3.2.2. SEM Observation

The SEM images showed in Figure 4 are the fracture surfaces of Al-Ga-In3Sn-Zn alloys with various content of Zn. All the Al grains in the prepared ingots present columnar shapes and directional growth during solidification. The Al grain sizes (column widths) range from 20 to 50 μm. According to statistics, the average size is approximately 35 μm, and the standard deviation is 6μm, which is similar to the Zn-free alloy (33 ± 5μm). Zn as an additive, which is similar to Bi [23,29], affects the grain size

slightly, but it is completely different from Ti [4], and the segregation has a greater effect on grain size [33–35]. Furthermore, the columnar surfaces are covered by a lot of small granular and irregular particles. Considering the solidification process, the GB phase begins to nucleate and grow at the grain boundaries, which is different from the nucleation time of Al grains. From the SEM images (Figure 4a–c), several cracks exit on the grain surfaces; these alloys easily oxidize in the air and lose their metallic luster. On the other hand, the grains with high Zn content (Figure 4d–f) are much more complete. This will delay the arrival of the fast hydrogen generation stage, because the alloy needs to split into small pieces to enable rapid hydrolysis.

**Figure 4.** SEM images of Al-rich alloys containing (**a**) 0 wt.% Zn, (**b**) 1 wt.% Zn, (**c**) 2 wt.% Zn, (**d**) 3 wt.% Zn, (**e**) 4 wt.% Zn, and (**f**) 5 wt.% Zn.

W. Wang et al. [19] had indicated that the changes in size and number of GB particles will lead to different hydrogen production performance of alloys. In their study, the larger number and size of particles contributed to increasing the rate of hydrogen production. A statistical analysis of grain-boundary phase particle areas, obtained from 40 images, is shown in Figure 5. As the doping amount of Zn is within the range of 0–5 wt.%, total area of GB particles in the observed area decreases rapidly (from 6.5% to 2.5%). The average area of individual GB particles has also changed and was

reduced from 15 to 12 μm. When the Zn content exceeded 2 wt.%, the particle size was stable (between 11 and 12 μm). The results indicate that the relative content of Zn and In3Sn has a certain effect on the formation of GB phase, and these phenomena need to be further explained in conjunction with EDX data.

**Figure 5.** Area ratio of GB particles to the total area and GB area versus Zn contents. Inset shows the GB particle numbers versus Zn content.
