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Top1. Introduction
Metal matrix Composites (MMC) are important class of materials, with the potential to replace a number of conventional materials, which is being, used in automotive, aerospace, defense & leisure industries, where the demand for lightweight & higher strength is increasing (Rhotagi, 2001). Reinforcing a metal matrix with ceramic particles, whiskers or fibers yields a composite material that displays some of the most useful properties of both the metal and the reinforcement. Discontinuous reinforcements, which include particulates, short fibers and whiskers, have gained significant attention since problems associated with continuous – fiber reinforcement such as extensive interfacial reactions can be avoided (Seah et al., 2003). Particulate reinforced polymer composite materials are used for various engineering applications to provide unique physical and mechanical properties with a low specific weight (Satheesh, 2013). The hardness, ultimate tensile strength and yield strength of composite found increasing with increased reinforcements in the composites (Ghazi, 2013).
Composite materials that are conventionally reinforced with micro structured particle reinforcements in MMCs have the prospect of obtaining the tailor-made material properties such as hardness, tensile strength, ductility, density, thermal and electrical conductivity, and wear resistance. With the advent of processing technologies to synthesize nanomaterials, nanocomposites are being developed, with properties that overcome the limitations for metals or composites that contain micron scale reinforcements. Metallic composites containing nano-structured particles offer distinct advantages over polymeric composites due to the inherent high temperature stability, high strength, high modulus, wear resistance, and thermal and electrical conductivity of the metal matrix. Aluminium matrix nano-composites are predicted to surpass the weight reduction currently realized through the use of polymer-based nanocomposites and polymer-based fiber composites in aerospace applications primarily because these metal matrices have higher strength, stiffness and better thermal stability.
Utilization of fly ash in producing novel materials provides opportunity for the positive reuse of this abundant material, thereby helping to reduce the environmental and economic impacts of its disposal.
Currently, research on the use of fly ash as a filler and reinforcement in both MMCs and polymer matrix composites (PMCs) has been growing (Yadong et al., 1998). The density of fly ash particles obtained from various sources has been found to range between 1.6 and 2.7 g/cm3. Their incorporation into aluminum alloys therefore leads to a significant reduction in the density of the composite and also promotes for the reuse of this low-cost waste by-product thereby creating the potential for conserving energy intensive aluminium and reducing the cost of aluminium products. Low density fly ash reinforced aluminum MMCs could be attractive for rotary parts in automobile and other transportation applications (Zhang et al., 2003, Kolukisa et al., 2003).
Nano-structured composites guarantee high strength, wear resistance, hardness and exceptional microstructure stability at high temperatures (Miracle, 2005). They are suitable for high-performance applications where cast alloys or precipitation strengthened material cannot be employed due to their limited properties. Moreover, nano composite materials ensure performances far superior than alloys strengthened by micro-size particles. The synthesis of nano-structured fly ash by using high energy ball milling process is relatively inexpensive and can also be produced in large quantities. In this mechanical treatment, fly ash powder particles are subjected to severe plastic deformation due to the repetitive compressive loads arising from the impacts between the balls and the powder.