## 3 Results and Discussion
It is clear that the rolled specimen exhibits the highest relative frequency of low KAM values (less than 0.5). In contrast, the annealed sample demonstrates the highest relative frequency of high KAM  
Figure 6.1 Kernel average misorientation (KAM) or local misorientation histogram of (**a**) the rolled and (**b**) the annealed specimens after FSP. Figure 6.2 EBSD mapping and frequency distribution of grain sizes for (**a**) the rolled and (**b**) annealed FSPed specimens. values. The high KAM values in the annealed sample imply that it has a high dislocation density [42]. As a result, it is evident that following FSP, the dislocation density in the annealed specimen gets higher than in the rolled specimen. Intergranular particles pin these dislocations in the hybrid composite grains. The engineering stress-strain curves for the FSPed hybrid composites are shown in Figure 6.3 The figure shows that the tensile strength for rolled and annealed composites is 146 and 155 MPa, respectively. Therefore, the annealing treatment before FSP resulted in the fabrication of a more refined structure during FSP, which significantly impacts the mechanical properties of composites. The microstructure modification during FSP is due to introducing an active powder mixture (Al and Fe\({}_{2}\)O\({}_{3}\)), and subsequently forming new nanoparticles of Al\({}_{2}\)O\({}_{3}\) and Al\({}_{13}\)Fe\({}_{4}\), elastic mismatch mechanisms, and cumulative nucleation of particles. The high ultimate tensile strength of the composite fabricated from the annealed sample is related to the presence of large amounts of reinforcing particles (Al\({}_{2}\)O\({}_{3}\) and Al\({}_{13}\)Fe\({}_{4}\)), and also the finer grain aluminium matrix and the high dislocation density in this composite. Figure 6.4 The KAM value vs. relative frequency of the rolled and the annealed specimens after FSP. Figure 6.5 The engineering stress–strain curves for the FSPed hybrid composites. Figure 14 shows the microhardness along the centerlines of both the rolled and the annealed specimens after FSP with powder addition. It is obvious that the FSP region is harder than the base metal in both specimens. This perception is intently associated with the development of microstructure. The hardness in both samples increased due to microstructure modification, along with an active reaction between aluminium and hematite and, thus, the fabrication of high-hardness products (\(\alpha\)-Al\({}_{2}\)O\({}_{3}\) and Al\({}_{13}\)Fe\({}_{4}\)). However, it is observed that the hardness is higher in the annealed sample. The alloy used in this research (aluminium alloy 1100) is not a precipitation-hardening alloy, so the amount of hardness depends on the grain size, density of dislocations, and hard particles in the matrix (resulting from the active reaction between aluminium and hematite). Intensive grain refinement combined with a higher density of dislocations in this sample resulted in higher hardness in the stir zone. It is noteworthy that it is reported that FSP on this alloy will reduce the density of dislocations in the worked samples. On the contrary, the dislocation density will increase in the annealed samples [43]. According to the Hall-Petch relationship, the grain size strengthening can be illustrated as kd\({}^{-1/2}\) where d is the grain size, and k is the strengthening coefficient ([PERSON] et al. [44] reported the k = 74 MPa\(\surd\)um for aluminium alloy 1100). In the case of the composite fabricated from an annealed sample, considering the fine aluminium grains size (2 microns) can contribute to the yield strength equal to 52 MPa.