## 5 Conclusions  
Because of the fact that FGA corresponds to a three-dimensional fine-grained layer, early models that are based on hydrogen-induced short crack propagation or the decohesion of carbides, that neither take grain refinement into account nor explain it can be regarded as disproven. As a result of the observed FGA-formation around artificial defects, it is evident that the model of vortex shaped flows is not able to explain the formation mechanism due to the lack of adhesion between the defect and the surrounding matrix. Similarly, the model based on FGA-formation before crack propagation by polygonization due to the stress-increasing effect of inclusions with subsequent coalescence of micro-cracks cannot explain the size of FGA in the crack propagation direction. A detailed analysis of the effect of load ratio in regards of compressional testing is important from the authors point of view. If the formation of the FGA is based on the dislocation rearrangement and dislocations cells formation resulting in grain refinement a symmetrical decrease in the FGA is expected when superimposed with positive or negative mean stresses. However, if the FGA-formation is based on the compressive forces between previously generated fracture surfaces, the decrease in FGA-formation which is observed for increasing positive mean loads should lead to a stronger formation under negative mean stress. The effect of cyclic creep, as well as the influence of the threshold value for crack propagation should be considered while comparing different load ratios. Additionally, investigations regarding the shape of the FGA are necessary since contradictory observations exist and the reasons for those different observations are still unknown. The favorable influence of stress ratio and vacuum-like conditions, and especially the local grain refinement being observed in front of a crack tip of VHCF-cracks, indicate, in the authors' view, that the grain refinement mechanism is based on dislocation rearrangement and interaction within the plastic zone and not based on cold welding or plastic deformation of the crack wakes due to repeated fracture surface contact. The very slow crack propagation in the early crack growth stage and the observed good agreement between the FGA layer thickness and the plastic zone size are indications for the evaluation of initially non-propagating long cracks, which is why the model of local grain refinement in front of the crack tip currently describe the VHCF behavior and the FGA-formation most accurately. Also, the relatively early crack initiation without further crack propagation observed via SR \(\mu\)-CT [100] suggests that the interaction of the crack with the surrounding microstructure is responsible for the fatigue failure in the VHCF-regime. This indicates from the authors point of view, that the grain refinement based on dislocation rearrangement into dislocation cells and the formation of new grain boundaries therefore reducing the local threshold for crack propagation enabling the propagation of the VHCF-crack might be the underlying mechanism. However, further investigations are necessary to perform a more detailed evaluation of the postulated grain refinement models based on dislocation rearrangement and to clarify the question of crack characterization as long or short cracks. Especially the understanding of the influence of microstructure on crack propagation during the crack initiation and early propagation phase seems to be of crucial importance. Investigations on artificial surface defects in vacuum, as well as further investigations using SR \(\mu\)-CT to evaluate crack propagation and initiation during the formation phase of the FGA seem to be promising. Also, newly developed EBSD-analyzing methods like the one described in [127] might be used for the characterization of the interactions between VHCF-cracks and the microstructure. Conceptualization, J.P.S. and E.K. ; writing--original draft preparation, J.P.S. ; writing--review and editing, E.K.