## 3 Composite Matrices and Applications
### Metals
Elemental powder peaks vanished after 40 h of milling, indicating the formation of an AlCoFeMnNi solid solution. The use of spark plasma sintering allowed for rapid consolidation of the powdered alloy, minimizing grain growth due to the short sintering duration. Different sintering temperatures (850 \({}^{\circ}\)C and 950 \({}^{\circ}\)C) demonstrated that higher temperatures helped reduce porosity significantly, thereby enhancing mechanical integrity and reducing defects within the sintered material [115]. As seen in Figure 12, the introduction of CNTs significantly increased the maximum shear strength and hardness of the HEA. The addition of CNTs significantly enhanced the ultimate tensile strength and hardness of the alloy from approximately 500 to over 650 VHN. The improvements are attributed to solid solution strengthening from carbon diffusion, dispersion hardening, the pinning effect of CNTs on dislocations, and crack bridging, all of which contribute to increased mechanical strength and ductility [115]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Matrix** & **CNT Ratio (wt\%)** & **Corrosion Rate (mm/yr)** \\ \hline AZ61 & Unreinforced & 0.0336 \\ \hline AZ61 & 0.1 & 0.0388 \\ \hline AZ61 & 0.2 & 0.0324 \\ \hline AZ61 & 0.5 & 0.0678 \\ \hline AZ91 & Unreinforced & 0.0396 \\ \hline AZ91 & 0.1 & 0.0267 \\ \hline AZ91 & 0.2 & 0.0178 \\ \hline AZ91 & 0.5 & 0.0718 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Corrosion properties of AZ61- and AZ91-based composites and unreinforced alloys calculated from PDS curves [114]. From the aforementioned studies, it is evident that the integration of CNTs into metal matrices represents a significant advancement in materials engineering, particularly for applications that demand high strength, stiffness, and enhanced functional properties. Metal matrix composites (MMCs) enhanced with micro and nanofillers, such as CNTs, are at the forefront of addressing these technological demands. The inclusion of CNTs in metal matrices not only boosts mechanical strength and stiffness but also improves high-temperature properties, electrical performance, and corrosion resistance while maintaining or reducing the overall weight of the composite. Studies have shown that even small additions of CNTs can significantly impact the wear resistance and mechanical integrity of composites. For instance, the incorporation of CNTs into aluminum matrices has dramatically reduced wear rates under high stress, demonstrating an 84% decrease in wear with higher CNT concentrations. Similarly, magnesium matrix composites reinforced with CNTs have shown varying mechanical and corrosion resistance based on the amount of CNTs added, with a noted increase in strength but a reduction in ductility as the CNT content increases. Moreover, the development of high entropy alloy (HEA) composites with 1% CNTs using advanced manufacturing techniques, like spark plasma sintering (SPS), highlights the ability to enhance alloy properties through fine control of microstructural features [101]. This includes significant improvements in shear strength and hardness attributed to the uniform dispersion of CNTs within the matrix, which enhances load transfer capabilities and increases dislocation density, thereby improving the overall mechanical properties of the alloy. Overall, the integration of CNTs into metal matrices not only meets but often exceeds the current technological requirements for high-performance materials, promising a new era of advanced composites with tailored properties for specific applications. The conclusions of the composites investigated in this section are summarized in Tables 6-8. Figure 12: Results of shear strength for AlCoFeMnNi and AlCoFeMnNi-1 wt% CNT nanocomposite samples sintered at (**a**) 850 and (**b**) 950 \({}^{\circ}\)C [115].