*2.2. Characterization*

The Mg–Al bimetal specimens were cut along their cross section. A Quanta 200 scanning electron microscope (SEM, FEI, Eindhoven, The Netherlands) equipped with energydispersive X-ray spectroscopy (EDS) was used to investigate the microstructure and chemical composition of the Mg–Al bimetallic interface. The thickness of the Mg–Al interface and the size of the precipitated phase were measured using the image-pro software, and the measure method and process are illustrated in supplementary materials, as shown in Figures S1–S11. The element distribution of the interface was tested by WDS equipped with EPMA (EPMA-8050G (Shimadzu, Tokyo, Japan)). An XRD-6100 X-ray diffractometer (XRD, Shimadzu, Tokyo, Japan) was employed to identify the phase compositions at the Mg–Al interface. Further investigation of the constitutive phases at the interface was performed using transmission electron microscopy (TEM; JEOL2100, Tokyo, Japan). The bonding properties of the Mg–Al bimetal were tested by a push-out experiment in the ZwickZ100 universal testing machine (Zwick, Roell, Germany) with the compression rate of 0.5 mm/min. The schematic diagram of the push-out experiment is shown in Figure 3. The bonding property of the Mg–Al bimetal was evaluated according to Equation (1) [33–35]:

$$S = F/(\pi dh),\tag{1}$$

where *S* is the shear strength of the Mg–Al bimetal, *F* is the maximum force loaded obtained from the testing machine during the compression process, *d* is the diameter of the aluminum rod, and *h* is the height of the specimen. The fracture behavior of the Mg–Al bimetal was analyzed using SEM equipped with EDS.

**Figure 3.** Schematic diagram of the push-out test for the shear strength testing.
