Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 31

or along the grain boundary resulting in the rate controlling step being the
climb of dislocations at the grain boundary.
Gifkins 56 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

2

p

⎛
⎜

⎞
⎟

⎛
⎜

⎞
⎟

b
dE D

σ

=

ε K

⎛
⎝

⎛
⎝

[3.25 ] s

,

where p = 2 or 3 and D = D L or D B - 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 defor-
mation in superplastic materials is supposed to be in accordance with the
mechanisms discussed in this section.

Microstructural features

Fiducial markers are generally employed to study the contribution of grain
boundary sliding to the total creep strain. 53 GBS leads to shearing of the
fi ducial markers and the shear offset provides a measure of the strain contri-
bution. Since the stress concentrations developed during sliding are relieved
by dislocation emission, dislocation activity can be expected in the vicinity of
the grain boundary. Recent work by Gollapudi et al . 57 shows increased dis-
location activity close to the grain boundary during deformation controlled
by GBS. At the same time, dislocations emitted from a grain boundary are
expected to travel across the grain until they encounter a grain boundary.
These dislocations subsequently pile up which is relieved by dislocation
climb. Dislocation pile-up close to the grain boundary was also observed
by Gollapudi et al . 57 Figure 3.9a and 3.9b provide microstructural features
associated with creep in the GBS regime.

￿ ￿ ￿ ￿ ￿ ￿

3.3.3 The n = 3 regime: viscous glide (class-A alloys)

The n = 3 regime, though in principle corresponding to the power-law con-
trolled ( n = 4-7) creep mechanism, differs from it at a mechanistic level.
The power law controlled creep mechanism (as will be discussed in the fol-
lowing section) is mostly dislocation climb-controlled commonly noted in
pure metals and class-II or metal-class alloys. In contrast the n = 3 regime
is dislocation glide-controlled creep usually exhibited by alloys known as