Metal discontinuously reinforced

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. 

Stress Distribution and High Temperature Creep Rate of Discontinuous Fiber Reinforced Metals, Acta Metallurgies et Materialia, 38, 1941-1953 (1990). 26. A. G. Evans, J. W. Hutchinson, and R. M. McMeeking, Stress-Strain Behavior of Metal Matrix Composites with Discontinuous Reinforcements, Scripta Metallurgica et Materialia, 25, 3-8 (1991). 

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. 

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. 

Baxter, W.J., The strength of metal matrix composites reinforced with randomly oriented discontinuous fibers, 1992, Metall Trans. 23A, 3045... 

Dow, N.F. (1963). Study of stresses near a discontinuity in a filament-reinforced composite metal. In Space Sci. Lab. Missile and Space Div., General Electric Co. Tech. Report, No. R63SD61. 

There are many ways to classify composites, including schemes based upon (1) materials combinations, such as metal-matrix, or glass-fiber-reinforced composites (2) bulk-form characteristics, such as laminar composites or matrix composites (3) distribution of constituents, such as continuous or discontinuous or (4) function, like structural or electrical composites. Scheme (2) is the most general, so we will utilize it here. We will see that other classification schemes will be useful in later sections of this chapter. 

Fiber-Matrix Composites. As shown in Figure 1.75, there are two main classifications of FMCs those with continuous fiber reinforcement and those with discontinuous fiber reinforcement. Continuous-flber-reinforced composites are made from fiber rovings (bundles of twisted filaments) that have been woven into two-dimensional sheets resembling a cloth fabric. These sheets can be cut and formed to a desired shape, or preform, that is then incorporated into a composite matrix, typically a thermosetting resin such as epoxy. Metallic, ceramic, and polymeric fibers of specific compositions can all be produced in continuous fashions, and the properties of the... 

The reinforcements amenable to RTM are similar to those used for pultrusion, except that they need not be continuous in nature. Thus, E-glass, S-glass, aramid, and carbon fibers are commonly used, as are discontinuous filaments such as wood fiber and polyesters. Even metal and ceramic fibers can be used in this technique. In one method, the preform is fabricated by spraying 12- to 75-mm-long chopped fiber rovings onto a preshaped screen. A binder sprayed with the fibers keeps them in place and holds the preform shape, which is then placed in the mold. 

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. 

Electrical continuity - poor metal to metal contact, poor reinforcement continuity within survey area or scanning across a discontinuity such as... 

SiC whiskers were initially developed in the early 1960s. However, their use was originally to be applied to the reinforcement of metal matrices, such as aluminum. These metal matrix composites were only a small commercial success, mainly because of the high cost of the whiskers. The first application of whisker reinforcement to ceramics, and in particular alumina, did not occur until the 1980 s. For the purposes of this article, SiC whiskers will be defined to be acicular or needle-like shaped, discontinuous, nearly single crystals. 

Fibrous reinforcement in popular usage is almost synonymous with fibreglass, although other fibrous materials (carbon, boron, metals, aramid polymers) are also used. Glass fiber is supplied as mats of randomly oriented microfibrils, as woven cloth, and as continuous or discontinuous filaments. Hand lay-up is a versatile method employed in the construction of large structures such as tanks, pools, and boat hulls. 

The constituents of a composite are generally arranged in such a way so that one or more discontinuous phases are embedded in a continuous phase. The continuous phase is the matrix. The discontinuous phase are called reinforcement and generally much stronger and stiffer than the matrix although exception dose exist (such as the use of robber particles). The matrix of composite could be polymer, ceramics, or metal. The main thrust of this chapter will be directed at polymeric matrix composites. These materials usually have exceptional mechanical properties and often termed high performance composites. They can be classified... 

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. 

Ceramic fiber reinforcements n. Non-metallic inorganic fibrous materials, available in a wide spectrum of forms, both continuous and discontinuous. 

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. 

The wires for making the reinforcement are used as received, but may be galvanically coated or covered with polymer coating to increase wire-matrix bonding and to reduce the danger of possible corrosion. Expanded metal lath and welded meshes perform better than woven ones because the latter causes discontinuities at nodes, which act as stress concentrators. 

---