# Advanced Metal Matrix Nanocomposites
## 2 Fabrication Methods
### Powder Metallurgy (PM)  
Powder metallurgy (PM), considered as a solid-state manufacturing method, is based upon three sequential steps: blending of the matrix metallic powders and the reinforcement particles in the desired composition, compaction to produce green compacts, and sintering using resistance or microwave (MW) energy. Upon sintering, another finishing process is applied to ensure consolidation of the produced composite which may involve equal channel angular pressing (ECAP) or an extrusion process at elevated temperatures. There are numerous advantages of PM over other manufacturing methods including:  
* Ability to produce \"near-net\" shape components;
* Capability to integrate higher volume fractions of the reinforcement; and
* Ability to produce large batches (i.e., for automotive applications). Recently, [PERSON] et al. [42] successfully produced aluminum and nickel matrix composites reinforced with carbon nanotubes (CNTs) using ultrasonication-assisted powder metallurgy as the dispersion/mixture process. They mixed different contents of CNTs (0.5 to 2.0 wt%) with the metallic powders in isopropanol; upon drying, the mixtures were uniaxially compacted and then sintered at elevated temperatures at different sintering times (i.e., 90 min at 640 \({}^{\circ}\)C for Al/CNT nanocomposites and 120 min at 950 \({}^{\circ}\)C for Ni/CNT nanocomposites). The PM technique, assisted with a novel \"MW sintering\" has been used extensively to produce different grades of Mg matrix nanocomposites [43; 44; 45; 46]; MW heating is used to rapidly and effectively heat up the mixed powder particles as heat is generated within the powders, internally, and radiates outward (core-to-surface), which is contrary to what occurs in the conventional sintering process (surface-to-core) [44; 45]. Recently, [PERSON] et al. [47] used the PM process involving MW sintering followed by hot extrusion to produce Al-Si\({}_{3}\)N\({}_{4}\) nanocomposites (silicon nitride with volume fraction of 0.5, 1.0, and 1.5%). They ball milled the mixture for 2 h at the milling speed of 200 rpm, then compacted the blended powders under 97 bar pressure and sintered the green product, using MW sintering, at 550 \({}^{\circ}\)C. Finally, an extrusion ratio of 20.25:1 was used to produce extruded rods of 8 mm diameter (Figure 6); the microstructure and mechanical properties of the produced nanocomposite were examined. Figure 5: Comparison of mechanical properties between liquid cast AC43A alloy, liquid cast AC43A/0.5 wt% SiC nanocomposite, and semi-solid cast AC43A/0.5 wt% SiC nanocomposites (30% solid fraction); (**a**) ductility comparison and (**b**) strength comparison [41]. Their results indicate that the Si\({}_{3}\)N\({}_{4}\) addition in the aluminum matrix leads to a linear increase in yield strength and UTS along with decrease in ductility (Figure 7). [PERSON] et al. [48] utilized the PM technique to fabricate a Cu/CNT nanocomposite (CNT fraction from 0 to 3.5 wt%) and studied the effect of sintering parameters (850-950 \({}^{\circ}\)C for 2 h) on the mechanical properties and microstructure of the formed nanocomposite. The admixed copper powders (2-3 mm in size) with multi-walled carbon nanotubes (50 nm in diameter and 50 \(\upmu\)m in length) were subjected to a high-energy ball milling (6 h at 300 rpm) which were then compressed and sintered at 850, 900, and 950 \({}^{\circ}\)C for 2 h in a pure argon atmosphere (Figure 8). An increase in the hardness trend with increase in the volume fraction of CNTs was observed up to 3 wt% which begins to drop, probably due to clustering/agglomeration when a larger volume fraction of the reinforcements (i.e., CNT) is added (Figure 9); based on Hall-Petch theory, and due to the grain grown at 950 \({}^{\circ}\)C sintering temperature, the hardness value of the sample processed at 950 \({}^{\circ}\)C was lower than the sample fabricated at 900 \({}^{\circ}\)C.  
Figure 6: Schematic representation of the fabrication of Al–Si\({}_{3}\)N\({}_{4}\) nanocomposites [47].