Superplastic Ceramics

However, it has often been observed that little or no hardening at all occurs. 

Superplastic materials are characterized by high ductility, preferably achieved at relatively low temperatures and high deformation rates. The strain rate is expressed in terms of stress and grain size, d, as  

Grain-boundary mobility, which is related to grain growth, may be seen in Fig. 5.12 with the effect of the same additives that was indicated in the stress-strain curves of specimens deformed as illustrated in Fig. 5.9. 

In Fig. 5.8, the microstructure of alumina is shown, indicating superplastic behavior (early experiments failed to show superplasticity). Plastic deformation to large strains was achieved, but the window for superplastic fabrication is very narrow, because sintering below 1330 °C is quite difficult and requires much experience for successful production. Above this temperature, grain growth is quite fast. Therefore, proper additives are needed to reduce grain growth above this 

SiaNq is another superplastic ceramic. Two polymorphs, a and f sUicon nitrides, exist. Both are hexagonal and capable of having a large range of solubility with various constituents. Strain hardening in Si3N4 depends on its composition 

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Superplastic ceramics have several obvious potential advantages for commercial application. These include net size and shape forming and the possibility of forming complex components from initially flat sheets. Whilst the practical problems of forming at temperatures in excess of 1200°C obviously... 

Nieh, T. G., Wadsworth, J., and Wkai, E, Recent advances in superplastic ceramics and ceramic composites. Int. Mater. Rev. 36,146 (1991). 

Structural modifications of engineered materials are caused by the incorporation of nanoparticles as passive basic building blocks and lead, for example, to superplastic ceramics or extremely hard metals. Functional applications, on the other hand, rely on the transformation of external signals, such as the filtering of light, the change of electrical resistance in different environments, or the occurrence of luminescence when electrically activated (Tab. 11.1). 

One of the important features of impact testing is the evaluation of the ductile-to-brittle transition temperature. What, then, is the purpose of discussing the impact testing of ceramics, since most are brittle at ambient temperature (and clearly at low temperatures) Yet, impact tests are also performed on classic, brittle materials in order to evaluate the energy absorbed during the fracturing process. Furthermore, some brittle ceramics are ductile at sufficiently elevated temperatures, so the brittle-ductile transition may still be of interest. Ductile and superplastic ceramics will be discussed in depth in Chap. 2 (on ductile ceramics), while the present section deals with the actual process of performing impact tests. 

Not unlike the case of superplastic ceramics, ductility and strength relations are influenced by strain rate. The conditions of the experiment must be above the DBT to observe plastic flow, which is different for various ceramics. An illustration of the effect of strain rate and temperature on the strain (ductility) at some stress level can be seen in monolithic Si-C-N. Silicon-nitride-based ceramics are quite promising candidates for mechanical applications at elevated temperatures. Specimens were prepared by hot isostatic pressure (henceforth HIP) of pyrolyzed powder compact at 1500 °C and 950 MPa, without any sintering additives. These compression tests were conducted at temperatures from 1400 to 1700 °C in a nitrogen atmosphere with a servo-hydraulic-type testing machine at constant crosshead speed in an induction heating furnace. In Fig. 2.5, stress-strain curves... 

As previously indicated, zirconia is a typical superplastic ceramic and was among the first oxide ceramics to be smdied. As early as 1986, Wakai et al. studied... 

Fig. 2.61 Tensile ductility of fine-grained iron carbide as function of the Zener-Hollomon parameter is compared with some superplastic ceramics doped with various impurities. The strain rate sensitivity parameter is in the range m = 0.5-0.6 [28]. With kind permission of Elsevier...

The tensile ductility of various superplastic ceramics are compared with that of iron carbide in Fig. 2.61. All the curves show the same tendency, namely that tensile ductility decreases with increased strain rate-temperature, sexp( ). This decrease has been explained by grain growth. It is possible to superimpose all the superplastic ceramics data shown in Fig. 2.51 on a common curve when sexp( ) is multiplied by A, which is unique for each ceramic. The results are shown in Fig. 2.62. 

Fig. 5.8 Scanning electron microscopy micrographs of ultrafine grains of superplastic ceramics a 2Y-TZP, b alumina, c silicon nitride, and d 2Y-TZP/alumina at equal volume fraction [3]. With kind permission of John Wiley and Sons...

Equation 6.29 may be expressed differently for superplastic ceramics, to include the effect of grain size, as seen in Eq. (6.31) ... 

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