Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 30

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 Nix 47 suggested
that grain switching cannot be entirely attributed to diffusional fl ow as the
diffusion paths are physically incorrect.

Accommodation through dislocation movement

The earliest model to explain GBS accommodated by dislocation movement
was proposed by Ball and Hutchison. 48 Later modifi cations to this model
were brought about by Langdon, 49 Mukherjee 50 and Arieli and Mukherjee. 51
The Ball-Hutchison model is well illustrated by Fig. 3.8. 52 As shown in the
fi gure, when the grains tend to slide under the application of a shear stress,
strain incompatibilities and stress concentrations are developed at triple
points 53 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 Illustration of the Ball-Hutchison model of GBS accommodated by
dislocation movement. 58