## 3 Results and Discussion
The improved distribution of microsilica particles after ultrasonic-assisted stir casting further synergizes with heat treatment by enhancing particle-matrix interfacial bonding, reducing localized stress concentrations. The XRD analysis indicates a stabilization of amorphous SiO\({}_{2}\) within the matrix, which acts as an energy-dissipating phase, preventing crack propagation. Furthermore, SEM and LM analyses confirm a decrease in porosity and microcracks due to the combined effects of ultrasonic processing and T6 treatment. Overall, these microstructural changes collectively contribute to the observed significant improvement in elongation, reaching 60.3%, while maintaining high strength (73 MPa), making the composite material suitable for forming processes. Our findings on the mechanical properties of AMCs reinforced with ultrafine microsilica particles (\(\sim\)128 nm, 0.36 wt%) align with and extend upon prior research. In the study by [PERSON] et al. [10], AMCs reinforced with nano-SiC particles (30-50 nm, 1 wt%) and fabricated via ultrasonic-assisted stir casting demonstrated an increase in tensile strength to 142 MPa and a modest elongation. This illustrates that the use of higher wt% SiC improves strength significantly, but at the expense of ductility. Furthermore, the porosity levels in our study (0.560% for S2) were significantly reduced compared to [PERSON] et al. 's results, demonstrating the effectiveness of ultrasonic processing and T6 treatment in achieving a homogeneous microstructure. A direct comparison with our previous work [35] involving the reinforcement of pure aluminum with boron carbide (B4C) particles reveals several noteworthy distinctions. While the earlier study demonstrated significant enhancements in the mechanical strength of aluminum through the incorporation of 0.5-2 \(\upmu\)m B\({}_{4}\)C particles, the present study focuses on achieving an optimal balance between strength and ductility using ultrafine microsilica (\(\sim\)128 nm) as a reinforcing material. In our earlier study, the addition of B4C particles increased the ultimate tensile strength (UTS) of aluminum to approximately 82 MPa, which is higher than the UTS of 73 MPa observed in the current microsilica-reinforced composites (S2-HT). However, the elongation to failure in the B\({}_{4}\)C-reinforced composites was limited to 45.1%, compared to the significantly higher elongation of 60.3% achieved in the microsilica-reinforced composite after T6 heat treatment. This indicates that while B\({}_{4}\)C offers superior strength, the ductility of the material is compromiseddue to the high stiffness and stress-concentrating effects of the B4C particles. In addition to superior ductility, the microsilica-reinforced composites offer significant environmental and economic advantages. Unlike B4C, microsilica is an industrial byproduct, making it a cost-effective and sustainable reinforcement option. This aspect aligns with the growing emphasis on eco-friendly material development, as microsilica utilization addresses waste management challenges while delivering high-performance composites. The performance of the AMCs developed in this study was compared with composites reinforced with Al2O3 nanoparticles as reported by [PERSON] et al. [50]. The study by [PERSON] et al. focused on the reinforcement of AA7150 alloy with 0.5 wt% Al2O3 particles, which achieved a notable improvement in mechanical properties, including a 55% increase in tensile strength and a 40% increase in hardness. In contrast, our AMCs reinforced with ultrafine microsilica (~0.36 wt%) achieved a 59.7% increase in Vickers microhardness and a 96.8% increase in tensile strength compared to unreinforced aluminum. These results highlight the competitive performance of our composites despite the lower reinforcement concentration. In terms of processing, the ultrasonic-assisted stir casting used in our study provided uniform particle distribution, which is critical for achieving superior mechanical performance. While the study by [PERSON] et al.