Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 26

Several models were proposed to understand the mechanism of H-D
creep. The models of high temperature H-D creep were discussed in detail
by Langdon and Yavari. 27 Barrett et al . 28 proposed a model based on the creep
strain resulting from dislocation glide with dislocation multiplication through
climb. Murty 29 suggested that H-D creep in Al-Mg solid solution arises from a
modifi ed viscous creep glide process (described later) with stress-independent
dislocation density. More recently Kumar et al . 30 summarized the experimental
results obtained on ceramic single crystals. Purity of crystals and a low initial
dislocation density were cited as necessary conditions to unequivocally estab-
lish the presence of H-D creep as a viable mechanism of deformation. A review
of the viscous creep with n = 1 was recently made by Lingamurty et al . 31

Spingarn-Nix slip-band model

The S-N model 32 is based on the fact that dislocation climb, when assisted
by grain boundaries, can occur at activation energies smaller than those of
lattice diffusion. Since grain boundary diffusion is much faster than lattice
diffusion, climb rates are increased in the proximity of a grain boundary.
The S-N model thus relates to the ideas of diffusional creep and dislocation
climb at grain boundaries. This model, also known as the slip-band model,
provides a physical mechanism to explain the observation of activation
energies equal to the grain boundary self diffusion and a stress exponent
equal to 1. According to this model, creep occurs by shearing along slip-
bands blocked by grain boundaries. The creep strain at the boundary is in
turn accommodated by diffusional fl ow. A schematic of the slip-band model
is shown in Fig. 3.5. Under the application of a shear stress, the slip-band/

τ

Grain boundary

￿ ￿ ￿ ￿ ￿ ￿

C

C

C

Slip-band

T
T
T

λ

Slip-band

C
C
C

τ

3.5 Schematic of the slip-band model.