Creep resistance temperature dependence

Talc is a hydrated magnesium silicate that is composed of thin platelets primarily white in color. Talc is useful for lowering the cost of the formulation with minimal effect on physical properties. Because of its platy structure and aspect ratio, these extenders are considered reinforcement. Polymers filled with platy talc exhibit higher stiffness, tensile strength, and creep resistance, at ambient as well as elevated temperatures, than do polymers filled with particulate fillers. Talc is inert to most chemical reagents and acids. The actual chemical composition for commercial talc varies and is highly dependent on the location of its mining site. 

Background At elevated temperatures the rapid application of a sustained creep load to a fiber-reinforced ceramic typically produces an instantaneous elastic strain, followed by time-dependent creep deformation. Because the elastic constants, creep rates and stress-relaxation behavior of the fibers and matrix typically differ, a time-dependent redistribution in stress between the fibers and matrix will occur during creep. Even in the absence of an applied load, stress redistribution can occur if differences in the thermal expansion coefficients of the fibers and matrix generate residual stresses when a component is heated. For temperatures sufficient to cause the creep deformation of either constituent, this mismatch in creep resistance causes a progres-... 

Fig. 5.6 Relationship between the creep rate of a composite and the stress and temperature dependence of the creep parameters of the constituents.31 (a) Temperature dependence of constituent creep rate, (b) Stress dependence of constituent creep rate, (c) Intrinsic creep rate of constituents as a function of temperature and stress illustrating the temperature and stress dependence of the creep mismatch ratio. In general, load transfer occurs from the constituent with the higher creep rate to the more creep-resistant constituent, (d) Composite creep rate with reference to the intrinsic creep rate of the constituents. The planes labeled kf and em represent the intrinsic creep rates of the fibers and matrix, respectively.

In view of the discussed composition dependence of the creep resistance, it is concluded that the effective diffusion coefficient is of primary importance for controlling the creep resistance (Sauthoff, 1993 b). This of course does not mean that the other parameters in Eq. (2) can be neglected. This is demonstrated by the temperature dependence of the creep of B2 (Ni,Fe)Al, as was discussed earlier (Sauthoff, 1991 a). In view of Eqs. (2) and (3), the apparent activation energy for creep is expected to correspond to that for diffusion since the other parameters depend less sensitively on temperature, and indeed this has been confirmed repeatedly in the case of conventional disordered alloys. However, in the case of B2 (Ni,Fe) Al, the apparent activation energy for creep only corresponds to that for diffusion at temperatures up to 900 °C, whereas at higher temperatures the apparent activation energy for creep is much higher. Acti-... 

Figure 25. Temperature dependence of creep resistance (in compression with 10 s secondary strain rate) for various single-phase intermetallic alloys NiAl, CoAl and related alloys with a B2 structure (Jung et al., 1987 Sauthoff, 1989), NijTiAl with an L2, structure (Strutt and Polvani, 1973), two NijAl variants with Ll structures, i.e., an advanced alu-minide (+)(Schneibel et at, 1986) and NijAl Fe(x) (Nicholls and Rawlings, 1977), FcjAIC with an L l structure (Jung and Sauthoff, 1989 b), and A Nb with a DOjj structure (Sauthoff, 1990a, b Reip, 1991).

Selecting ceramics for use at high temperatures or under applied load requires consideration of their long-term stability. Time dependent deformation is known as creep, and creep resistance is a critical design parameter. Even if creep does not lead to failure, a change in shape or size may render a component useless. The mechanism responsible for creep depends on temperature, stress, and the microstructure of the ceramic. 

In addition, the flow characteristics of the material depending on the local strain rates are affected by local temperatures. Consequently, the initial yield point drops as the temperature rises, and rises as the strain rate increases as the strain rate increases, the heat generated from forming also increases, thereby compensating for the rise in the initial yield point the thermal and elastic properties of tool and workpiece are influenced by the temperature as the temperature increases, there is often a drop in creep resistance. 

In general, the maximum temperature/time/stress capability of the more creep-prone fibers is limited by the fiber tendency to display excessive creep strains (for example, > 1%) before fracture. On the other hand, the temperature/time/stress capability of the more creep-resistant fibers is limited by fiber fracture at low creep strains (<1%), the values of which are often dependent on the environment. These limitations are illustrated in Table 3, which shows the approximate upper use-temperature for some SiC fibers, as determined from the... 

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