It is observed in Figure 1, the microstructures for samples with Aluminum plus different CuO₂ additions, where in Figure 1(a) it is observed a dendritic behavior with 0.15 g additions of CuO₂, being the dendrite size of approximately 45 µm. For the case of sample with 0.25 g of additions of CuO₂ in Figure 1(b) it can be observed a dendritic behavior, where dendrites present an enlargement in its structure being the dendrite size of 90 µm average. On the other hand for sample with 0.50 g it can be observed its microstructure in Figure 1(c) that exist an evolution of the dendritic structure to equiaxed grains. Hence, the effect of the oxide additions it is clearly observed for the three different compositions where copper oxides promote de grain growth [1] . Microstructural variations are explained considering that oxides additions may induce the modification of the dendritic structure and grain growing under the mechanism that particles during the solidification process acts as nucleation points on which the aluminum grains solidify [2] [3] [4] .

The X-Ray Diffraction patterns are presented in Figure 2, in it can be observed different peaks corresponding to the compounds formed during the melting process.

The proposed chemical reactions developed during the melting-casting-solidification process are as follow:

4Al + 3CuO₂ → 3Cu + 2Al₂O₃ (1)

Al + 2CuO₂ → 2CuO + AlO₂ (2)

2Al + 2CuO₂ → Cu + Al₂CuO₄ (3)

It can be observed in the three possible reactions that Al and copper oxide produces the aluminum oxide plus copper, due to the dissociation of the oxygen present in the CuO₂. The proposed reaction mechanism of formation is the interfacial reaction of the oxygen of the CuO₂ with the liquid aluminum [6] . It is noticeable that the aluminum oxide (Al₂CuO₄) in Equation (3) is product of an overheating in the system and the agitation produced by the stirring process that permit a constant external detachment of the superficial layer of the CuO₂ particles.

In Figure 2(a) it is observed the diffraction peaks for samples with different CuO₂ particles additions without heat treatment, where is observed that that during the melting process are generated mainly two types of oxides; CuO and AlO₂. On the other hand, when alloys with different CuO₂ particles are exposed to annealing treatment (see Figure 2(b)), predominantly Al₂CuO₄ is generated. Finally, as can be seen in Figure 2(c) the change of atmosphere of heat treatments allows the creation of new metal oxides in very small quantities such as AlO₃ and Al₂O₃, this effect is attributed to an absence of oxygen in the furnace chamber [3] .

The particles size obtained from the X-ray diffraction spectrum was determined by measuring the average percentage of the width at half height (FWHM) of the characteristic peaks, for each of the alloys reinforced with different CuO₂ additions and with different heat treatments. Figure 3 shows the percentage of the width of the peaks corresponding to each of the alloys; for as-cast alloys (BA) it was observed that the higher the amount of oxide additions, the higher the amounts of resultant aluminum and copper oxides.

After annealing for 12 h, the oxides maintain the same behavior and in the sample AA 0.50 a lower amount of AlO₂ is generated, but a peak corresponding to copper and Al₂CuO₄ is shown with a width of 2.10% which is attributed to be affected by the amount of CuO₂ existing in the alloy [7] . Annealing in an argon atmosphere reduces the creation of different oxides in all alloys; Compared to BA, CuO₂ decreased for the alloy AAA 0.15 by 40.33%, AAA 0.25 by 58.94% and AAA 0.50 by 54.28% and for AlO₂ 90.08%, 75.32% and 79.41% respectively. The

previous data, allowed us to obtain as a result the average crystallite sizes calculated for each alloy, supposing that the fluctuation in hardness is inversely proportional to the average crystal size.

It is well known that electrical resistance of a material is proportional to the electrical resistivity and is inversely proportional to the electrical conductivity [21] . In plot of Figure 7 it can be observed a horizontal line that denotes value of the electrical conductivity of aluminum, this resulted 3.5 S/m at 20˚C, taking this value as a reference, it can be observed that copper dioxide addition in combination with the different heat treatments tends to increase the electrical conductivity. As it is well known, copper oxides are included in the family of superconductors [22] [23] and are called high temperature superconductors since they exhibit superconductivity at approximately 195˚C. For the OQ samples with 0.15g CuO₂ addition oil tempered it is observed that it keep the electrical conductivity close to the value of unalloyed aluminum. The decrease in electrical conductivity for the rest of the samples with this addition has been attributed as a result of the gradual increase in the number and size of particles of CuO₂, which leads to a decrease in the conductivity of the composite material due to the electrons dispersion; this effect has been correlated with impurities in aluminum matrix [21] [22] .

For the samples with the addition of 0.25 and 0.50, it is observed that the AA, AAA and OQT samples the conductivity increases, this is attributed to the low index of precipitates in the surface area that which produces a partially freeway effect on the matrix [22] , for consequence this phenomena generates an increment in electrical conductivity values.

On the other hand, the non-linear correlation of the electrical conductivity with the hardness perhaps is the result of the differences in response of the hardness with the thermal treatments, which has been affected via generation of copper and aluminum precipitates with different size and randomly dispersed, created during the different aging stages [22] [23] .