# Advanced Metal Matrix Nanocomposites
## 6 Mechanical Properties  
Enhanced mechanical property is the main goal for most of MMNCs. Among different techniques, ultrasonic-assisted fabrication of metal matrix nano-composites (MMNCs) offers significant opportunities for developing structural materials possessing improved mechanical properties which are not achievable by conventional casting process. For example, ultrasonic treatment can provide uniform dispersion, as well as finer Si particle in the Al-17 wt% Si alloy, as shown in Figure 19[105]. The wide selection of matrices (predominantly Al, Mg, and Cu), and reinforcements (carbides, oxides, and carbonaceous particles) permits the development of MMNCs with attractive mechanical properties, such as high tensile strength, creep, and fatigue resistance [117; 126; 129; 131; 156; 117]. [PERSON] et al. [131] produced the SiC/Mg alloy (AZ91D) composites using ultrasonic-assisted casting. A good dispersion of the 30 nm SiC was observed although some clusters (\(\sim\)300 nm) of SiC still existed, which authors contend may be due to the partial oxidation of SiC leading to this clustering during casting. However, the 5 wt% SiC reinforcement improved the microhardness by 75%. The same Mg alloy (AZ91D) matrix reinforced with 1 wt% of nano AlN improved the yield strength by 44% and 21% at room temperature and 200 \({}^{\circ}\)C, respectively, compared with unreinforced AZ91D alloy while the ductility remained the same [26]. In the microstructure of the composite, the finer lamellar \(\beta\) phase has also been observed that suggests that nanoparticles can also refine the microstructure and improve the mechanical properties. [PERSON] et al. [157] revealed the improved ductility in the extruded AZ91D Mg alloy reinforced with 0.5 wt% of 50 nm SiC. The yield strength, UTS, and elongation were increased for  
Figure 19: Importance of ultrasonic treatment to reduce the size of Si particle in Al-17%Si alloy [105]. the composites extruded at different temperatures, as shown in Figure 20; extrusion at 350 \({}^{\circ}\)C was the most beneficial because of higher UTS and yield strength (YS), as well as improved ductility. In yet another similar fabrication process, the addition of 1 wt% SiC in Mg-4 Zn alloy increased the UTS and elongation twice compared to unreinforced alloy attributed to the presence of nano-SiC as grain refinement; similar improvements in mechanical properties due to the presence of nanoparticles and grain refinement by ultrasonic treatment has been documented in other studies [126; 129; 158; 159]. TiB\({}_{2}\) is also a promising reinforcement due to its high melting point and thermodynamic stability, especially in Al matrix where it has no interface reaction with Al. Ultrasonic-assisted casting of 5 vol% TIB\({}_{2}\)/Al-4.5 Cu improved the yield strength by 100% while the ductility is reduced by 100%. Ultrasonic dispersion has also been able to disperse carbon nanotubes (CNTs) in Mg alloy (AZ91D) that was not attainable by stir casting without ultrasonic assistance and using a dispersion agent due to the strong interface between CNTs and matrix [156]; the ensuing composite with 1.5 wt% CNTs has improved tensile strength, yield strength, and elongation by 22%, 21%, and 42%, respectively, compared with AZ91D magnesium alloy. Additional studies on MMNCs via ultrasonic-assisted fabrication are listed in Table 5 along with the reinforcement material content and percentage improvement in the tensile properties. The introduction of ultrasonic waves also depends on the casting process, the most common being stir-casting and squeeze-casting process. [PERSON] et al. [160] compared these two processes on the mechanical properties of AA 6061 Al alloy reinforced with nano Al\({}_{2}\)O\({}_{3}\) particles (Figure 21). It can be seen that the tensile strength (Figure 21a) and hardness (Figure 21b) of composites fabricated by ultrasonic-assisted squeeze-casting is higher than the stir casting process.