Accommodation Processes in Superplasticity

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. 

In the model employed, bulk diffusion through the grains and diffusion along the grain boundaries must be considered. These two processes are always dominant at low stresses, because the rates of diffiisional flow are usually linear 

In this situation, Ashby and Verrall considered a group of four grains deformed at constant pressure and stress. During this flow, four irreversible processes took place  

By considering the entropy change in these four processes, the constitutive equation of the A-V model, when lattice and grain boundary diffusion are taken into account, can be written as  

Although this explanation might seem adequate for metals, it cannot be applied to ceramics where dislocations at grain boundaries have never been observed, neither at the boundaries nor in the bulk during superplastic deformation. 

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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... 

For instance, dislocations have been shown to play a key role in the accommodation process in YTZP, justifying the threshold stress in YTZP, in contrast with the hypothesis that this threshold stress is due to the electric field created by impurity segregation. However, dislocations are not systematically observed in YTZP furthermore it was shown that in yttria-stabilized tetragonal zirconia single crystals, the stress necessary to activate dislocations at 1400°C was over 400 MPa, one order of magnitude higher than the stresses used during superplastic deformation of YTZP at the same temperature. It will be necessary to conduct a systematic study of the microstructure of the monolithic ceramics such as YTZP before and after deformation and to correlate their relationship with the superplastic features. 

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. 

When accommodated by some of the mechanisms involving dislocation movement or the diffusion of point defects, GBS forms the basis of the structural superplastic behavior of these materials (see Section 15.2). By taking advantage of the processes involved in superplasticity, it is possible to join ceramics super-plastically. For example, when two pieces of the same ceramics in contact are deformed within a superplastic regime (i.e., as soon as GBS is activated), the grains of one part interpenetrate those of the other part. This produces a rapid and perfect junction of the two, in such a way that a shorter time and a lower temperature can be used than are commonly required in other conventional process for ceramics joining [90]. 

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