## 3 Results and Discussion
In addition, reinforcement improves the interfacial bonding between the matrix and microsilica particles, which in turn increases the strength of the material. The microhardness value for S2 was 29.44 HV0.1, which is 36.6% higher than S1. The highest microhardness was observed for S2-HT, which combined the effects of T6 heat treatment and microsilica reinforcement, reaching 34.41 HV0.1. This value was 59.7% higher than the microhardness of untreated aluminum (S1) and 16.9% higher than that of S2. This synergy is conditioned by the  
Figure 7: Average microhardness values of S1-HT, S2, and S2-HT specimens. formation of a more uniform dispersion of reinforcing material particles because of the homogenization of the matrix during T6 heat treatment and additional strengthening owing to the formation of more stable strengthening phases. The obtained results agree with the data from the literature, indicating the effectiveness of ultrasonic mixing for the uniform distribution of reinforcement particles [9; 10; 11; 44; 45] and the high efficiency of T6 heat treatment in improving the mechanical properties of aluminum alloys and composites [46; 47]. This combination of processing methods has significant potential for the development of high-performance AMCs. The stress-strain curves and changes in the tensile properties of the specimens are presented in Figure 8.  
The obtained data on the mechanical properties of cast aluminum materials and composites showed a significant effect of both microsilica reinforcement and T6 heat treatment on the strength and plastic properties of the samples. A comparison of S1  
Figure 8: Stress-strain curves and changes in tensile properties of S1, S1-HT, S2, and S2-HT specimens. and S1-HT specimens makes it possible to evaluate the effect of T6 heat treatment on pure cast aluminum. T6 heat treatment increased the yield strength (YS) from 18.5 MPa to 22 MPa and the ultimate tensile strength (UTS) from 37.1 MPa to 44.2 MPa. This is explained by the reduction in the internal lattice defects and increase in the structural uniformity after quenching and aging. Simultaneously, the elongation decreased from 64.3% to 53.5% because of partial hardening of the material and reduced ductility. The addition of microsilica (-0.36 wt%) to aluminum (S2) led to a significant increase in strength properties: YS increased to 36 MPa and UTS increased to 73 MPa compared to untreated aluminum (S1). This is because the microsilica particles act as dispersion-hardening centers and prevent dislocation movement. However, the elongation decreased up to 45.2%, indicating a decrease in ductility [52; 53] owing to hard particles causing stress concentration. The S2-HT specimen showed an interesting effect: after T6 heat treatment, the yield strength slightly decreased to 34 MPa, while the UTS remained at 73 MPa, but the elongation increased significantly to 60.3%. This is because the heat treatment improved the bonding between the reinforcing particles of the microsilica and aluminum matrix and reduced the internal residual stress level. Consequently, despite a partial reduction in strength, the material ductility was significantly improved. As shown in the results, microsilica as the reinforcing material can improve the mechanical performance of pure aluminum with initially poor mechanical properties at a relatively low concentration of reinforcing components. The SEM and EDS images of S2 and S2-HT specimens reveal a uniform distribution of microsilica particles in the aluminum matrix, with minimal agglomeration due to ultrasonic-assisted stirring. The fine dispersion of particles, as seen in Figure 4, contributes to enhanced mechanical properties by impeding dislocation movement and improving strain hardening. The interface bonding between microsilica and the matrix appears strong, as evidenced by the absence of significant debonding or microcracks in the micrographs.