Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 33

stress exponent changes from 5 (region I) to 3 (region II) and back to 5
(region III).
The creep behavior illustrated in Fig. 3.10 is a consequence of a compe-
tition between two rate controlling mechanisms: dislocation climb and dis-
location glide. Once the dislocations are generated from Frank-Read (FR)
sources on parallel glide planes, the leading edge dislocations fi rst glide
and then climb to annihilation. In pure metals dislocation glide is relatively
faster compared to the diffusion-controlled climb and thus climb becomes
the rate controlling process resulting in n = 5. In class-A alloys, the rate of
glide is controlled by the diffusion of the solute atoms, thereby leading to
a relatively slower rate of glide compared to that of climb whereby the vis-
cous glide of dislocations becomes the rate controlling process with n = 3;
this mechanism is known as Weertman microcreep. 54 Region II, the three
power-law creep regime, is also known as the viscous glide regime. Viscous
glide is described by

3

035

⎛
⎛

⎞

σ

=

ε D s

[3.26 ]

⎝

,

s s

E A

where A is an interaction parameter that depends upon the viscous process
controlling dislocation glide and D s is the solute diffusivity.
The viscous process can be of different types. According to Cottrell and
Jaswon, 58 the dragging force could be due to the segregation of solute atmo-
spheres to moving dislocations. The dislocation speed in this case is con-
trolled by the rate of migration of the solute atoms. Fisher 55 suggested that
the viscous process had its origin in the destruction of the short range order
in solid solution alloys. The disorder created by dislocation motion would
result in the formation of a new interface thereby the interfacial energy
becomes the rate controlling process. Suzuki 59 suggested that the drag-
ging force was an outcome of solute atoms segregating to stacking faults.
There are suggestions that the obstacle to dislocation motion could be the
stress-induced local ordering of solute atoms. The ordering of the region
surrounding a dislocation reduces the total energy of the crystal pinning the
dislocation.
The three power-law creep region has usually been observed to occur
in solid solutions with a large atom size mismatch. Alloys with higher con-
centrations of the solute atoms seem to prefer the three power-law creep
regime as a viable creep mechanism. In fact, for very high concentrations of
the solute atoms, regime II could be suppressed. In addition, class-A alloys
usually exhibit either no or little primary creep or a region characterized by
an increasing slope (increasing strain rate). This is in sharp contrast to pure
metals and class-M alloys that exhibit a distinct primary creep curve with a
decreasing strain rate; distinguishing features of class-A and class-M alloys

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