## 3 Results and Discussion
### Mechanical Properties of the Nanocomposites and Aluminum Matrix  
Tensile tests initially evaluated the mechanical properties of the Al/CNT nanocomposites. The Al matrix was also evaluated and produced under the same conditions for comparison purposes and to evaluate the effects of the presence of the reinforcement in the matrix. Figure 6 shows tensile curves for the nanocomposites and aluminum matrix tensile specimens performed at room temperature. For each condition, four samples were tested. The aluminum matrix samples showed lower yield strength, with a maximum attained  
Figure 3: Full analysis of the Al powder as received showing (**a**) Scanning electron microscopy (SEM) image of Al powders, (**b,c**) exemplifying particle analyzed with electron backscatter diffraction (EBSD) with an (**a**) inverse pole figure map and (**b**) Kernel average misorientation map, (**d**) size distribution and (**e**) average values of length and width of the particles. The high spread in results especially observed in the nanocomposite tensile curves can be associated with the impossibility of obtaining a totally uniform dispersion of carbon nanotubes into the matrix, as well as the existence of pores. These features result in some microstructure heterogeneities (local grain size and crystallographic misorientation, for example) that consequently influence the mechanical behavior of the samples [8; 21]. This different mechanical behavior observed between the nanocomposites and the Al matrix can be explained with a detailed mechanical characterization associated with a microstructural characterization of the mechanically tested samples. The samples that showed a higher yield strength value (curves 1 for both samples) were chosen for the detailed microstructural characterization. Nanoindentation experiments were performed to evaluate hardness and reduce Young's modulus (Er) distribution maps to evaluate the effect of CNTs on the Al matrix. Figure 7 shows the hardness and Er maps for Al/CNT nanocomposites. Based on these maps, it is possible to observe significant differences closest to the CNTs' clusters. Nanocomposites are characterized by high hardness values and corresponding high values of Er, which corresponds to the presence of CNTs. This hardness variation may help explain what is observed in the tensile curves. The detail in the OM image shows an SEM image, where clusters of CNTs are visible [22]. This observation of reinforcement at the grain boundaries in the form of clusters that promotes an increase in hardness in some areas of the matrix at the nanometric scale could be one of the reasons for the significant decrease in the elongation observed in the tensile curves for the nanocomposites than the Al matrix. The fracture surface after tensile testing was observed in detail to understand better the mechanical behavior of the samples. Figure 8 shows the fracture surface of the Al matrix and Al/CNT samples observed by digital optical microscopy. Figure 6: Tensile curves of tensile specimens of aluminum matrix and the nanocomposites produced under the same conditions. With the observation of the images, great differences between the samples are not detected. The two samples present surfaces with a high topography, which agrees with the results observed in the Al tensile curve but not with what is observed for the nanocomposites. With the observation of the profiles, Al exhibits a greater height between the salience observed in the lower plane compared to the higher one. Still, the difference with nanocomposites is relatively low. A detailed observation was conducted by SEM in the fractured tensile samples and is present in Figure 9. Based on these images of Al/CNT samples, it is possible to detect that fragile features are characteristics. However, some areas also revealed deformed grains corroborating with a fracture surface characterized by mixed regions, not completely brittle or ductile. This can be explained due to the microstructure of the nanocomposites that present CNTs clusters, well-dispersed CNTs and pores.