Whisker-reinforced MMCs

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. 

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 ... 

The next generation of fibre-reinforced materials were the metal matrix composites (MMC) of ductile alloys reinforced with hard carbon or ceramic fibres and whiskers. The MMC offer serious advantages over other types of composites, mainly in the following aspects ... 

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). 

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. 

Ceramics in the form of fibers and whiskers are often used as reinforcing phases in composites. We described the different methods used to produce whiskers and fibers and how they are incorporated into PMCs, MMCs, and, particularly for our interest, CMCs. One of the current directions in the production of CMCs is to produce the matrix and fiber in situ. 

The use of whiskers as reinforcement in metallic matrix composites (MMCs) is described in Section 4.4.1.10. 

Description and General Properties. Metal matrix composites (MMCs) consist of a metal or an alloy matrix with a reinforcement material (e.g., particulates, monofilaments, or whiskers). The matrix alloy, the reinforcement material, the volume and shape of the reinforcement, the location of the reinforcement, and the fabrication method can all be varied to achieve required properties. Most of the metal-matrix composites are made of an aluminum matrix. But aluminum-matrix composites must not be considered as a single material but as a family of materials whose stiffness, strength, density, and thermal and electrical properties can be tailored. Moreover a growing number of applications require improved matrix properties and therefore, metal matrices of magnesium, titanium, superalloys, copper, or even iron are now available commercially. Compared to bulk metals and their alloys, MMCs offer a number of advantages such as ... 

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 SiC/aluminum (SiC/Al) MMCs are reinforced with SiC particles (P) (Fig. 4), whiskers (W), fibers, or monofilament (MF). In 1988, an aluminum tube in the catamaran Stars and Stripes 88 was replaced by a lighter SiCp/Al MMC tube having a 30.5-cm outer diameter [3]. Lightweight mirrors for telescopes have also been fabricated from SiCp/Al MMC foam [47]. 

MMCs consist of metals reinforced with a variety of ceramic and carbon fibers, whiskers, and particles. There are wide ranges of materials that fall in this category. An important example is a material consisting of tungsten carbide particles embedded in a cobalt matrix, which is used extensively in cutting tools and dies. This composite, often referred to as a... 

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. 

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. 

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... 

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