As demonstrated above, while there have been numerous studies on secondary phases of aluminum alloys, the mechanism of crack initiation and the effect of the stress ratio of the different secondary phases in the base material and joints are not well studied in a systematic manner.
The major cracking phase of the BM specimens was Mg2Si, which was the only stable intermetallic phase in the Mg-Si binary alloy. Although the Mg2Si phase has excellent mechanical properties, it is highly brittle and has low ductility, as shown in Figure 9 [22]. The backscattered electron images in Figure 10 illustrate the status of the coarse secondary-phase Mg2Si on the fracture surface, which is fragmented and debonded from the matrix. Figure 10b shows the small craters owing to the Mg2Si debonding from the substrate and the serrated cracks at the edges, which indicates that cracks tended to initiate in the bulk Mg2Si, with the subsequent coalescence and deflection of microcracks along the interface leading to serrated cracks. Although the Si content in the 7050 aluminum alloy was only 0.10%, its size was similar to that of Al2CuMg, and most of the Mg2Si phase observed at the broad transverse side was internally broken. However, the non-Mg2Si phase was tightly bound to the matrix, as shown in Figure 9b, causing an interfacial stress concentration or singularity owing to the elastic deformation inconsistency. This type of crack nucleation exhibited distinct fatigue cracks and extension marks on the bond surface on the microscopic level, as shown in Figure 11.
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Coarse secondary phase in the initial state of the BM: (a) microcracks in the BM of the Mg2Si phase; (b) Mg2Si phase with internal microcracks and various non-Mg2Si phases.
Figure 12a,b show the craters generated following the debonding of the Mg2Si phase, which indicates that the crack initiation mechanism of the Mg2Si phase in the joints was the same as in the BM. The thermomechanical deformation of FSW might have worsened the bonding strength between the Mg2Si phase and the matrix. Figure 13a shows the internal cracking of the large non-Mg2Si phase (50 μm), which was first subjected to external forces. Subsequently, the cracks gradually spread across the interface into the matrix as a result of the cyclic loading. As shown in Figure 13b, the crack growth in the non-Mg2Si phase can be divided into two distinct regions: (1) a smooth region created by instantaneous fracture close to the specimen surface and (2) a rough region with signs of stable crack extension far away from the specimen surface.
Distribution of the coarse secondary phase, during different stages of the fatigue crack extension: (a) early expansion stage; (b) stable expansion stage; (c) boundary between the stable and unstable expansion stages; (g) unstable expansion stage I; (h) unstable expansion stage II; (i) unstable expansion stage III. (d) processed image, (e) processed image, (f) processed image, (j) processed image, (k) processed image, (l) processed image.
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