steel 0 Cr17 Ni4 Cu4 Nb (AISI630) under variable amplitude loading (see Figure 6). A discontinuous graded layer structure (consisting of some areas with ultrafine and coarse grains and other areas with coarse grains only) as shown in the EBSD image in Figure 6c) has been reported. To further analyze and describe the FGA secondary cracks has been analyzed via EBSD. The propagation of the crack within the FGA as well as along the interface between the fine-grained area and the surrounding matrix was observed. In addition, it is reported that the FGA is not continuously formed on both sides of the fracture surfaces and that a gradient layer with respect to grain size is also formed locally before the crack tip. Therefore, grain refinement is evaluated to be a result of the interaction of dislocations in the immediate vicinity of the crack-initiation site or crack tip because of local high stress. Based on those observations, [PERSON] and [PERSON] [104] analyzed the microstructure around a previously introduced fatigue crack after an additional compressive fatigue test for the same material, to clarify the influence of repeated compressive stresses between fracture surfaces on the FGA-formation process, by means of EBSD (see Figure 7). The martensitic stainless steel 0 cr17 ni4 cu4 nb (AISI630), which showed FGA-formation under variable amplitude loading at an amplitude of 430 MPa after \(5\times 10^{6}\) cycles with a stress ratio of R = \(-1\)[88], shows no FGA-formation on a previously created crack after the same number of cycles during compressive load (\(6_{\text{max}}\) = 0 MPa \(6_{\text{min}}\) = \(-430\) MPa). [PERSON] and [PERSON] mention that the test was done in air environment, but refers to experiments of [PERSON] et al. [85] and state that according to [PERSON] et al. the FGA feature and the fine-grained layers are also observed for fracture surface in air for cold-rolled stainless steels in VHCF-regime. Since no grain refinement was observed after the compressive fatigue test, they therefore conclude that the FGA is not formed by a large number of repeating fracture surface contacts due to compressive loadings, but that FGA is rather formed during the crack-initiation process. Because of the uniqueness of [PERSON] et al. 's results, from the authors point of view, the tests should also be performed under vacuum conditions to confirm this conclusion. [PERSON] [91] recently published a study to clarify the influence of the FGA on crack propagation during crack initiation and early crack propagation. For this purpose, a VHCF fatigue test was carried out on a sample with an artificial surface defect in a vacuum environment and the microstructure of the FGA was analyzed by means of EBSD (see Figure 8). Figure 5: Measured FGA thickness \(t_{\text{FGA}}\) in comparison to the plastic zone size (plane strain) perpendicular to the crack plane \(r_{\text{py}}\) calculated by different plastic zone size models [90]. Figure 6: (**a,b**) SEM images of the cross-section samples cut along the loading direction in the “tree ring” region of a specimen failed at a total fatigue life of \(8.567\times 10^{7}\) under variable amplitude loading (\(\alpha_{a}^{H}=650\) MPa, \(\text{n}_{\text{H}}=4\times 10^{3}\), \(\sigma_{a}^{H}=430\) MPa, \(\text{n}_{\text{L}}=3\times 10^{6}\)), (**c**) EBSD image of the rectangle region in (**b**) with a scanning step of 10 nm for the enlarged EBSD image in the green rectangle, 40 nm for the upper part and 65 nm for the lower part of the large EBSD image [88]. Figure 7: _Cont._  
Figure 8a shows the indexing confidence map as a greyscale image, with a high degree of agreement in the light areas.