Superplastic behaviour

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. 

The commonly used equation for the steady-state strain rate e characterizing superplastic behaviour is written as ... 

The values of the creep parameters (p, n and Q) identifying the superplastic behaviour of ceramic-related materials are not unique to such materials, nor to the same type of materials. As shown in the review papers, these parameters are very similar in tension as in compression in zirconia-based materials (probably the most widely studied ceramics in the widest experimental conditions), although that depends strongly on the purity of the ceramics 5,7 however, their behaviour seems to be very different in compression than in tension when an aid-sintering phase is necessary during the processing, as in silicon carbide and silicon nitride ceramics.8... 

Sato, E., Morioka, H., Kuribayashi, K., and Sundararaman, D., Effect of small amount of alumina doping on superplastic behaviour of tetragonal zirconia , J. Mater. Sci., 1999, 34, 4511-18. 

The natiocrystalline Ceo specimen exhibits a behaviour similar to that of superplasticity and demonstrates a large strain at room temperature, which may be due to grain boundary sUding that originates from the weak bonding of molecular forces. In the present experiments, however, the stress-strain curve and the fractured surface indicate an absence of the superplastic behaviour. This may be due to the decomposition of the C o phase into the amorphous phase during the heat treatment. 

This phenomenon has been postulated as an explanation for the compressive superplastic deformation of a SiAlON which undergoes a transition from n = 1 (Newtonian behaviour) to n = 0.5 for a characteristic critical stress (<7C) independent of both temperature and composition of the secondary phase.35... 

Finally, the forthcoming comprehension of superplasticity will demand a well-settled justification of the basic equations for this phenomenon. In order to achieve this goal, first-principles calculations should be conducted. This is a very challenging task, because the mechanical behaviour of grain boundaries requires an understanding of the physics involved at many different scales. At this point, simulations at microscopic as well as mesoscopic levels can become a useful tool. 

Morita, K., Hiraga, K., and Kim, B.N., Effect of minor Si02 addition on the creep behaviour of superplastic tetragonal Zr02 , Acta Mater., 2004, 52, 3355-64. 

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