Diffusional creep mechanism

Figure 13.10 A comparison of tensile and compressive creep for SN 281 indicates that creep in tension and compression is equal. In addition, the slope ofthe creep curve is close to 1, suggesting a classical diffusional creep mechanism [45].

Figure 16.5 Deformation mechanism map for Ag polycrystal a = applied stress, p = shear modulus, grain size = 32 pm, and strain rate = IGF8 s 1. The diffusional creep field is divided into two subfields the Coble creep field and the Nabarro-Herring creep field. From Ashby [20].

Finally, mechanisms besides diffusional transport of mass between internal interfaces can contribute to diffusional creep. For instance, single crystals containing dislocations exhibit limited creep if the dislocations act as sources and sinks, depending on their orientation with respect to an applied stress (see Exercise 16.3). 

Fine-grained materials, when subjected to high temperatures and low applied stresses, deform by mutual accommodation of grains assisted by grain boundary sliding and transport of matter (diffusion). Under conditions where lattice diffusion dominates, the diffusional creep rate is reasonably well characterized by the Nabarro-Herring creep process. (For a review of this and other classical creep mechanisms, see Refs. 5 and 6.) Here the strain rate is expressed as... 

Many structural ceramics often contain significant amounts of glassy phases at the grain boundaries, and it is now well established that for many of them the main creep mechanism is not diffusional, but rather results from the softening and viscous flow of these glassy phases. Several mechanisms have been proposed to explain the phenomenon, most notable among them being these three ... 

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. 

Figure 7.15 Diffusional creep in AI2O3 (schematic). The solid lines show the creep rate dependence on grain size for each assumed species and path. The dotted line traces the rate-limiting mechanism, given by the slower diffusing species along its fastest path.

Practical consequences of Eg modification in polymer films include significant changes of dissolution, diffusional and etching characteristics, mechanical creep behavior, and adhesion. Figure 17.30 shows a plot of the effective diffusion coefficient of perfluorooctane sulfonate photoacid as a function of film thickness of partially protected poly(4-t-butyloxycarbonyloxstyrene). The profile shows asymptotic behavior at 600 A, below which diffusion slows down remarkably, probably due to interfacial and confinement effects. Clearly, the interaction of the first few hundred angstroms of the film with the substrate determines its adhesion and can alter its electrical and optical properties as well as its topographical and surface characteristics. ... 

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]. 

The term viscous creep is often used for creep at high temperatures with low stresses. Two mechanisms have been proposed to describe such creep in polycrystalline materials. The one known as Nabarro-Herring creep conceives of a stress-directed, diffusional migration of vacancies, while the other, originally suggested by Mott and subsequently elaborated by Weertman, is based on a dislocation-climb model [66]. Extensive experimental evidence also exists to support a dislocation-climb model . 

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