Metal tungsten fibers

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. 

An important application field for stainless steel fibers is the textile sector, in which 0., i to 6% of these fibers are incorporated to endow carpets, protective clothing etc. with an antistatic finish. A further application is protection against electromagnetic pulses, interference and charging. Tungsten fibers with a diameter of 12 pm are used for boron or SiC deposition and as light bulb filaments. Furthermore, metal fibers are used in the filtration of polymer melts and corrosive liquids, as well as for electrodes with high surface areas. 

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]. 

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. 

Kelly, A. and Tyson, W. R., Tensile properties of fiber reinforced metals copper tungsten and copper/molybdenum,. /. Mech. Phys. Solids, 13, 329 (1965). 

More recently, mngsten oxide fibers have been detected in a hard metal production plant. Subsequent in vitro experiments showed that the tungsten oxide fibers were cytotoxic to human lung cells. The role of tungsten in the development of hard metal dust pulmonary fibrosis remains unclear. 

As noted above, the range of fibers employed does not precisely overlap with those employed for organic composites. Because the formation of the MMCs generally requires melting of the metal-matrix, the fibers need to have some stability to relatively high temperatures. Such fibers include graphite, silicon carbide, boron, alumina-silica, and alumina fibers. Most of these are available as continuous and discontinuous fibers. It also includes a number of thin metal wires made from tungsten, titanium, molybdenum, and beryllium. 

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