Tungsten, elastic properties

Elastic Properties [1.30,1.31,1.35]. In regard to elasticity, at least below room temperature, tungsten behaves nearly isotropically the anisotropy coefficient at 24 °C is = 1.010 [1.35]. The elastic constants for polycrystalline tungsten at 20 °C are given below. Their temperature dependence as well as the respective values for singlecrystal elastic constants are shown in Fig. 1.10 [1.40], based on ultrasonic measurements [1.30,1.31]. 

Most metals (Table L.2) show elastic anisotropy. Note that a thin film is polycrystalline, that is, formed of monocrystalfine grains, the size and orientation of which depend on deposition conditions. They will exhibit elastic macroscopic anisotropy only when texture is present, that is, when crystallites show a preferential orientation. Tungsten is an archetype since the crystallites are elastically isotropic. Hence tungsten films show microscopic and macroscopic elastic isotropy. For A = 1 the elastic properties are completely defined by two elastic constants E and v and we have... 

Tensile and shear forces are not the only types of loads that can result in deformation. Compressive forces may as well. For example, if a body is subjected to hydrostatic pressure, which exists at any place in a body of fluid (e.g. air, water) owing to the weight of the fluid above, the elastic response of the body would be a change in volume, but not shape. This behavior is quantified by the bulk modulus, B, which is the resistance to volume change, or the specific incompressibihty, of a material. A related, but not identical property, is hardness, H, which is defined as the resistance offered by a material to external mechanical action (plastic deformation). A material may have a high bulk modulus but low hardness (tungsten carbide, B = 439 GPa, hardness = 30 GPa). 

Another material of this kind is tungsten disulphide, originally developed by NASA for aerospace applications, now also applicable to specialty industries [58]. Ti3SiC2 is a thermodynamically stable, nano-layered, ternary carbide and part of a family of over 50 ternary carbides and nitrides, the MAX phases [62]. These phases are a new class of solids possessing unique combinations of properties they are readily machinable, relatively soft for ceramics, but elastically stiff, and electrically and thermally conductive. They combine the good properties of both metals and ceramics that could lead to this technology contributing to future lubricant developments. 

Elements such as chromium, manganese, nickel, tungsten, vanadium, silicon, and molybdenum are added to steels to obtain alloys with improved properties, for example strength, elasticity, hardness, abrasion resistance, rust resistance, and chemical resistance. 

Table 2.3 contains an overview of the elastic constants for some metals and ceramics. As can be seen, the anisotropy factor of tungsten is 1.0, so it is (almost) isotropic even as a single crystal. For most other materials, almost isotropic properties can only be found in a polycrystalline state. The direction dependence of Young s modulus for selected materials is plotted in figure 2.10. 

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