## 3 Results and Discussion  
It is well known that PED is generally used as a traditional metal cutting method, which can instantly heat the surface of Al alloys far beyond their boiling points by the spark [14]. Under such high temperatures, the surface elements of Al alloys are vaporized and then carried away by the water fluid flow, as shown in Figure 1a. Then, the melted surface rapidly cools to room temperature by high-speed flow and the nature of the thermal diffusion to Al alloy substrate. The PED equipment and the samples in our case are presented in Figure 1b. Obviously, the surface color of the Al alloy became gray after PED treatment due to the formation of a recast layer. As shown in Figure 1c, the recast layer on the Al alloy was about 25 \(\upmu\)m without obvious cracks and had an amorphous state. \begin{table}
\begin{tabular}{c c c c c} \hline
**Alloys** & **Zn (at. \%)** & **Mg (at. \%)** & **Cu (at. \%)** & **Al** \\ \hline Al-Cu-Mg & - & 1.6 & 1.66 & Balance \\ Al-Cu-Mg-Zn & 2.88 & 1.75 & 1.11 & Balance \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the alloys. The electoral electron diffraction provided similar results (inset in Figure 1d). The ultra-fast heating and cooling rates led to the formation of the MG layer on the surface of the Al alloys through the PED treatment. Figure 2 shows that the characteristic peaks of the Al and AlCu phases disappeared after PED treatment, which proves the complete vitrification on their surfaces during the ultra-fast cooling during the PED treatment. As reported, the extreme heat generated and the gas produced by the electrolysis of water by the PED treatment led to the generation of bubbles (with diameters ranging from 10-100 \(\upmu\)m) near the surface of the Al alloys [15; 16], as shown in Figure 1a. The explosion of these bubbles bounded the Al alloy surface, which caused a shallow pit-shaped recast layer without obvious orientation, as shown in Figure 1c. Meanwhile, the heat generation induced rapid transformations of the three phases (vapor, liquid, and solid), simultaneously causing significant material evaporation and the formation of MG layers at the Al alloy surface. The bubble percussion and surface evaporation on the surface have a great influence on the element proportions and distribution in the MG layers of Al alloys. To understand the PED-induced changes of the main element proportions and distributions in Al-based metallic glass, element mappings of the Al-Cu-Mg alloys as cast and after PED treatment were carried out by EPMA. As shown in Figure 3a, Cu and Mg in the Al-Cu-Mg alloy tended to be concentrated together at the solidification phase, whereas there does not seem to have been a segregation of Al and alloying elements (Cu, Mg) at the conventional solidification phase. It is easy to accept that the Al content in the region of the solidified phase is poorer in Al alloys, according to our common metallography cognition. However, the proportions and distributions of the Al-Cu-Mg alloys after PED treatment showed significant differences (Figure 3b). Serious element segregation of Al and Cu was observed on the MG layer of Al-Cu-Mg alloys after PED treatment, which shows almost an opposite position with that of the \(\alpha\)-Al and micro-alloying element Cu in the solidified phase. The element Mg was uniformly distributed in the Al matrix without element segregation. The serious reductions in the element proportions of Cu and Mg (Figure 3b) show significant differences in the atomic diffusion-induced element concentration in the traditional casting process [17; 18]. Such PED-induced changes in the main element proportions and distributions in Al-based metallic glass cannot be understood from the traditional diffusion kinetics in solidification processes. Figure 1: The schematic diagram of the pulsed electrical discharge equipment (**a**); the inset is Al-Cu-Mg-Zn samples with and without pulsed electrical discharge (PED) treatment (**b**).