## 3 Results and Discussion
EDS point and elemental mapping analyses were employed to identify the displayed structure. In this image, both types of particles, ellipticals and irregular shapes, are extant together. According to the EDS point and elemental mapping analyses, it could be concluded that the elliptical-shaped particle is iron-aluminium when the irregular-shaped particle has a core-shell structure. The core structure has a chemical composition of iron-oxygen, surrounded by a shell with a combination of aluminium-oxygen. These results can confirm  
Figure 4: Optical micrographs of the rolled (**a**) and the annealed (**b**) specimens after FSP. Figure 5: Shows the temperature profiles during the fourth pass FSP for wrought and annealed aluminium samples. the previous findings, especially the X-ray diffraction ones because the outcomes of the XRD pattern showed that iron-aluminium intermetallic compounds, aluminium oxide and iron oxide, are present in the fabricated composite. Additionally, the presence of sediments with a chemical composition of oxygen/aluminium and aluminium/iron is a confirmation of the in situ reaction between aluminium and hematite during the FSP. Eleptical-shaped particles were detected as Al\({}_{13}\)Fe\({}_{4}\) particles with an approximate size of 100 nm. As mentioned earlier, the results of EDS and XRD patterns for these particles are consistent with Al\({}_{13}\)Fe\({}_{4}\) intermetallic composition. It was also found that the irregular-shaped particles are \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\) particles (Figure 9). During the FSP, the rotating tool breaks and crushes the initial Fe\({}_{2}\)O\({}_{3}\) particles, which will be dispersed into the aluminium matrix. This will encourage the reaction between Fe\({}_{2}\)O\({}_{3}\) particles and the aluminium base, resulting in the formation of Al\({}_{13}\)Fe\({}_{4}\) and Al\({}_{2}\)O\({}_{3}\) intermetallic compounds as reaction products during the FSP. Based on our previous studies [19; 34], during FSP, the exothermic reaction between Al and Fe\({}_{2}\)O\({}_{3}\) can initiate from the heat produced by the rotating tool. In fact, because of the consolidation of high temperature and severe plastic strain related to FSP, the Al-Fe\({}_{2}\)O\({}_{3}\) system's exothermic reaction started and advanced. It is also noteworthy that our previous findings indicated that mechanical milling of the powder mixture before FSP enhances the kinetics of the exothermic reaction [34]. Therefore, the amount of in situ products, Al\({}_{13}\)Fe\({}_{4}\) and Al\({}_{2}\)O\({}_{3}\), resulting from the reaction increases. Those particles act as reinforcements, and thus the matrix grains will be refined by the pinning mechanism of the grain boundary. Figure 10 illustrates the EBSD mapping and frequency distribution of grain sizes for the rolled and annealed FSPed specimens with an Al + Fe\({}_{2}\)O\({}_{3}\) milled powder mixture. Recrystallised grains of about 3 \(\upmu\)m are seen in the wrought alloy SZ (Figure 10a). The FSP zone of the annealed samples (Figure 10b), like that of the wrought alloy, comprises fine recrystallised grains of 2 \(\upmu\)m that are slightly smaller than that of the wrought specimens. Figure 6: The optical and SEM micrographs related to the microstructure of the wrought (**a**,**b**) and annealed (**c**,**d**) AA1100 alloy after FSP with Al + Fe\({}_{2}\)O\({}_{3}\) milled powder mixture. Various researchers have investigated mechanisms that lead to grain refinement during FSP [5; 8; 30; 35]. Based on their report and the finding of our previous study, materials have been subjected to hot deformation. As a result, dynamic recovery (DRV) and dynamic recrystallisation (DRX) of the highly distorted grains occur and cause grain refinements. The annealing treatment and the recrystallisation phenomena almost controlled the final microstructure after FSP.