Composites metal-matrix

The metal matrix composites can be described as materials whose microstructure comprises a continuous metallic phase into which a second phase (ceramic materials) has been artificially introduced during processing, as reinforcement. 

The most common matrices are the low-density metals, such as aluminum and aluminum alloys, and magnesium and its alloys. Some work has been carried out on lead alloys, mainly for bearing applications, and there is interest in the reinforcement, for example, of titanium-, nickel- and iron-base alloys for higher-temperature performance. However, the problems encountered in achieving the thermodynamic stability of fibers in intimate contact with metals become more severe as the potential service temperature is raised, and the bulk of development work at present rests with the light alloys. 

The principal reinforcements for metal matrices include continuous fibers of carbon, boron, aluminum oxide, silica, aluminosilicate compositions and 

Transverse tensile strength Tensile elastic modulus, E Shear modulus, G (calculated) Density 

Strength-to-density ratio Modulus-to-density ratio Poisson s ratio, m (calculated) 

The maximum interfacial shear stress in polymer matrix composites is determined by the adhesion between fibre and matrix, not by the yield strength of the matrix. 

A further increase in dislocation density occurs during plastic deformation because plastic deformation is usually limited to the matrix, leading to a formation of dislocation loops around the fibres (see also section 6.4.4). The Orowan mechanism (see section 6.3.1 and figure 6.45), which would impede dislocation movement, is not relevant, though, because the fibre diameter and distance are too large. 

Due to their high specific strength and stiffness, long-fibre reinforced aluminium matrix composites are attractive in aerospace apphcations. The high-gain antenna boom of the Hubble Space Telescope, for example, is made from a carbon-fibre reinforced aluminium matrix composite [114]. Aluminium oxide reinforced aluminium matrix composites are also suitable for push rods in motorcycle engines and for electrically conductive and mechanically loaded connectors on power poles [1]. 

Short-fibre reinforced metal matrix composites are significantly less expensive than long-fibre reinforced materials and can thus be used in automotive engineering or in sports equipment. For example, short-fibre reinforced aluminium-silicon carbide composites can be used as pistons in diesel engines at elevated temperatures [49]. Golf clubs and bicycle components can also be manufactured from aluminium matrix composites. Frequently, whiskers (see section 6.2.8) are used as short fibres because of their high strength and favourable aspect ratio. 

The stiffness and strength of metals can be increased not only by adding fibres, but also using particles. In contrast to fibres, load is transferred also at the front and back end of the particle, not only by shear stresses. In an aluminium-silicon carbide composite, for example, the tensile strength can be as high as 700 MPa. 

As may be noted in Table 16.8, the material of choice (i.e., the least expensive) is the standard-modulus carbon-fiber composite the relatively low cost per unit mass of this fiber material offsets its relatively low modulus of elasticity and required high volume fraction. 

Fiber Type Fiber Volume (cm ) Fiber Mass (kg) Fiber Cost ( US) Matrix Volume (cm ) Matrix Mass (kg) Matrix Cost ( US) Total Cost ( VS) 

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. 

Fiber Matrix Fiber Content (yol%) Density (g/cm ) Longitudinal Tensile Modulus (GPd) Longitudinal Tensile Strength (MPa) 

Source Adapted from X W. Weeton, D. M. Peters, and K. L. Thomas, Engineers Guide to Composite Materials, ASM International, Materials Park, OH, 1987. 

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Metal-matrix composites Metal membranes Metal-metal bonds Metal naphthenates Metal oleates... 

KETENES, KETENE DIhffiRS AND RELATED SUBSTANCES] (Vol 14) -metal-matrix composites [METAL-ITATRIX COMPOSITES] (Vol 16)... 

Molybdenum hexafluoride is used in the manufacture of thin films (qv) for large-scale integrated circuits (qv) commonly known as LSIC systems (3,4), in the manufacture of metallised ceramics (see MetaL-MATRIX COMPOSITES) (5), and chemical vapor deposition of molybdenum and molybdenum—tungsten alloys (see Molybdenumand molybdenum alloys) (6,7). The latter process involves the reduction of gaseous metal fluorides by hydrogen at elevated temperatures to produce metals or their alloys such as molybdenum—tungsten, molybdenum—tungsten—rhenium, or molybdenum—rhenium alloys. 

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

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. 

A composite material (1) is a material consisting of two or more physically and/or chemically distinct, suitably arranged or distributed phases, generally having characteristics different from those of any components in isolation. Usually one component acts as a matrix in which the reinforcing phase is distributed. When the continuous phase or matrix is a metal, the composite is a metal-matrix composite (MMC). The reinforcement can be in the form of particles, whiskers, short fibers, or continuous fibers (see Composite materials). 

There are three kinds of metal-matrix composites distinguished by type of reinforcement particle-reinforced MMCs, short fiber- or whisker-reinforced MMCs, and continuous fiber- or sheet-reinforced MMCs. Table 1 provides examples of some important reinforcements used in metal-matrix composites as well as their aspect (length/diameter) ratios and diameters. 

Table 1. Typical Reinforcements Used in Metal-Matrix Composites...

Fig. 1. Typical microstmctures of some metal-matrix composites (a) continuous alumina fiber/Mg and (b) siUcon carbide particle/Al composites.

There are several important fabrication processes for metal-matrix composites. 

Strength. Prediction of MMC strength is more compHcated than the prediction of modulus. Consider an aligned fiber-reinforced metal-matrix composite under a load P in the direction of the fibers. This load is distributed between the fiber and the matrix ... 

An important example of an MMC in situ composite is one made by directional solidification of a eutectic alloy. The strength, (, of such an in situ metal-matrix composite is given by a relationship similar to the HaH-Petch relationship used for grain boundary strengthening of metals ... 

Thermal expansion mismatch between the reinforcement and the matrix is an important consideration. Thermal mismatch is something that is difficult to avoid ia any composite, however, the overall thermal expansion characteristics of a composite can be controlled by controlling the proportion of reinforcement and matrix and the distribution of the reinforcement ia the matrix. Many models have been proposed to predict the coefficients of thermal expansion of composites, determine these coefficients experimentally, and analy2e the general thermal expansion characteristics of metal-matrix composites (29-33). 

Electronic-Grade MMCs. Metal-matrix composites can be tailored to have optimal thermal and physical properties to meet requirements of electronic packaging systems, eg, cotes, substrates, carriers, and housings. A controUed thermal expansion space tmss, ie, one having a high precision dimensional tolerance in space environment, was developed from a carbon fiber (pitch-based)/Al composite. Continuous boron fiber-reinforced aluminum composites made by diffusion bonding have been used as heat sinks in chip carrier multilayer boards. 

T. W. Clyne and P. J. Withers, M Introduction to Metal Matrix Composites, Cambridge University Press, Cambridge, U.K., 1993. 

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