The earliest model to explain GBS accommodated by dislocation movement was proposed by Ball and Hutchison.4 8 Later modifications to this model were brought about by Langdon,49 Mukherjee50 and Arieli and Mukherjee.51 The Ball-Hutchison model is well illustrated by Fig. 3.8.52 As shown in the figure, when the grains tend to slide under the application of a shear stress, strain incompatibilities and stress concentrations are developed at triple points53 and grain boundary ledges.54 Dislocation emission from these ledges and triple points is a natural consequence of the stress concentration. The emitted dislocations traverse the grain diameter until they encounter the opposite grain boundary at which point the dislocations start piling up and generate a back stress that prevents the further emission of dislocations. To enable further deformation, the lead dislocation at the pile-up climbs into

3.8 I llustration of the Ball-Hutchison model of GBS accommodated by dislocation movement.58

or along the grain boundary resulting in the rate controlling step being the climb of dislocations at the grain boundary.

Gifkins56 presented a similar but slightly different model to explain the mechanism of GBS known as the ‘core and mantle’ model and considered the grain as the core and the regions adjacent to the grain boundary as the mantle. All deformation was assumed to occur only in the mantle region of the grain. This model and the rest of the models predicted strain rates which had an n = 2 dependence of the applied stress. According to this model

wherep = 2or3 and D = DLorDB — depending on whether the motion of the dislocations is along the lattice or along the grain boundary respectively. Superplasticity — the ability of a material to exhibit high tensile elongations before failing — is primarily attributed to GBS. The mechanism of defor­mation in superplastic materials is supposed to be in accordance with the mechanisms discussed in this section.