## 3 Composite Matrices and Applications
### Metals
Aluminum is one of the most widely used metals after steel due to its high electrical conductivity and corrosion resistance; however, due to its relatively fragile nature, different wt% of copper, magnesium, and silicon are mixed with it to create alloys commonly used in the automotive industry and electrical goods [107; 108; 109]. The use of both SWCNTs and MWCNTs as reinforcement in aluminum matrices is important as carbon nanotubes provide improved strength and stiffness, high-temperature resistance, reduced density, controlled thermal expansion, enhanced electrical performance, and good wear and abrasion resistance [110; 111; 112]. [PERSON] et al. investigated the tribological and mechanical properties of aluminum matrix composites reinforced with CNTs and titanium carbide (TiC), prepared using the stir casting process [84]. In the initial (100% aluminum) composition (AlCTS02), only a few particles of CNTs and TiC were visible at 100\(\times\) magnification. However, as the reinforcement content (wt% of CNT and wt% of TiC) increased in subsequent compositions (AlCTS03-AlCTS05), more particles were observed with uniform dispersion. According to the authors, higher reinforcement contents contributed to enhanced mechanical and tribological properties of the composites, as seen in Figure 11. The study investigated wear behavior using dry sliding pin-on-disc equipment with varying loads (10 N, 20 N, 30 N, and 40 N). The wear rate dropped dramatically as the concentration of CNTs increased. For instance, when 40N force was applied, AICTS01 had a wear rate of \(\sim\)0.038 mm\({}^{3}\)/km, whereas AICTS05 had a wear rate of \(\sim\)0.006 mm\({}^{3}\)/km, which is an 84.2105% decrease [113]. Another study led by [PERSON] et al. investigated the effect of CNT reinforcement on the mechanical properties and corrosion resistance of magnesium matrix composites [85]. Magnesium alloys AZ61 (6.5 wt% Al, 1 wt% Zn, and 0.1 wt% Mn) and AZ91 (8.5 wt%  
Figure 11: Vickers hardness and density for the samples: AICTS01: 100% Al (aluminum); AICTS02: 99 wt% Al + 0.5 wt% CNT + 0.5 wt% TiC; AICTS03: 98.5 wt% Al + 0.5 wt% CNT + 1 wt% TiC; AICTS04: 98 wt% Al + 0.5 wt% CNT + 1.5 wt% TiC; and AICTS05 97.5 wt% Al + 0.5 wt% CNT + 2 wt% TiC) [113]. Al, 0.8 wt% Zn, and 0.25 wt% Mn) were reinforced with varying amounts of carbon nanotubes (0.1, 0.2, and 0.5 wt%) using the powder metallurgy technique. The CNTs were synthesized using the chemical vapor deposition method [85]. It was observed that as the percentage of CNTs increases, the strength of the composites rises while elongation decreases. However, the addition of 0.1 wt% CNTs leads to a decrease in compression strength due to increased porosity in the matrix. The effect of CNT reinforcement is more significant in AZ61 series samples compared to AZ91 series, with a higher increase in compression strength observed [114]. The composites' corrosion resistance varied with the amount of CNTs. Lower concentrations of CNTs (0.1 and 0.2 wt%) generally provided better corrosion resistance compared to higher concentrations (0.5 wt%). Specifically, composites with 0.2 wt% CNTs showed the best corrosion resistance across the tests (Table 5). Microstructural analysis reveals that CNT distribution was critical, with a more homogeneous distribution leading to better mechanical properties. Dislocations in the matrix, influenced by CNT distribution, played a significant role in strengthening the composites [114]. Another study created a high entropy alloy (HEA)/CNT nanocomposite by combining a high entropy alloy (HEA) made of AlCoFeMnNi with 1% CNTs using spark plasma sintering (SPS) [115]. Mechanical alloying effectively transformed the initial elemental powder mixtures into a solid solution of HEA within 40 h. The addition of CNTs during the final 5 h of alloying contributed to a finer crystalline structure in the nanocomposite (Figure 12). The crystallite size decreased from about 40 nm to around 10 nm after 50 h of milling, while lattice strain increased from 0.25 to 0.7 during the same period.