# Advanced Metal Matrix Nanocomposites
## 6 Mechanical Properties
The authors suggested that the application of pressure during the squeeze-casting process combined with ultrasonic waves provided finer dendritic structures which culminated in higher mechanical properties compared to ultrasonic-assisted stir-casting. This study also revealed that the ultrasonic waves during both the casting processes were able to avoid agglomeration of nano-Al\({}_{2}\)O\({}_{3}\) particles. However, for those samples reinforced by higher weight fraction (>2 wt%), tensile strength begins to decrease due to the clustering and agglomeration of nanoparticles. Figure 20: Tensile properties of AZ91-SiC nanocomposites fabricated by ultrasonic-assisted casting followed by extrusion at different temperature [157]. It has been also observed that the bonding between the matrix and reinforcement plays an important role for mechanical properties, such as \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) having a higher lattice mismatch compared to \(\alpha\)-Al\({}_{2}\)O\({}_{3}\). The higher lattice mismatch of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) resulted in a better nucleation catalyzer in A356 alloy during ultrasonic-assisted casting because it requires less under-cooling [161]. It has also been observed that uniform dispersion of nanoparticles in the metal matrix by ultrasonic-assisted casting provides a finer microstructure that helps to either maintain or improve the ductility of composites. Other mechanical properties, such as fatigue, creep, etc., have not been studied significantly to understand the influence of ultrasonic-assisted casting. One of the studies on 1 wt% AlN reinforced Mg alloy (Elektron21, Mg-2.8 Nd-1.2 Gd-0.4 Zr-0.3 Zn) showed improvement in the creep properties, as shown in Figure 22[128]; the reinforced Mg alloy (El21) showed a lower creep rate compared to unreinforced Mg alloy. However, further studies have to be conducted on different matrices and reinforcements to bring out the beneficial effect of ultrasonic treatment during casting that can alter the other mechanical properties, including fatigue, creep, damping, etc. \begin{table}
\begin{tabular}{c c c c} \hline
**Matrix** & **wt\%** & **\% Improvement** & **Reference** \\ \hline A356 & SiC = 1.5 wt\% & UTS = 100\% & [117] \\ Mg-2 Al-1 Si & SiC = 2 wt\% & UTS = 15\% & [129] \\
2024 Al alloy & Al\({}_{2}\)O\({}_{3}\) = 1 wt\% & YS = 45\% & [126] \\ Mg-6 Zn & SiC = 1.5 wt\% & UTS = 55\% & [162] \\ Al7075 & Al\({}_{2}\)O\({}_{3}\) = 1.5 wt\% & UTS = 59.6\% & [163] \\ Al7075 & SiC = 1 vol\% & YS = 12\% & [164] \\ A356 & Al\({}_{2}\)O\({}_{3}\) = 1 wt\% & UTS = 15\% & [165] \\ A356 & SiC = 1 wt\% & UTS = 24\% & [165] \\ Al-4.4 Cu & TiB\({}_{2}\) = 2 wt\% & YS = 65\% & [166] \\ AA6061 & SiC = 1 wt\% & UTS = 3\%, Elongation = 100\% & [167] \\ AA6061 & Al\({}_{2}\)O\({}_{3}\) = 1 wt\% & UTS = 6\%, Elongation = 100\% & [167] \\ AA2219 & SiC = 2 wt\% & UTS = 36.5\% & [168] \\ \hline \end{tabular}
\end{table} 
Table 5: Percentage improvement in mechanical properties of MMNCs fabricated by ultrasonic treatment during casting. Figure 21: Comparison of (**a**) tensile strength and (**b**) hardness of AA6061/Al\({}_{2}\)O\({}_{3}\) composite fabricated by ultrasonic-assisted squeeze casting and ultrasonic-assisted stir casting processes [160]. Casting processes are beneficial in industrial settings due to easy adaption on large-scale production. However, the potential nano additives for MMNCs, namely graphene (Gr) and carbon nanotubes (CNTs), have poor wetting behavior during liquid mixing [95], a limitation that leads to utilization of the powder metallurgy (PM) route that involves solid-state processing. Several Al, Cu, Ni, and Ti matrix composites reinforced with graphene and CNT have been produced via powder metallurgy with superior mechanical properties [8; 169]; some composites are listed in Table 6 along with their mechanical and tribological properties.