## 3 Results and Discussion
Scanning electron microscopy (SEM) image of the surface and the cross-section (**c**) of the Al-Cu-Mg-Zn after PED treatment; the inset is the cross-section view of the recast layer. (**d**) TEM image of the recast surface layer within the rectangular boxes and its selected area electron diffraction (SAED) pattern. Besides the atomic diffusion, the reason for the change of surface composition can only be the detachment of elements from the surface of Al alloys. In our case, the detachment of surface elements of Al alloys can be attributed mainly to two possible ways. First, the bubble percussion peels the surface uniformly and cannot lead to the segregation of surface elements. Second, from the surface evaporation perspective, the boiling point-dependent co-evaporation provides us a possible way for understanding such abnormal element segregation after PED treatment. As shown in Table 2[19], the boiling points of Cu and Al are similar and are much higher than Mg. As a result, Mg would evaporate more easily than Al and Cu from the alloy's surface. Meanwhile, the similar high boiling points of Cu and Al lead to the co-evaporation of Al and Cu azeotrope during the PED heating and could cause significant element segregation of the Al and Cu. Therefore, element evaporation rather than atomic diffusion caused the element proportion and distribution changes in the MG layers on an Al alloy surface during the PED treatment. Figure 3: (**a**) Original element distribution of Al-Cu-Mg alloys by electron microprobe analysis (EPMA); (**b**) element distribution of Al-Cu-Mg alloys after PED treatment. Figure 2: X-ray diffraction (XRD) patterns of the Al-Cu-Mg and Al-Cu-Mg-Zn alloys by pulsed electrical discharge (PED) treatment with pure Al as reference. To confirm that evaporation induced the element proportion and distribution changes, Zn as a more volatile metal with 2.88 at. % was added in the Al-Cu-Mg alloys. Significant element segregation of Al and Cu was also observed after Zn addition, as shown in Figure 4b. In contrast to Figure 4a, the residual quantity of Zn was quite low, which indicates that metals with a lower boiling point evaporate more easily from the alloy's surface with PED treatment. Significant enhancement in corrosion resistance of the Al-Cu-Mg-Zn alloy has also been found after PED treatment. As shown in Figure 5a, two Al-Cu-Mg-Zn alloy samples with and without MG layers were dipped into 10 vol % HCl solution. The sample with the MG layer showed less bubble generation than the sample without the MG layer. From the full immersion corrosion loss presented in Figure 5d, the sample with an MG layer showed significantly higher corrosion than without. It is well known that the corrosion behavior of an alloy depends primarily on the chemical activity of the alloy elements within the solution. Mg and Zn elements are more prone to corrosion in HCl solutions. As a result, the reduction in the element proportions of Mg and Zn in MG layers caused by surface evaporation significantly improved their corrosion resistance. Meanwhile, the dense atomic arrangement of MG layers on an Al-Cu-Mg-Zn alloy led to relatively mild and uniform corrosion, which also improved their corrosion resistance. The average corrosion rates of the Al-Cu-Mg-Zn alloys with a MG layer were reduced to about 2% of that without an MG layer. The formation of an MG layer not only provided a corrosion-resistant layer on the Al alloy surface but greatly enhanced the surface hardness. As shown in Figure 5b,c, the microhardness of the Al-Cu-Mg-Zn alloy was greatly enhanced from 90.086 (\(\pm\)2) (mean, standard deviation) HV\({}_{0.1}\) to 537.173 (\(\pm\)7) HV\({}_{0.1}\) after the PED treatment. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Elements** & **Zn (\({}^{\circ}\)C)** & **Mg (\({}^{\circ}\)C)** & **Cu (\({}^{\circ}\)C)** & **Al (\({}^{\circ}\)C)** \\ \hline Melting point & 419 & 650 & 1083 & 660 \\ Boiling point & 907 & 1108 & 2580 & 2500 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Melting point and boiling point of the elements.