Particulate-reinforced MMCs

The particulate-reinforced MMCs are lightweight and demonstrate a significant increase in modulus and tensile strength at both ambient and elevated temperatures of the unreintorced material. The process was announced in late 1992 by Magnesium Elektron Ltd.. Manchester. U.K. 

Spray deposition The spray deposition process, which was developed by Osprey Ltd during the late 1970s for producing monolithic alloys [365], has been adapted by several manufacturers to produce particulate-reinforced MMC billets with a residual porosity of 5% [366]. The porosity is eliminated by a secondary processing, such as extrusion or rolling. A spray gun is used to produce an atomized stream of aluminum alloy, into which heated SiC particles are injected. An optimum particle size is... 

MMC with large thermal strain shows nonuniform and localized plastic deformation. The strengthening in the metal matrix in particulate-reinforced MMCs can be determined by (Dieter, 1961),... 

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 ... 

Although the feasibility of fibrous MMCs is technically demonstrated, the cost of the fibers and of the manufacturing processes, e.g., squeeze casting, is still too high. Thus, particulate reinforcements are presently preferred. 

One group of collisions occurring in the penetration regime is the main source for the incorporation of particulate reinforcements in the interior region of MMC particles. The incorporation efficiency can be roughly described by the volume fraction of these particles to the droplet. 

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. 

In some applications the lack of toughness of ceramics or CMCs prohibits their use. In cases where enhanced stiffness, wear resistance, or elevated temperature capabilities greater than those provided by metals are necessary, metal matrix composites (MMCs) offer a reasonable compromise between ceramics or CMCs and metals. Typically, MMCs have discrete ceramic particulate or fiber reinforcement contained within a metal matrix. In comparison to CMCs, MMCs tend to be more workable and more easily formed, less brittle, and more flaw tolerant. These gains come primarily at the expense of a loss of high-temperature mechanical properties and chemical stability offered by CMCs. These materials thus offer an intermediate set of properties between metals and ceramics, though somewhat closer to metals than ceramics or CMCs. Nonetheless, like ceramic matrix composites, they involve physical mixtures of different materials that are exposed to elevated temperature processes, and therefore evoke similar thermodyamic considerations for reinforcement stability. 

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. 

Depth of attack can be used to assess corrosion damage in MMCs. It may be more difficult, however, to assess the penetration depth in MMCs compared to that in monolithic metals because reinforcement constituents that are left in relief can prevent a depth gage x>m contacting the matrix surface, and thus, prevent the true depth of penetration from being measured. In particulate MMCs, the interference may be attenuated since reinforcement particles are free to detach from the matrix as they are undercut by corrosion. When reinforcements interfere with mechanical measurements, depth of penetration can be assessed metal-lographicaUy. Samples may be mounted and polished, and penetration measurements can be made with a microscope and a calibrated eyepiece. Details for determining penetration depth are available in ASTM G 46, Recommended Practice for Examination and Evaluation of Pitting Corrosion. 

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. 

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 (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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