Superplastic ceramic composites

A DOMINGUEZ-RODRIGUEZ D GOMEZ-GARCIA, Universidad de Sevilla, Spain and F W A K A I, Tokyo Institute of Technology, Japan 

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. 

This chapter discusses the following leading topics  

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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. 

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... 

As can be inferred from the equations outlined above, none of the different models can adjust the creep parameters for all the different ceramics, especially in the case of YTZP,7 explaining why there is still controversy over the accommodation process controlling superplasticity. The same conclusions can be outlined for ceramic composites, although more experimental work should be done.20,31... 

Chokshi., A.H., Superplasticity in fine-grained ceramic and ceramic composites current understanding and future prospects , Mater. Scl. Eng., 1993, A166, 119-33. 

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

N owadays, the list of ceramics and ceramics composites with superplastic behavior is very wide [4—9], and ceramics such as high-temperature superconductors YBaCuO behave superplastically (see Ref. [10] and the cover page of the Jourrudofthe American Ceramics Society, volume 97, May 1996). 

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... 

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. 

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. 

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. 

C. K. Yoon and I.-W. Chen, Superplastic Flow of Two-Phase Ceramics Containing Rigid Inclusions-Zirconia/Mullite Composites, J. Am. Ceram. Soc., 73[6], 1555-1565 (1990). 

Another common die alloy for tem-peratxires up to about 1000 °C (1830 °F) is a wrought stainless steel alloy of nominal composition Fe-22Cr-4Ni-9Mn. This alloy exhibits both creep and oxidation resistance for satisfactory operation up to 1000 °C (1830 °F) and is appropriate for forming of most titanium alloys. For higher temperatures, such as is necessary for superplastic forming of y-TlAl, ceramic dies must be used. 

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