Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 59

(a) σ (b) σ

-

D gb
+
D gb

D g -

D + D +

D L -

3.25 (a) Transport paths in ceramics as suggested by Gordon where the
total fl ux from horizontal to vertical boundaries is in the appropriate
stoichiometric ratio. (b) Transport paths in ceramics as suggested by
Chokshi where total fl ux along each transport path is in the appropriate
stoichiometric ratio.

anion grain boundary diffusion ( C − ) to anion lattice diffusion ( N − ). Figure
3.26b, corresponding to transport paths proposed by Chokshi, indicates that
over the same temperature range, there will only be a single transition from
Coble creep controlled by cation grain boundary diffusion to N-H creep
controlled by anion lattice diffusion.

3.7. 3 Presence of a second phase and effect on creep
behavior

The constitutive relationships identifi ed in the previous sections are appli-
cable for a wide variety of metals and alloys. However the strain rates of
deformation might assume values different from model predictions even
while the parametric dependencies remain the same. This was discussed in
case study (3.7.2) and was attributed to the presence of impurities. On the
other hand, there are cases where the strain rates of deformation as well as
parametric dependencies can turn out to be different. Such instances are
encountered while dealing with two- or multi-phase alloys that exhibit pre-
cipitation or dispersion strengthening. Strain compatibility issues as well as
differences in deformation rates of individual phases contribute to discrep-
ancies in experimental observations and traditional creep model predic-
tions. To this end, analytical models have been proposed to understand the
creep behavior of multi-phase alloys. 103 - 105 Here we present a case where
the second phase is rigid and is added to enhance the overall strength of
the alloy. For particle strengthened alloys, stress exponents higher than
those predicted by established creep models 106 and/or anomalous varia-
tion of stress exponent with stress are observed. 107 This is rationalized by
the introduction of a friction or resisting stress also known as back stress

￿ ￿ ￿ ￿ ￿ ￿