Metal matrix composites mechanical properties

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. 

The amount of incorporated particles is the parameter characterizing a metal matrix composite. As discussed in the previous section it largely determines the composite properties. In order to obtain a composite exhibiting certain properties, the effect of process parameters on the particle composite content has therefore to be known. Apart from the practical significance knowledge of these effects is also a prerequisite for the understanding of the mechanism underlying particle codeposition. 

Finally, in chapter 6, another direction of applied electrochemistry is treated by Hovestad and Janssen Electroplating of Metal Matrix Composites by Codeposition of Suspended Particles. This is another area of metals materials-science where electroplating of a given metal is conducted in the presence of suspended particles, e.g. of A1203, BN, WC, SiC or TiC, which become electrodeposited as firmly bound occlusions. Such composite deposits have improved physical and electrochemical properties. Process parameters, and mechanisms and models of the codeposition processes are described in relation to bath... 

In recent years, there has been a growing interest in the electrochemical synthesis of composite materials consisting of metal matrix with embedded particles of oxides, carbides, borides, etc. Metal-matrix composites offer new possibilities in fabrication of ftmctional coatings with radically improved durability and performance [1], However, in spite of the efforts of many researches, the overall picture of the processes occurring during co-deposition of metal with dispersed phase and mechanism of particle-induced modification of mechanical and chemical properties still remain unclear. In this study, we focused on the kinetics and mechanism of the electrochemical co-deposition of nickel with highly dispersed oxide phases of different nature and morphology. 

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. 

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). 

J.P. Lucas, L.E. Toth and W.W.Geberich, ANovelTechnique for ProducingFine Metal Fibres forEnhancing Mechanical Properties of Glass Matrix Composites, J. Am. Ceram. Soc. 63, 280-285 (1980). 

Hamouda, A. M. S., Hashmi, M. S. J. Mechanical properties of aluminium metal matrix composites under impact loading. International Journal of Materials Processing Technology 1996 56 743-56. 

The criteria for designing fibers for use in ceramic matrix composites (CMCs) are different from those for designing fibers for use in poiymer or metal matrix composites. The key properties are thermal stability and mechanical properties at high temperatures [43]. As a consequence, relatively coarse microstructures are obtained at elevated temperatures, corresponding to somewhat lower failure strengths (-2 GPa), but high thermal stability and creep resistance are preferable to ultrafine microstructures. 

The thermal characteristics of NR-metal composites are close to the properties of metals, whereas the mechanical properties and the processing methods are typical of polymers.Thermally conducting, but electrically insulating, polymer-matrix composites are increasingly important for electronic packaging because the heat dissipation ability limits the reliability, performance and miniaturization of electronics.Thermal properties such as thermal conductivity, thermal dilfusivity and specific heat of metal (copper, zinc, Fe and bronze) powder-filled polymer composites are investigated experimentally in the range of filler content 0-24% by volume. ... 

I nformation on the properties of carbon-fiber, metal-matrix composites is still scanty with little or no suitable comparative data available. I n many reports, important variables such as fiber-matrix ratio and fiber orientation are not mentioned. As a rule, the mechanical properties of present composites are still far short of the potential predicted by the rule-of-mixtures. 

These polymers can be moulded by injection or compression and can be cast as films fi om organic solvents. Their isotropic tensile moduli are well beyond the range of known isotropic polymers. They have outstanding mechanical properties, considerably better than those of the best polymeric materials, and can even be compared to structural metals [66] (see Mechanical properties). The rigid PPP backbone is presented as the microscopic equivalent of the fibre in a fibre-polymer composite, such as oriented carbon fibres embedded in a suitable polymer matrix the pendant groups play the role of the suitable polymer matrix. Applications in the design and construction of military and commercial aircraft, sports and industrial equipment and automobile components are proposed. 

Ceramic fibers. The other fibers shown in Table 4.6 have varying uses, and several are still in development. Silicon carbide continuous fiber is produced in a chemical vapor deposition (CVD) process similar to that for boron, and it has many mechanical properties identical to those of boron. The other fibers show promise in metal matrix composites, as high-temperature polymeric ablative reinforcements, in ceramic-ceramic composites, and in microwave transparent structures (radomes or microwave printed wiring boards). 

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