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.
Max Payne 3-Crack Only
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