## 3 Results and Discussion
### Mechanical Properties of the Nanocomposites and Aluminum Matrix
The surface fracture observation results are according to the behavior perceived in the tensile curve. CNTs clusters are observed, which confirms that these are present at the grain boundaries that can significantly decrease the ductility of the nanocomposites. However, there was also the presence of CNTs on the fracture surface, agglomerated in the pores and embedded in the matrix. Additionally, some CNTs seem elongated in the tensile test direction, which can have acted as a bridge, hindering the crack propagation, as seen in other works [23, 24, 25, 26, 27, 28, 29, 30]. This means that the matrix can transfer the load to the CNTs, during the tensile test, which prevents crack propagation. As the cracks expand into the CNTs, they act as a bridge between two points in the matrix, restraining crack growth. However, these CNTs that act as a bridge will fracture. These protruding CNTs on the surfaces of some grains can be related to the increase in yield strength to the reinforcement mechanism associated with the load transfer from the matrix to the reinforcement during the tensile test. [PERSON] et al. [26] observed the load transfer mechanism that resulted in a 60% increase in the yield stress of Al/CNTs nanocomposites. The authors observed elongated and fractured CNTs on the fracture surface analyzed by SEM. Although the authors have identified different strengthening mechanisms acting simultaneously, they consider that load transfer was the dominant mechanism. Figure 10 shows the Vickers microhardness maps of the fractured tensile samples of the Al matrix and the nanocomposites. Significant differences can be observed between samples in the evolution of hardness. For the Al matrix, the highest hardness value is observed near the fracture surface, while the nanocomposite is observed in the zone furthest from the fracture. A detailed microstructure analysis was performed in the two areas (closer to the fracture surface and further away from the fracture surface) to better understand the  
Figure 7: OM image with an SEM detail and with the nanoindentation matrix marked and hardness and reduced Young’s modulus (Er) maps in the nanocomposites. hardness variations. This characterization involved the observation of the crystallographic orientation, the average misorientation of the grains and the determination of the estimated density of dislocations. Figure 8: Surface analysis of the fracture surface of the samples after the tensile test: (**a,b**) digital optical microscope image of the area selected of Al and Al/CNT samples; (**c,d**) 3D image of the area of the (**a,b**); (**e,f**) profile of the line selected in the surface image. A detailed analysis close to the fracture surface can be seen in Figure 11. Vickers hardness maps and average geometrically necessary dislocations (GNDs) density can be seen for the Al matrix and the nanocomposites. The Al matrix revealed a higher hardness  
Figure 10: Hardness distribution maps (HV 0.01) of (**a**) Al and (**b**) Al/CNT of fractured tensile test specimens. Figure 9: Secondary electrons image obtained by scanning electron microscopy (SEM) of different areas of the nanocomposite fracture surface of tensile samples, being (**a**) low magnification (**b**) higher magnifications and (**c**) a detail showing elongated CNT. in this region. However, the difference in hardness cannot be influenced by the grain size of the samples, since they show a similar grain size: 7.2 and 8.3, respectively. On the contrary, with the observation of these results, the aluminum matrix reveals a higher density of dislocations near the fracture surface than the nanocomposites. These results agree with the hardness map, which reveals a higher hardness value for the matrix than the nanocomposites for the same characterized area and the results observed in the tensile curves. The Al matrix showed a greater ductility than the nanocomposites and, therefore, a greater elongation, resulting in a greater deformed area and, consequently, a greater plastic deformation.