# Microstructure Study on Very High Cycle Fatigue of an
## 3 Results and Discussion
### Vhcf-Induced Fracture and Microstructure Features  
For the grain structure, the results reported in this paper are not consistent with those in the literature [4; 5; 9; 17; 29; 33; 34; 35; 36; 38; 45]. There are no observed nanograins in the crack initiation region of Vhcf at \(R=-1\), but a few nanograins formed underneath a very local area with a size of about several micrometers on the fracture surface adjacent to the Vhcf origin under \(R=0\). One probable explanation is that the nanograin thermal stability of aluminum alloys [46] differs from the cases of high-strength steels or titanium alloys, and the failure cycle is not sufficient to satisfy NCP (numerous cyclic pressing) conditions proposed by [PERSON] et al. [35]. In accordance with Ref. [17], Vhcf life beyond \(4\times 10^{8}\) cycles is necessary for microstructure refinement at \(R=-1\). Nevertheless, it cannot explain why nanograin formation can occur in Vhcf of \(R=0\), although there are only a few number of nanograins and within the very limited small area at the marginal region around the crack initiation site.  
Figure 3: TEM characterizations for sample A1 cut from specimen A: (**a**) bright-field image, (**b**) dark-field image. TEM characterizations for sample A2 cut from specimen A: (**c**) bright-field image, (**d**) dark-field image. Specim A printed horizontally and failed by fatigue loading of \(R=-1\), \(\sigma_{\text{m}}=1\) MPa, \(\sigma_{\text{a}}=93\) MPa, and \(N_{\text{f}}=3.67\times 10^{8}\) cycles.  
For the distribution of chemical elements, Si networks are broken and disappeared in the profile surface layer of TEM sample A1. As shown in Figure 3b, it seems that Si precipitates with various orientations sparkle along the subsurface of dark-field image. Outside of the surface layer, the Si networks and Al cells in sample A1 are not distinctly different from other TEM samples, except for shapes and orientations. The shapes and orientations of Si networks and Al cells are closely tied with the original AM microstructure, depending on the location of TEM sampling. In addition, the rearranged Si is preferred to precipitate in the region near the fracture surface on the profile microstructure of VHCF loaded specimens.  
The phenomena of Si networks partially dissolved, rearranged, and precipitated are most pronounced in sample A1, followed by sample B1, again in sample B2, and lastly in sample A2. This ranking is completely identical to that of the granulation degrees for the local fracture surfaces around the samples, as described in Section 3.3. These characterizations confirm that the granulation of the fracture surface and the redistribution of Si elements near the fracture surface are the main characteristics of crack initiation in the AM aluminum alloy experienced VHCF. These behaviors of granulation and redistribution have little to do with the stress ratios \(R\), but are positively correlated with the distance between the location and the crack source. It is implied that the granulation and redistribution are more dominated by the loading cycle. Only when the loading cycles are similar does the  
Figure 4: TEM characterizations for sample B1 cut from specimen B: (**a**) bright-field image, (**b**) dark-field image. TEM characterizations for sample B2 cut from specimen B: (**c**) bright-field image, (**d**) dark-field image. Specim B printed vertically and failed by fatigue loading of \(R=0\), \(\sigma_{\text{m}}=40\) MPa, \(\sigma_{\text{a}}=40\) MPa, and \(N_{\text{f}}=3.82\times 10^{8}\) cycles.  
effect of negative stress ratios emerge, causing the significant Si rearrangement in sample A1 and the maximum roughness around the TEM sample location on the fracture surface of specimen A under VHCF loading at \(R=-1\).