Metal matrix composites reinforcing phase

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. 

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). 

The effect of dispersoids on the mechanical properties of metals has already been described in Section 5.1.2.2. In effect, these materials are composites, since the dispersoids are a second phase relative to the primary, metallic matrix. There are, however, many other types of composite materials, as outlined in Section 1.4, including laminates, random-fiber composites, and oriented fiber composites. Since the chemical nature of the matrix and reinforcement phases, as well as the way in which the two are brought together (e.g., random versus oriented), vary tremendously, we shall deal with specific types of composites separately. We will not attempt to deal with all possible matrix-reinforcement combinations, but rather focus on the most common and industrially important composites from a mechanical design point of view. 

Besides these developments, which are directed at specific applications, NijAl alloys are used for the development of inter-metallic matrix composites which contain reinforcing particles or fibers of borides, carbides, oxides or carbon (Fuchs, 1989 Lee et al., 1990 Tortorelli et al., 1990 Al-man and Stoloff, 1991 Kumar, 1991 McKamey and Carmichael, 1991 Muk-herjee and Khanra, 1991 Brennan etal., 1992). Apart from the mechanical properties and the necessary corrosion resistance, the chemical compatibility of the used phases is of primary importance with respect to the long-term stability. It was found that SiC, B4C, and TiBj react extensively with Nij Al alloys, whereas very little reaction has been observed with AljOj or Tie in NijAl (Fuchs, 1989 Lee etal., 1990 Brennan etal., 1992). It should be noted that NijAl alloys are used not only as the matrix material, but also as a reinforcing phase in, e.g. an Al alloy to form a metal-matrix composite (Metelnick and Varin, 1991). 

Metal-matrix composites using TiC as the reinforcing phase have also been used as tool materials for copper alloys (Ref 26). Both sintered TiC Ni W and hipped TiC Ni Mo alloys were used to friction stir copper alloys. However, both TiC-containing alloys produced brittle tools that fractured during the tool plunge. 

Composites may use ceramics as the matrix phase and/or the reinforcing phase. The purpose of a composite is to display a combination of the preferred characteristics of each of the components. In CMCs one of the principal goals has been to increase fracture toughness through reinforcement with whiskers or fibers. When ceramics are the reinforcement phase in, for example, metal matrix composites the result is usually an increase in strength, enhanced creep resistance, and greater wear resistance. Three issues must be solved ... 

Metal matrix nanocomposites are those having metal as the continuous phase or matrix and other nanoparticles like carbon nanotube as the reinforced materials. These types of composites can be classified as continuous and noncontinuous. One of the more important nanocomposites is Carbon nanotube reinforced metal matrix composite, which is an emerging new material with the high tensile strength and electrical conductivity of carbon nanotube materials. In addition to carbon nanotube metal matrix composites, boron nitride reinforced metal matrix composites and carbon nitride metal matrix composites are the new research areas on metal matrix nanocomposites [9,10]. 

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. 

Composites usually consist of a reinforcing material embedded in various matrices (binder). The elfective method to increase the strength and to improve the overall properties of composites is to incorporate dispersed phases into the matrix which can be an either polymer or engineering materials such as ceramics or metals. Hence, metal matrix composites (MMCs), ceramic matrix composites (CMCs) and polymer matrix composites (PMCs) are obtained. Besides, hybrid composites, metal/ceramic/polymer composites and carbon matrix composites can also be obtained. MMC and CMC composites are developed to withstand high temperature applications. MMCs are also used in heat dissipation/electronic transmission applications due to the conductive nature of metals (electrically and thermally). 

In contrast to conventional metal-matrix composites, Au-based metal-metal composites have been developed, in which both the metal and the reinforcing phase are ductile metals that in principle are soluble in each other. They are prepared by powder metallurgy techniques from pure metal powders and subsequently processed by extensive deformation (reductions in area greater than 99.99%), for... 

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... 

By the term particulate composites we are referring to composites reinforced with particles having dimensions of the same order of magnitude. Particulate composites are produced from a polymeric matrix, into which a suitable metal powder has been dispersed, and exhibit highly improved mechanical properties, better electrical and thermal conductivity than either phase, lower thermal expansivity, and improved dimensional stability and behaviour at elevated temperatures. 

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