## 2 Properties of Fine Granular Area
### Characteristics of the FGA
#### 2.2.5 Characteristics Observed by Transmission Electron Microscopy and Selected Area Diffraction  
Selected area diffraction (SAD) of transmission electron microscopy (TEM) lamellas prepared by FIB is commonly used to analyze the grain refined regions resulting from VHCF tests. [PERSON] et al. [12] observed that the diffraction patterns inside the FGA displayed ring-like structures, that indicate the presence of many small grains in the analyzed area, while the corresponding image in an area outside the FGA displayed spot-like patterns peculiar to the original relatively coarse martensitic structure. The fine granular area observed at the fracture surface via SEM was shown to be a three-dimensional layer of refined grains with a thickness of about 40 nm. [PERSON] et al. [15] reported similar findings and confirmed a drastic grain size reduction inside the FGA in comparison to neighboring areas. The analysis of the ring-like patterns (Debye Rings) gave an average grain size estimation of about 70 nm inside the FGA and the patterns indicated, that those grains still consisted of martensite and austenite phase. While describing a high dislocation density, which made the observation of the smallest grains via TEM difficult, [PERSON] et al. state that the high miss orientation observed via TEM inside the FGA suggest large grain boundaries. Additionally, an arrangement of dislocations was observed within a martensitic needle, which could be a pre-stage of dislocation cells. [PERSON] et al. [105] report the finding of special dislocation structures formed in the FGA during VHCF fatigue by means of high resolution TEM, too. The observed structures correspond to a special type of dislocation sub cells with the size of about a few hundred nanometers. The dislocation density inside those sub cells is relatively low, while the density inside the cell boundaries is neither constant nor homogeneous. Some areas of the cell boundaries show a very high dislocation density, while other areas display a low dislocation density. The grain refinement inside the FGA by means of SAD has been since confirmed in several studies [14; 83; 85], with observed thickness of the fine-grained layers ranging from 100-300 nm [85], 150-300 nm [83] up to a range of 500-800 nm [14]. In addition, a gradient in regards to the grain size inside the FGA has been found. The smallest grains are observed in the direct vicinity of the crack-initiation site with an increasing grain size along the crack path [83; 106]. This gradient is also reported at the border between the fine- and coarse-grained volume, corresponding to a mostly  
Figure 9: Map of the highest Schmid factors for the sliding system [110]/\(<\)1–11\(>\) and the traces of the [110] sliding planes in the areas 0 to 9, whereby the planes with the highest Schmid factor are marked in red and labelled with the corresponding values [91]. smooth transition rather than a harsh cut [83]. Furthermore, grain refinement in front of a VHCF-crack tip has been observed by means of SAD [83; 88]. [PERSON] et al. [88] carried out TEM and SAD investigations of samples after variable amplitude loading and resulting \"tree ring pattern\" consisting of alternating rough fracture surface areas (generated at low stress amplitude) and smooth fracture surface areas (generated at high stress amplitude). Grain refinement could be detected locally isolated under the smooth areas. In contrast to the reported continuous fine grain layers on both sides of VHCF-cracks [14; 83; 85], [PERSON] et al. observe a discontinuous grain refinement under both smooth and rough areas, they therefore conclude that the grain refinement occurs during the low-stress amplitude in front of the crack tip before the smooth fracture surface is generated by the high stress amplitude and the resulting high crack propagation rate.