# Advanced Metal Matrix Nanocomposites
## 1 Introduction  
Composites are a unique class of materials invariably favored by nature in its creation and humans from prehistoric times. Composites provide a unique capability of enhancing the selected properties of monolithic materials which remains superior to the monolithic counterparts [1, 2]. The enhancement in the targeted property of a metallic matrix can be controlled through the selection of type, size, and the amount of reinforcement which forms the core of composite design. The research in metal-based composites gained pace primarily in 1970s when researchers attempted, stir casting, rheocasting, and powder metallurgy routes to unify metals/alloys with ceramic reinforcements [3, 4] with captivating results. Improvements in mechanical properties, such as elastic modulus, creep, damping, and wear resistance, were typically reported [5, 6]. Researchers used both continuous and micron-scale discontinuous reinforcements to enhance the properties of different metallic matrices until approximately the end of the 20 th century. The development of continuously-reinforcedmetal matrix composites (MMCs) was limited for highly specialized sectors, such as the space segment, due to the high cost and complexity involved in the synthesis process. On the other hand, the synthesis of discontinuously-reinforced MMCs was successfully attempted using a range of processing techniques [1; 7; 8]. Around the year 2000, the interest in nano-materials was revived and the research gained stride in exploring the influence of nano-scale materials as reinforcement in metallic matrices [9; 10]; Ni, Cu, Ti, Mg, and Al were particularly targeted. The use of nano-scale reinforcements led to hitherto unrealized promising results in improving a wide spectrum of properties pertaining to metallic matrices; often deployed reinforcements were primarily selected from oxide, carbide, boride, and nitride families. Moreover, carbon-based reinforcements, such as carbon nanotubes, buckyballs, graphene, and carbon nanoplatelets, were also critically investigated not only in metal matrix composites but also in polymer composites [11]. The following properties were enhanced due to the use of nano-scale reinforcements:  
1. Tensile and compressive behavior. 2. Ductility or elongation to failure, a must for bend than break design philosophy. 3. High-temperature mechanical properties. 4. Creep. 5. Dynamic mechanical properties. 6. Wear resistance, including scratch resistance. 7. Coefficient of thermal expansion. 8. Damping. 9. Machining. 10. Ignition resistance. 11. Dry/wet corrosion resistance. 12. The property improvement in the case of MMCs ensues from a combination of factors, such as:
1. Judicious selection of matrix and reinforcement (including its length scale). 13. Careful selection of primary and secondary processing routes. 14. Heat treatment. In the case of nano-MMCs, another important attribute that assists in property improvement is the minimization of particle-based damage mechanisms which are prevalent in micron-scale ceramic reinforcement particles. Particle damage mechanisms include:  
1. Particle breakage under the application of stress triggering the initiation of cracks. 2. Matrix-reinforcement interfacial debonding under the application of stress. Particle-based damage mechanisms lead to inferior mechanical properties of MMCs, especially the elongation to failure and fracture toughness, which is not desirable in engineering/biomedical applications. Accordingly, nano-MMCs enjoy a distinct advantage over micro-MMCs in many applications due to their superior combination of properties. In addition to the choice of matrix and reinforcement, the improvement in properties of MMCs crucially depends on the processing route and the processing parameters that can provide a uniform distribution of reinforcement and a good matrix-reinforcement interface.