Ashby and Verall46 proposed a model to describe the process of GBS accom­modated by diffusional flow. According to this model [3.23]

where

and a0 is the threshold stress. As shown in Fig. 3.7, a natural outcome of diffusional flow during GBS is grain switching. The grains change their neighbors during the process of sliding and such a change is assisted by dif — fusional flow. The threshold stress term present in Equation [3.23] appears

due to an increase in grain boundary area, a resultant of the grain switching process. Though this model was successful in explaining the experimentally observed switching of grains during deformation, it failed to predict the stress dependence of strain rate. Moreover, Spingarn and Nix47 suggested that grain switching cannot be entirely attributed to diffusional flow as the diffusion paths are physically incorrect.