*3.1. Microstructure of As-Cast Alloy*

Figure 1 shows the XRD patterns of as-cast alloy and three ECAP alloys. Two phases, α-Mg and Al2Ca phases, are indexed from the diffraction peaks for all alloys, which suggests no obvious phase transformation occurs during hot deformation. Since the Ca/Al ratio of this alloy is lower than 0.8, Al2Ca phase becomes the main second phase [24]. Figure 2a,b show the optical and SEM micrographs of as-cast Mg-3.7Al-1.8Ca-0.4Mn alloy. Dendritic α-Mg grains and continuous-network interdendritic second phases (marked by A in Figure 2b) could be observed. Seen from the enlargement of the network phase shown in Figure 2c, it exhibits a lamellar shape, consistent with the morphology of a typical eutectic structure. The corresponding EDS result (Figure 2e) reveals high contents of Al and Ca elements, indicating they could have a Al2Ca + α-Mg eutectic structure. Figure 3 shows the TEM image of the eutectic structure. Black and white stripes are arranged alternately. The index of the corresponding selected area electron diffraction (SAED) pattern of the black stripes (inset of Figure 3) further demonstrates the existence of Al2Ca phase.

**Figure 1.** X-ray diffractometer (XRD) patterns of as-cast and ECAP-ed alloys.

**Figure 2.** Microstructure of the as-cast alloy. (**a**) Optical image; (**b**,**c**,**d**) scanning electron microscope (SEM) images and enlargements of (**c**) the eutectic structure and (**d**) cubic particle; The corresponding EDS results of (**e**) the eutectic and (**f**) the particle.

**Figure 3.** Transmission electron microscope (TEM) micrograph of the eutectic structure and the corresponding selected area electron diffraction (SAED) pattern of the black lamellar phases.

Moreover, some cubic particles (marked by B in Figure 2b) are observed near the eutectic network. EDS analysis on one particle (Figure 2d) was performed and the result (Figure 2f) suggested that it

contains much higher Mn and Al elements, which should be Al8Mn5 phase [19]. However, Al-Mn particles were not detected from the XRD patterns due to the fact that the content is extremely low.
