## 3 Results and Discussion
### Measurement of Mechanical Properties of Composite Al-Nano-Al\({}_{2}\)O\({}_{3}\)  
Experimental methods to study mechanics of deformable \"Al-nano-\(\alpha\)-alumina\" composites included hardness studies and tensile testing of the material with data registration in the form of \"stress-strain\" curves. For the in-depth mechanical studies, samples with an aluminum oxide concentration of 10 and 14 wt.% were selected. The instrumented indentation data on the specific features of Al and Al-Al\({}_{2}\)O\({}_{3}\) behavior under elastoplastic strain are given in Table 4. It is evident that the \"in situ\" implementation of uniformly distributed nano-\(\alpha\) Al\({}_{2}\)O\({}_{3}\) (10 and 14 wt.%) into the aluminum matrix decreased in the maximum and residual indentation depth h\({}_{\text{max}}\) and h\({}_{\text{p}}\) and increased in the Martens hardness H\({}_{\text{M}}\) and the indentation hardness at all loads HIT. Such simultaneous change in the characteristics h\({}_{\text{max}}\), h\({}_{\text{p}}\), H\({}_{\text{M}}\), H\({}_{\text{HT}}\), W\({}_{\text{e}}\), and W\({}_{\text{t}}\) measured in the indentation process is commonly caused by the material hardening and mainly is observed in aluminum composites with uniformly distributed alumina nanopowders [40; 41; 42]. In particular, for the samples \"Al- nano-alumina \", an average increase in H\({}_{\text{IT}}\) was nearly 20-25%. Contact elasticity modulus E * decreased for Al-14% Al\({}_{2}\)O\({}_{3}\) composite at the load 1000 mN and did not change for all other composites. The AMMC was characterized by higher resistance to elastoplastic strain as evidenced by an increase in the calculated parameters R\({}_{\text{e}}\) by factor 1.22-1.52, H\({}_{\text{IT}}\)/E* by factor 1.2-1.5, and H\({}^{3}\)IT/E*2 by the factor 2.1-2.7 (Table 5). The plasticity index \(\delta_{\text{A}}\) did not change at the increased hardness and remained relatively high in the range of 0.972-0.982. Figure 7: DSC curves (**left**) of initial aluminum (**1**), AMMCs with 7.79% (**2**), 18.4% (**3**), and 4.61% (**4**) of Al\({}_{2}\)O\({}_{3}\). It is evident that the yield strength did not increase with the increase of nano-\(\alpha\)-alumina content up to 14 wt.% in the composite (Figure 10). This is in opposition to conclusions drawn in [43] that the increase in tensile strength was observed only up to \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) concentrations of about 5 wt.% and the tensile strength decreased at other concentrations. The composites with an addition of 7-14 wt.% of \(\alpha\)- Al\({}_{2}\)O\({}_{3}\) possessed tensile strengths of 67.56 and 78.16 MPa, which were 1.10 and 1.27 times greater than that of the aluminum matrix sample (61.38 MPa). As can be seen from Figure 8, a further increase in the content of aluminum oxide in the composite to 19 wt.% neither improved, nor worsened, the mechanical properties of the composite. Thus, it can be considered that the aluminum oxide concentration of 14 wt.% was optimal from the point of view of the combination of all properties. A great simultaneous increase in the sample elongation accompanied this strong improvement of tensile strength. At the same time, significant hardening was accompanied by the elongation of the samples with the addition of 7-14 wt.% of \(\alpha\)-Al\({}_{2}\)O\({}_{3}\), which increased from 1.64 to 3.06 times with the greater addition of alumina and reached values of 21.32% and 39.77%, compared with 13% for pure aluminum. This means that the hardness, elasticity, and ductility of the aluminum-nano-\(\alpha\)-Al\({}_{2}\)O\({}_{3}\) composite improved simultaneously according to the oxide concentration with uniformly distributed \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) nanoparticles inside the aluminum grains.