Viscous creep cavitation

Figure 12.4 (a) Intergranular fracture of an alumina sample showing creep cavitation due to compressive creep at 1600°C. Note closely spaced cavities along the two-grain facets. (b) Schematic of cavity formation in viscous grain boundary films as a result of applied tensile stress. 

Interfacial cavitation, microcracking, diffusion and crack branching due to the viscous flow of the glass phase during creep-fatigue. 

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. 

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