## 3 Results and Discussion  
The XRD pattern of the Al\({}_{20}\)Zn\({}_{80}\) powder mixture mechanically processed for 48 h is shown in Figure 1a and compared with the reference XRD patterns of pure Al and Zn. The mechanical processing made Al reflections disappear and Zn ones broaden. The Rietveld analysis indicates that broadening can be ascribed to the reduction of grain size from the initial value of about 100 nm to the final one of about 70 nm. A small downward shift of about 0.1\({}^{\circ}\) in the angular position of Zn peaks is detected. Based on the Al and Zn atomic radii [31], it can be ascribed to a terminal solid solubility of Al in Zn [32], with the two elements remaining substantially segregated. Therefore, it can be concluded that the mechanical processing of the Al\({}_{20}\)Zn\({}_{80}\) powder mixtures induces the formation of a nanocomposite. The SEM micrographs, such as the ones shown in Figure 1b,c, provide support for this conclusion. In particular, the SEM-BSE imaging indicates that the nanocomposite consists of finely dispersed nanocrystalline phases with diverse atomic numbers. Specifically, the zones appearing brighter have a higher atomic number, i.e., a higher Zn percentage. As shown by the SEM micrographs reported in Figure 2, the annealing at 550 \({}^{\circ}\)C under high-vacuum conditions for 1 h drastically changes the material microstructure. As shown in Figure 2a, the sample surface exhibits the typical appearance of NP metals with nanosized ligaments and pores (approximately 10 nm). The SEM micrograph in Figure 2b reveals that the NP architecture is maintained inside the sample, although the ligament size appears to be larger. EDS measurements shown in Figure 2c and Figure S1 indicate that the Zn atomic percentage, normalized with respect to the sum of the Al and Zn ones, mostly averages around 1% near the pellet surface, while it increases up to about 3% near the opposite surface. Therefore, Zn has been almost completely removed from the sample. The O content is related to the oxidation of the Al surface, with consequent formation of a thin passivation layer [33]. The experimental evidence heretofore presented definitely suggests that we have been able to fabricate almost pure NP Al. The architecture we observe can be well described in terms of open cells with irregular morphologies that tessellate the entire sample volume. Therefore, the NP Al can be considered a truly NP metal with ligaments characterized by a relatively high aspect ratio, which gives them their stocky appearance. It is possible to note that the sample shows a porous structure over its whole volume. However, relatively  
Figure 1: (a) XRD pattern and Rietveld refinement of the Al\({}_{20}\)Zn\({}_{80}\) nanocomposite, compared with reference reflection peaks of Al and Zn elements. SEM micrographs of the nanocomposite collected using (b) SE and (c) BSE. Figure 2: SEM-SE micrographs of (a) the NP Al surface and (b) the NP Al cross-section; (c) EDS spectrum corresponding to the cross-sectional area included between 37 and 74 μm from the pellet surface with \(\sim\)3% of Zn content. The spectra of the other areas of the sample are reported in Figure S1. large voids, such as the one shown in Figure 3a, have been observed. Our first hypothesis is that they are due to the incomplete compaction of the nanocomposite powders during the preparation of pellets. The shape of the N2 physisorption isotherms shown in Figure 3b is associated with the occurrence of a hysteresis loop and does not strictly match with a specific classification, supporting the NP Al structure's inhomogeneity [34]. The NP Al specific surface area, as obtained by BET analysis, is nearly 73 m2 g-1, a remarkably large value that compares well with the highest literature values for NP metals [35; 36]. Although limitations associated to surface area determination by physisorption routes have been reported, the correlation with the structural features has been pointed out [35; 36; 37].