Tungsten-Fiber-Reinforced Composites

Tungsten metal and tungsten alloy wires have attracted interest for fiber reinforcement of high-temperature materials, in particular of superalloys. However, until now, they have remained exotic materials with only a few specific applications or for model studies of mechanical behavior and are not produced on a large scale. This is at least partially due to the difficulties encountered during composite fabrication and the resulting high cost of the material. 

Other interesting matrices are niobium alloys (investigated for long-term, high-temperature applications in space power systems) and copper and silver for electrical applications and heat exchangers [6.50,6.51]. 

Tungsten fiber composites can be produced either by infiltration of the fiber bundle by the molten matrix, or by bringing together the fibers and matrix in the solid state and 

Second Line E. Pink and R. Eck, Refractory Metals and their Alloys, in Materials Science and Technology (R. W. Cahn, P. Haasen, and E. J. Kramer, eds.), Vol. 8, pp. 591-638, VCH, Weinheim (1996). 

FIGURE 6.19. Micrographs of a tungsten-wire reinforced superalloy sectioned perpendicular to the wire direction (left) strength and ductilities of typical tungsten-wire q reinforced superalloys with 40 vol% fibers (right) [6.50]. By courtesy of R. Warren, Lulea University of Technology, Sweden.  

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Boron/tungsten fiber applications include the use of filaments and of boron/tungsten fiber reinforced prepreg tape, aluminum matrix composites, and boron/graphite structures. The major applications for these structures are found in the aerospace market and about 25% in sporting goods markets [36]. SiC/carbon fiber reinforced products include aluminum, titanium, and ceramic matrix composites. Major applications for these structures are also found in the aerospace market, minor uses in the industrial market [37]. 

In these expressions, E and V denote the elastic modulus and volume fraction, respectively, and the subscripts c, m, and p represent composite, matrix, and particulate phases, respectively. Figure 16.3 plots upper- and lower-bound c-versus-Vp curves for a copper-tungsten composite, in which tungsten is the particulate phase experimental data points fall between the two curves. Equations analogous to 16.1 and 16.2 for fiber-reinforced composites are derived in Section 16.5. 

Both molybdenum and tungsten can be worked in air without ductiHty loss. AH refractory metals can be made into tubing by extmsion, and most refractory metals, except chromium, are available as wine. Tungsten wines were attempted as fiber reinforcement for experimental nickel-base composites. 

Boltzmann s constant, and T is tempeiatuie in kelvin. In general, the creep resistance of metal is improved by the incorporation of ceramic reinforcements. The steady-state creep rate as a function of appHed stress for silver matrix and tungsten fiber—silver matrix composites at 600°C is an example (Fig. 18) (52). The modeling of creep behavior of MMCs is compHcated because in the temperature regime where the metal matrix may be creeping, the ceramic reinforcement is likely to be deforming elastically. 

Finally, metallic fibers find some limited applications as reinforcement in composites. They are generally not desirable due to their inherently high densities and because they present difficulties in coupling to the matrix. Nonetheless, tungsten fibers are used in metal-matrix composites, as are steel fibers in cement composites. There is increasing interest in shape memory alloy filaments, such as Ti-Ni (Nitanol) for use in piezoelectric composites. We will discuss shape-memory alloys and nonstructural composites in later chapters of the text. 

Murphy, C., Byrne, G. and Gilchrist, M.D. (2002) The performance of coated tungsten carbide drills when machining carbon fiber-reinforced epoxy composite materials, PI MECH ENG B-J ENG, 216 143-52. 

Metal matrix composites (MMCs) are metals that are reinforced with fibers or particles that usually are stiff, strong, and lightweight. The fibers and particles can be metal (e.g., tungsten), nonmetal (e.g., carbon or boron), or ceramic (e.g., silicon carbide (SiC) or (alumina) AljOj). The purpose for reinforcing metals with fibers or particles is to create composites that have properties more useful than that of the individual constituents. For example, fibers and particles are used in MMCs to increase stiffness [/], strength [f ], and thermal conductivity [2], and to reduce weight [f], thermal expansion [3], fiiction [4], and wear [5]. 

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... 

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