Creep superplastic

W. Beere. Stress redistribution during Nabarro-Herring and superplastic creep. Metal Sci., 10(4) 133—139, 1976. 

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... 

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... 

At high temperatures the glassy phase may become less viscous and even liquid and as a consequence may account for the plastic deformation. However, viscous flow creep is not regarded as a viable creep mechanism for superplasticity due to its limited deformation, which corresponds to the redistribution of the glassy phase and therefore to the squeeze of these secondary phases from grain boundaries subjected to compression.8... 

Duclos, R., Direct observation of grain rearrangement during superplastic creep of a fine-grained zirconia , J. Eur. Ceram. Soc., 2004, 24, 3103-10. 

Langdon, T.G., The significance of grain boundary sliding in creep and superplasticity , Metal Forum, 1981, 4, 14-23. 

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. 

This superplastically deformed silicon nitride also showed substantially improved creep resistance at high temperatures, when the stress was applied along the extruding direction. For example, the creep rates of the deformed body in tensile creep tests conducted at 1200 °C was found to be about one order of magnitude lower... 

Depending on which of the above factors dominates during deformation, the accommodation mechanism may be regarded as viscous flow, solution-precipitation, or cavitation creep. In general, cavitation creep can be discarded as an accommodation mechanism for superplasticity, as the strain-to-failure afforded by this mechanism is rather small. Therefore, only viscous flow and solution-precipitation mechanisms are important. Obviously, too, whether these mechanisms apply depends on the presence or absence of a liquid phase. Solution-precipitation requires a liquid phase to envelop the grains, while viscous flow is facilitated by the fast diffusion path of the liquid, although it may also occur in a dry polycrystal via diffusional creep. 

The values of the creep parameters (p, n, and Q) identifying the superplastic behavior of ceramic-related materials are not unique, neither are they similar for the same types of material. Several factors can affect these parameters, among them the... 

The mechanism of GBS accommodated by diffusional flow has been successfully used to explain the superplastic behavior of YTZP. In the case of YTZP, values of p between 1 and 3, of n between 2 and higher than 5, and of between 450 and 700 kj mol have been reported during creep [8]. 

Shear-thickening creep was postulated to explain the compressive superplastic deformation of SiAlON, which undergoes a transition from n=l (Newtonian behavior) to n=0.5 for a characteristic critical stress (oj, independent of both temperature and the composition of the secondary phase (88). 

The subject of superplasticity has been extensively discussed in Chaps. 2 (Sect. 2.2) and 5 (Sect. 5.23) therefore, only material relevant to creep will be discussed here. The power law given in Eq. (5.2), rewritten here as ... 

This relation determines the activation energy. A is the material constant, tr is the steady-state flow stress, d is the grain size, n and p are the stress and grain exponents, respectively, and Q is the activation energy for superplastic flow. Thus, it may be seen from this equation that the parameters n, p and Q play a role in the deformation mechanism. Superplasticity is considered to be similar to diffusion creep . 

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