*3.3. E*ff*ects of Prebending Radii on Microstructure*

The TEM images of 7075 aluminum alloy at different prebending radii are shown in Figure 4, where the data indicate that the prebending radii have a significant influence on the microstructure of 7075 aluminum alloy. By measuring over 100 precipitates in TEM images, the average precipitate size

was 60 nm in width and 80 nm in length in the matrix, which was the largest at a prebending radius of 500 mm, and the width of the precipitate free zone (PFZ) was the widest, approximately 100 nm, as shown in Figure 4c. The PFZ width and precipitate size decrease as the prebending radius increases. When the prebending radius was 1000 mm, the width of PFZ was 81 nm, and the average precipitate size was 55 nm in width and 64 nm in length, as shown in Figure 4b. When the prebending radius was 1500 mm, the width of PFZ was only half of that when the radius was 500 mm, and the average precipitate size was 50 nm in width and 60 nm in length, as shown in Figure 4a.

**Figure 4.** TEM microstructure of 7075 aluminum alloy at different prebending radii. (**a,d**) 1500 mm; (**b,e**) 1000 mm; (**c,f**) 500 mm.

Previous studies [23] have shown that stress can increase dislocation density, thus accelerating the rate of phase transformation and growth. In addition, the dislocations have a significant effect on fatigue performance: the higher the number of dislocations, the greater the interface energy, and the greater the stress concentration around the dislocations. As can be seen from Figure 4, at a small stress (the prebending radius was 1500 mm), the precipitated phase in the matrix is a finer dispersion than in the other cases. On the one hand, the smaller the size and the more dispersed the precipitated phase, the stronger the combining capacity between the precipitated phase and the matrix, which can result in stronger fracture toughness, hardness and strength [24]. In this experiment, with the increase of the prebending radius, the size of the precipitated phase becomes smaller and the distribution becomes more homogeneous. Therefore, the hardness and tensile properties of the alloy increased. On the other hand, the smaller the size and the more dispersed the precipitated phase, the better the fatigue performance.

In this experiment, the average number and average length of dislocations were measured on an area of 0.93 μm × 0.93 μm of TEM images more than ten times. The stress was the largest at prebending radius of 500 mm, and thus the average density of dislocations was 7.996 <sup>×</sup> 109 cm−<sup>2</sup> and their average length (77.1 nm) of dislocations were also the largest, as shown in Figure 4f. This, together with the fact that the size of the precipitated phase was also the largest, causes the conditional fatigue limit to be the lowest. The average density of dislocations was 5.239 <sup>×</sup> <sup>10</sup><sup>9</sup> cm−<sup>2</sup> and the average length was 58.6 nm when the prebending radius was 1000 mm, as shown in Figure 4e. When the prebending radius was 1500 mm, the average density of dislocations was 3.86 <sup>×</sup> 109 cm−<sup>2</sup> and the average length (49 nm) of dislocations were smaller than those at the prebending radius of 500 mm and 1000 mm, and the size of precipitated phase was smallest as well, as shown in Figure 4d, so the fatigue life was the highest.

### *3.4. E*ff*ects of Prebending Radii on Fracture Morphology*

Under 260 MPa, the fatigue fracture topographies of 7075 aluminum alloy after creep age forming with different prebending radii are shown in Figure 5. The data show that there were three typical regions on fatigue fracture topographies, which are marked by A, B and C, where A is the fatigue crack initiation region, B is the fatigue stable propagation region, and C is the fatigue transient fracture region. Some shear lips appeared at the edge of the specimens, which are represented by the white arrow mark.

Under 260 MPa, the morphologies of the fatigue crack initiation regions of 7075 aluminum alloy after creep age forming with different prebending radii are shown in Figure 6. The data show that radial stripes appear in the fatigue crack initiation regions under low magnification (Figure 6a–c), while under high magnification (Figure 6d–f), many fine steps appear in the fatigue initiation regions, which have smooth surface and many tiny cracks. Those steps are the secondary cracks formed in the nucleation process of the crack, but which lose power.

**Figure 5.** Fatigue fracture topography of 7075 aluminum alloy at different prebending radii (*S*max = 260 MPa). (**a**) 1500 mm; (**b**) 1000 mm; (**c**) 500 mm.

**Figure 6.** SEM morphologies of fatigue crack initiation regions of the 7075 aluminum alloy at different prebending radii (*S*max = 260 MPa). (**a,d**) 1500 mm; (**b,e**) 1000 mm; (**c,f**) 500 mm.

Under 260 MPa, the SEM morphologies of the fatigue stable propagation regions of 7075 aluminum alloy after creep age forming with different prebending radii are shown in Figure 7. The data indicate that the fracture shows many fatigue striations and furrows. When the prebending radius was 1500 mm, the average width of the fatigue striation was 0.426 μm, obtained by measuring over 5 strips in SEM images. The average fatigue striation width was 0.308 μm when the prebending radius was 1000 mm and 0.368 μm when the prebending radius was 500 mm.

Under 260 MPa, the SEM images of the fatigue transient fracture regions of 7075 aluminum alloy after creep age forming with different prebending radii are shown in Figure 8. As can be seen from Figure 8, the fracture transient fracture regions show that there were both intergranular fractures and transgranular fractures. Intergranular fracture forms the stratified fracture planes on the fracture surface, while transgranular fracture forms dimples on the fracture surface [25]. When the prebending radius was 1500 mm, the dimples of 7075 aluminum alloy fracture were the largest. As the prebending radius decreases, the dimples become progressively smaller. It indicates that the toughness of the alloy decreases with the decrease of the prebending radius, which was consistent with the decrease in the elongation of the alloy.

**Figure 7.** SEM morphologies of fatigue stability propagation regions of 7075 aluminum alloy at different prebending radii (*S*max = 260 MPa). (**a**) 1500 mm; (**b**) 1000 mm; (**c**) 500 mm.

**Figure 8.** SEM morphologies of fatigue transient fracture region of 7075 aluminum alloy under different prebending radii (*S*max = 260 MPa). (**a**) 1500 mm; (**b**) 1000 mm; (**c**) 500 mm.
