## 3 Results and Discussion  
### Structure and Hardness  
The structural modification of the carbon phase in the volume of Al-Fe matrix was studied using Raman spectroscopy. The Raman spectrum of the carbon inclusions shown in Figure 1 contains a characteristic G peak at 1576 cm\({}^{-1}\), a D peak at 1347 cm\({}^{-1}\), and a 2D peak at 2699 cm\({}^{-1}\), meaning that the carbon inclusions can be identified as graphene [33]. The analysis of carbon peaks' intensity ratios allows for the determination of the structural properties of graphene [34]. The intensity ratio of the 2D to G bands, I\({}_{\rm 2D}\)/I\({}_{\rm G}\), was found to be 0.48, which implies the formation of triple-layer graphene. This type of graphene has very \begin{table}
\begin{tabular}{c c c} \hline  & **Fe** & **C** \\ \hline AlFe & 2.5 & 0 \\ AlFeGn & 2.4 & 0.12 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the samples (wt.%). few defects since the intensity ratio of D peaks, characterizing the deficiency of the carbon film, to G peaks, ID/IG, was found to be 0.29, which is far below unity. The presence of a small kink (D\({}^{\prime}\) at 1600 cm\({}^{-1}\)) indicates the appearance of five-seven-membered rings in the graphene structure. A comparison of the results of the metallographic studies of the macrostructures of the cross-sections of the AlFe alloy and AlFeGn composite ingots is shown in Figure 2.  
Figure 2 shows that the microalloying of aluminum with graphene changes the nature of crystallization. The ingot of the AlFe alloy has a zonal structure, consisting of columnar and equiaxed grains (Figure 2a), and in the ingot of the composite, a zone of columnar grains is absent (Figure 2b). The grain size is practically the same (400-600 \(\upmu\)m), i.e., graphene is not an inoculant and does not modify the as-cast structure of the aluminum alloy, and this finding agrees with the results previously obtained for other aluminum matrix composites incorporating graphene [29; 30; 31; 32]. The uniformity of the composite structure may be due to its higher thermal conductivity resulting from the addition of graphene, which can clearly be seen in Figure 3. All the values of the thermal conductivity of the matrix alloy lie below the ones for the thermal conductivity of the composites. The higher the thermal conductivity of the metal, the faster the melt temperature equalizes over the volume of the bath. As the temperature difference of the melt near the walls of the crucible and in its center becomes smaller, the melt cooling rate decreases, and supercooling at the crystallization front increases. Such thermo-physical conditions of crystallization contribute to the suppression of the growth of the columnar structure [35]. Figure 1: Raman spectrum of the AlFeGn composite. Figure 2: The macrostructures of the cross-sections of the ingots: the AlFe alloy (**a**) and the AlFeGn composite (**b**). Based on these data, it can be supposed that during the crystallization of the composite, the temperature gradient in the volume of the melt quickly equalizes; consequently, the nucleation and growth of the solid phase proceed at the same temperature and rate along the cross-section of the ingot, resulting in the formation of a uniform macrostructure. A comparison of the hardness values of the matrix alloy and the composite shows that the hardness of the as-cast composites differs from the hardness of the matrix alloy by no more than 10-15% and equals 39-42 HV. A similar effect was previously observed in experiments investigating the effect of graphene on the structure of aluminum matrix composites incorporating magnesium and manganese [30,31]. According to the relevant phase diagram, Al-2.5%Fe has a hypereutectic composition (1.8%Fe is the eutectic point) [36]. Since the cooling conditions are not homogeneous when the melt is cast in the mold, the alloy compositions close to the eutectic point have a non-homogeneous, non-uniform structure with several constituents: an Al solid solution, primary iron aluminides, and eutectics.