Superplasticity creep mechanism

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... 

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 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]. 

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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