Ceramic superplasticity

In the immediate future, the main objective in ceramic superplasticity will be the search of the right conditions to achieve high strain rate superplasticity (HSRS) ((e > 1CT2 s 1). Although this phenomenon has been found in several ceramic compounds and several inputs have been outlined to achieve it, we are still far from knowing what to do to obtain this effect systematically. This HSRS will enlarge the applications for ceramics. 

In this chapter, the macroscopic and microscopic aspects of superplasticity, the accommodation processes, the applications and the future prospects of ceramic superplasticity vdll be addressed. 

In 1986, Wakai et al. (1986) in Japan discovered that ultra-fine-grained ceramics can also be superplastically deformed they may be brittle with respect to dislocation... 

Nieh, T.G., Wadsworth, J. and Sherby, O.D. (1997) Superplasticity in Metals and Ceramics (Cambridge University Press, Cambridge). 

There apparently exists a critical amount of liquid phase for the optimization of grain/interface boundary sliding during superplastic deformation. The optimum amount of liquid phase may depend upon the precise material composition and the precise nature of a grain boundary or interface, such as local chemistry (which determines the chemical interactions between atoms in the liquid phase and atoms in its neighboring grains) and misorientation. The existence of an equilibrium thickness of intergranular liquid phase in ceramics has been discussed [14]. This area of detailed study in metal alloys has not been addressed. 

Chen IW, Hwang SL (1993) Superplastic SiAlON-A Birds Eye View of Silicon Nitride Ceramics. In Chen IW, Becher PF, Mitomo M, Petzow G, Yen TS (eds) Silicon Nitride Ceramics, Mat Res Soc Symp 287 Mat Res Soc, Pittsburgh, p 209... 

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... 

Wakai, F. A review of superplasticity in Zr02-toughened ceramics , Brit. Ceram. Trans. 88 (1989) 205-208. 

Yoon, C.K. and Chen, I.W. Superplasticity of two-phase ceramics containing inclusions - zirconia mullite composites , J. Am. Ceram. Soc. 73 (1990) 1555-1565. 

The application of ceramics has infiltrated almost all fields in the last 20 years, because of their advantages over metals due to their strong ionic or covalent bonding. But it is just this bonding nature of ceramics that directly results in their inherent brittleness and difficulty in machining. In other words, ceramics show hardly any macroscopic plasticity at room temperature or at low temperatures like metals. Hence, superplasticity at room temperature is a research objective for structural ceramics. In recent years, many researches have been carried out to investigate nanophase ceramic composites. 

Since the early report of superplasticity in a ceramic material in 1986, a variety of such materials have been shown to exhibit superplastic behavior... 

Wang, J.G., Raj, R., Mechanism of superplastic flow in a fine-grained ceramic containing some liquid phase, J. Am. Ceram. Soc., 1984, 67(6) 399. 

Superplasticity is macroscopically defined as the ability of a polycrystalline material to exhibit large elongations at elevated temperatures and relatively low stresses. It is commonly found in a wide range of materials from metals to ceramics (bioceramics or high-temperature superconductors, among others) when the grain size is small enough a few micrometres for metals and less than a micron in ceramics. 

Superplasticity is a very promising property, not only because, like in metals, the superplastic formation opens a way for the manufacturing of complex ceramic pieces for industrial applications, but also because the combination of GBS and diffusional processes makes superplasticity an interesting tool for joining ceramic pieces in shorter times and lower temperatures than the diffusional joining technique. 

Several parameters can influence strongly the superplastic behaviour of ceramics, i.e. the strain rate at which the material can be superplastically deformed. Between them can be mentioned the grain size, second phases and segregation of impurities at the grain boundaries, etc. 

Future tendencies in the field, with a special emphasis on up-to-date information about the most outstanding and promising superplasticity in related ceramic materials. 

The first observation of superplasticity in a 3 mol% yttria-stabilized tetragonal zirconia polycrystal ceramic (YTZP) with a grain size of 0.4 pm was reported by Wakai et al in 1986 (Fig. 16.1). Since then, a large number... 

Today, from an engineering point of view, the name superplasticity is ascribed to a polycrystalline material pulled out to very high tensile elongations prior to failure with necking-free strain. This phenomenon is usually found in many metals, alloys, intermetallics, composites and ceramics (recently in high-temperature superconductor ceramics) when the grain size is small enough, less than 10 pm for metals and less than 1 pm for ceramics. 

Although superplasticity is defined as the ability of a polycrystalline material to exhibit large elongations, in many ceramics-related materials and ceramic composites superplasticity is also said to occur even though the polycrystal is deformed in compression, or in three- or four-point bending conditions, as long as GBS is the primary deformation process.4-7... 

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