## 4 Discussion  
Given the above stated observed characteristic of the FGA, as well as the framework conditions that favor its development and the postulated models of the formation mechanism, an evaluation of these underlying mechanisms will now be carried out within this chapter. Concerning the model first postulated by [PERSON] et al. [9], which is based on hydrogen-induced short crack growth, resulting in the granular area, the following questions arise in consideration of the present state of knowledge; while trapped hydrogen is observed around non-metallic inclusions, influencing the crack propagation of short cracks, only the rough surface can be explained by hydrogen-induced short crack propagation. The actual observed grain refinement however is neither considered nor explainable by this model. Furthermore, the FGA-formation is also observed around artificial defects under vacuum conditions where no hydrogen is present [83] and there are results from [PERSON] et al. [53] observing decreasing FGA-formation for bainitic as well as martensitic structures with increased hydrogen content. For all investigated conditions, hydrogen decreased the fracture toughness and therefore reduced the critical stress intensity factors with increased hydrogen content. Since the characteristic grain refinement process cannot be explained and the presence of hydrogen is not needed for the formation of the FGA, while hydrogen embrittlement might obviously influence the crack propagation (even in the VHCF-regime), from the authors point of view it is not possible to be the root cause for the formation of the FGA nor the failure in VHCF-regime. A very similar problem arises for the model first postulated by [PERSON] et al. [10], which is based on dispersive decohesion of spherical carbides. Although the rough surface within the FGA observed via SEM would be explained, the formation of the fine-grained three-dimensional layer is inexplicable. In addition, carbides around analyzed VHCF fracture surfaces do not show decohesion, nor is an influence of carbides on the crack path observed [91]. Although the model postulated by [PERSON] et al. [12] based on polygonization with nucleation and coalescence of micro-debondings involves the formation of a fine-grained layer, the size of the FGA, which far exceeds that of the stress-raising effect of inclusions, cannot be explained. Furthermore, there are observations of secondary cracks propagating in the middle of the FGA instead of the interface of FGA and original microstructure [85; 88]. If the coalescence of micro-cracks would be the underlying formation mechanism, a widespread network of micro-cracks should be observed in the FGA, which is not supported by the majority of observations. Also, the slow crack propagation rate evaluated for the crack initiation and early crack propagation is inexplicable by the propagation of short cracks. The theoretical model for the formation of FGA based on the superplastic effects and vortex shaped flow resulting in FGA by [PERSON] [120] is based on a high adhesion between the inclusion and the surrounding matrix. This adhesion however is not present in the case of artificial defects and therefore the formation of an FGA around them when tested in vacuum [84] are inexplicable by this model. The model for the FGA-formation based on NCP by [PERSON] et al. [14] give an explanation for the formation of a fine-grained layer, as well as the observed size of it. The fact that FGA-formation can only be observed in the VHCF-regime is attributed to the cycles required to form the fine-grained layer by repeated surface contacts. Also, the tendency of the FGA being less pronounced with increasing stress ratio is explained because of the decreasing compressive stresses between the fracture surfaces. However, the formation of the FGA for positive stress ratios is only explicable by assuming crack closure effects overcompensating the loading conditions.