Accommodation processes controlling superplasticity

As mentioned above, several mechanisms can be responsible for the grain boundary sliding accommodation however, so far there is no consensus on a general single mechanism to accommodate GBS, nor one concerning a particular ceramic. In this section the different mechanisms for accommodation will be analysed. For the sake of clarity, the accommodation process will be described for each type of ceramic, whether monolithic, with secondary glassy phases or composite. 

The constitutive equation of the A-V model, when lattice and grain boundary diffusion are taken into account, is written  

As can be observed in Eq. (16.4), the strain rate is a linear function of the stress (n = 1) in disagreement with experimental observations for many monolithic ceramics5,7,28 for which n is 2. For YTZP, the grain size dependence fits to 2 and the activation energy corresponds to that of cationic lattice diffusion. 

In the case of YTZP, on which a large number of studies have been performed, the data could be fitted to a constitutive equation, which is identical to that found in metals when lattice diffusion is the rate-controlling mechanism 29 

When GBS is accommodated by the movement of dislocations, the strain rate can be deduced in a similar way as for recovery creep the dislocations generated at the grain boundaries move either along the grain boundaries or 

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The different accommodation processes controlling superplastic output... 

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... 

Before discussing the possible accommodation processes controlling superplasticity, it is important first to discuss how the nature of the grain boundaries can influence superplastic behavior. 

The accommodation processes are responsible for the rate-control of superplasticity, and no single mechanism exists to accommodate GBS, even with regards to a particular ceramic system. As noted above, several factors can affect the different mechanisms, among which should be included the nature of the impurities present in the grain boundaries, the secondary phases, and the testing conditions. The different mechanisms for accommodation will be analyzed in the following sections. 

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