Discontinuous reinforced MMCs

Particle or discontinuously reinforced MMCs have become important because they are inexpensive compared to continuous fiber-reinforced composites and they have relatively isotropic properties compared to the fiber-reinforced composites. Figures la and b show typical microstmctures of continuous alumina fiber/Mg and siUcon carbide particle/Al composites, respectively. 

Laminated MMCs. There are three types of laminated MMCs (/) metallic matrix-containing fibers oriented at different angles in different layers, similar to that of polymeric laminates (2) two or more different metallic sheets bonded to each other and (3) laminated metal/discontinuously reinforced MMC. 

Discontinuously Reinfbiced MMCs Discontinuously reinforced MMCs (particulates, whisker, platelets) are produced by either liquid- or solid-state processing [358-360]. Three different processes are used commercially used for hquid-state processing ... 

Discontinuous fiber MMCs are isotropic and are superior in terms of specific strength, elastic modulus, toughness, and thermal conductivity compared to conventional alloys. They are used for brackets and fittings that are subject to multiaxial loads. The ability to engineer the CTEs of discontinuously reinforced aluminum (DRA) composites makes it possible to save substantial weight by replacing Invar or Kovar in critical design situations where a low GTE material is needed. 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

Normally the processing of MMCs involves at least two steps consohdation or synthesis (i.e., introduction of reinforcement into the matrix), followed by a shaping operation. A host of consolidation techniques are available, some of which are relatively sophisticated discontinuous-fiber MMCs are amenable to shaping by standard metalforming operations (e.g., forging, extrusion, rolling). 

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Ceramic reinforcing fibers are utilized both in a continuous form (endless fibers) and in a discontinuous form (e.g. whiskers, short fibers). Most of the continuous fibers are utilized in the manufacture of composites with polymer matrices (PMC), where they are in competition with other high performance fibers (boron, carbon fibers), mainly for military or aerospace applications. Discontinuous fibers are generally used for the manufacture of metal matrix (MMC) and ceramic matrix (CMC) composites. 

FRCs can be classified based on matrix and fibres. Based on fibre source, FRCs may be natural fibre reinforced and synthetic fibre reinforced. Based on fibre length, they can be continuous fibre reinforced and discontinuous fibre reinforced. But FRCs are generally classified based on matrix component. Thus according to the types of matrices stated earlier, composites are of three types (i) ceramic matrix composites (CMCs), (ii) metal matrix composites (MMCs) and (iii) organic matrix composites (OMCs). Organic matrix is subdivided into two classes, namely polymer matrix and carbon matrix. A short description of all these types of composites are discussed below. 

MMCs are usually reinforced by either monofilaments, discontinuous fibers, whiskers, particulates, or wires. With the exception of wires, which are metals, reinforcements are generally made of advanced ceramics such as boron, carbon, alumina and silicon carbide. The metal wires used are made of tungsten, beryllium, titanium, and molybdenum. Currently, the most important wire reinforcements are tungsten wire in superalloys and superconducting materials incorporating niobium-titanium and niobium-tin in a copper matrix. The most important MMC systems are presented in Table 18.5. 

Both nature and man have made extensive use of composite materials in which two or more different materials are joined in such a manner that they maintain their identity but work together to add their strengths and decrease their weaknesses. Composites can be classified into three categories (1) Laminates, in which sheets of different materials are laminated together (2) particle-reinforced composites, in which particles of one material are imbedded in a matrix of a second material and (3) fiber-reinforced composites, in which fibers of one material are encapsulated in a matrix of a second material. Particle-reinforced composites can be subdivided into small particle composites, where the particles are incorporated into the microstructure, such as dispersion-hardened alloys, and large particle composites, where the matrix simply supports the particles. Fiber-reinforced composites may have continuous versus discontinuous fibers and aligned versus randomly oriented fibers, which can provide anisotropic versus isotropic properties. Composites combine all combinations of metals, ceramics, and polymers into MMCs, where a metal... 

Corrosion Proporlios. Marine corrosion of silicon carbide/aluminum composites is much less severe than that observed on graphite/aluminum MMCs. Discontinuous silicon carbide/aluminum MMCs, however, are susceptible to localized corrosion. Mild-to-moderate pitting has been reported on SiC whisker- and particulate-reinforced composites containiirg 6061 and 5000 series aluminum matrices exposed for a maximum of 42 months in splash/spray and marine atmospheric environments. The d ree of corrosion present on the composites is slightly accelerated compared to that on unreinforced alutrtittum alloys. 

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