Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 27

grain interfaces slide, generating compressive (C) and tensile (T) tractions
at the grain boundary. In order to relieve these tractions, atoms fl ow from
regions under compression to regions experiencing tensile stresses. This
atomic fl ow is accompanied by grain boundary sliding that causes the shear
of a continuous slip-band. A mathematical analysis using Fick's law for dif-
fusion of vacancies yields the following expression for the strain rate due to
the absorption of dislocations into the boundary: 33

l 50

4

Ω

σ

ε D δ

[3.22 ]

=

SB b

,

λ

kT

where λ is the slip-band width, l is the slip-band length, Ω is the atomic

volume, is the grain boundary thickness and the rest of the terms are as

defi ned before. The slip-band length, l , can be considered equal to the mean
linear intercept grain size, d . Recently Gollapudi et al . 17 have studied the
feasibility of the slip-band model as a viable creep mechanism in a titanium
based alloy.

δ

Microstructural features

The fi rst microstructural evidence for diffusional creep was provided
by Squires et al . 34 who carried out creep studies on Mg-0.5Zr at 723 and
773K. The initial microstructure had a uniform distribution of inert ZrH 2
particles and investigation of the microstructure of the crept specimen, as
shown in Fig. 3.6a, depicts the presence of regions denuded of the inert par-
ticles. These denuded zones mostly formed near transverse grain boundar-
ies. Squires et al . 34 attributed the formation of denuded zones to diffusional
creep of the Mg alloy. Under the application of a stress, Mg atoms diffuse
from parallel grain boundaries to the transverse grain boundaries causing
a slight elongation of the grains. The inert particles do not travel along with
the Mg particles and their absence adjacent to the transverse grain bound-
aries causes the formation of denuded zones. In subsequent years, denuded
zones have been observed by other groups in Mg-Zr 35 and Mg-Mn 36 alloys.
Even though the formation of denuded zones as a consequence of diffu-
sional creep appears reasonable, it has been a matter of regular debate. 37 - 40
Jaeger and Gleiter 41 carried out experiments on a bamboo structured cop-
per coated with Al 2 O 3 fi lm. Diffusional creep experiments were carried
out on copper at a temperature of around 1348 K. At the conclusion of
the creep experiment, it was observed that the alumina fi lm fractured in a
few places. The fracturing of the alumina fi lm was ascribed to the deforma-
tion incompatibility of the alumina fi lm and the copper beneath. The copper
grains elongated under the application of the stress whereas the alumina
fi lm did not deform to the same extent causing fracturing of the fi lm. The

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