Continuous reinforced MMCs

Continuously Reinforced MMCs For the production of continuous fiber-reinforced MMCs, the above-described methods of preform infiltration also apply. 

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. 

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. 

In aerospace appHcations, low density coupled with other desirable features, such as tailored thermal expansion and conductivity, high stiffness and strength, etc, ate the main drivers. Performance rather than cost is an important item. Inasmuch as continuous fiber-reinforced MMCs deUver superior performance to particle-reinforced composites, the former are ftequendy used in aerospace appHcations. In nonaerospace appHcations, cost and performance are important, ie, an optimum combination of these items is requited. It is thus understandable that particle-reinforced MMCs are increa singly finding appHcations in nonaerospace appHcations. 

A number of parallel future efforts are required in the Ti-B alloy system. The primary objective of future work is to eliminate primary TiB. Efforts to optimize processing to achieve the balance of first and second tier mechanical properties required for fracture-critical applications are also underway. Ti-B alloys may be considered as a matrix for continuously reinforced Ti MMCs, where the exceptional specific strength and stiffness of Ti-B alloys may help overcome current weaknesses in Ti-MMCs. In addition to the Ti-B alloys discussed here, exploration and development of similar in-situ eutectic composites are underway. Specifically, extensive efforts on the Ti-Si system are being pursed [12], and results similar to those reported here are now being obtained. These results are discussed in detail elsewhere in this volume. 

Uniform corrosion of MMC matrices can be expected in environments that attack the matrix metal uniformly. In continuous-fiber MMCs, however, fibers will be left in relief as the matrix corrodes, whereas, in particulate MMCs, reinforcement particles fall free as they are undercut. Uniform corrosion rates of MMCs may be greater than that of the monolithic matrix alloys, due to galvanic action between the matrix and reinforcement constituents. 

Studies on stress-corrosion behavior of MMCs have been limited. Results for some aluminum and magnesium MMCs are summarized in the subsections on Effect of Reinforcement Constituents on Stress Corrosion in Al MMCs, and Effect of Alumirui in Magnesium MMCs respectively. The type of tests that can be used will depend to some degree on the form in which the MMC is produced emd whether or not the MMC can be precracked. Some types of MMCs are produced only in the form of relatively thin panels (e.g., less than about 0.25 cm), whereas other types are available in bars or tubes. It is sometimes impossible to proporiy precrack unidirectional, continuous-fiber MMCs because cracks do not always grow pjerpjendicular to the direction of applied stress [146]. 

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). 

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. 

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. 

In the field of MMCs, continuous carbon and ceramic fibers compete with other reinforcements on a cost/performance basis. Continuous, and hence costly, fibers can be used in applications where high specific performance, e.g., in space applications, is required. In other potential applications, e.g., in automotive engines, metal matrix composites are not cost effective. 

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. 

Stress corrosion studies have been conducted on continuous-fiber reinforced and particle-reinforced Al MMCs. 

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... 

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. 

The principle of incorporating a high-performance second phase into a conventional engineering material to produce a combination with features not obtainable from the individual constituents is well known. In an MMC, the continuous, or matrix, phase is a monolithic alloy, and the reinforcement consists of high-performance carbon, metallic, or ceramic additions. 

Metal matrix composites (MMCs) are a group of materials (such as metals, alloys or intermetallic compounds) incorporated with various reinforcing phases, such as particulates, whiskers or continuous fibres. Based on the mechanical properties of the reinforcing phases, the composite materials could be simply divided into two categories [1]. In the first category, the matrix is reinforced with a ductile component, typically a refractory, such as... 

---