Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 23

(S-N) mechanism with a stress exponent value of 1 could also be a viable
creep mechanism. 17 This mechanism could be identifi able through micro-
structural evaluation following creep deformation revealing the limitation
of only considering the creep parameters. In this section, the mechanism of
Coble, N-H, H-D and S-N creep will be discussed.

N-H and Coble creep

The possibility of creep occurring by stress-assisted diffusional mass trans-
port through the lattice was fi rst considered by Nabarro 18 in 1948 and
Herring 19 in 1950. A few years later, Coble 20 proposed that grain boundaries
could also provide an alternative path for stress-directed diffusional mass
transport to take place. Figure 3.4 provides schematics of N-H and Coble
creep mechanisms.
As Fig. 3.4 indicates, under the application of a stress, grain boundar-
ies normal to the applied stress will develop a higher concentration of
vacancies. On the other hand, grain boundaries parallel to the applied
stress (lateral grain boundaries) will experience compressive stresses and
will have a reduction in vacancy concentration. This causes a concentra-
tion difference between the two boundaries leading to a fl ux of vacan-
cies diffusing from the normal grain boundaries to the parallel or lateral
grain boundaries (atoms diffuse in the opposite direction). The diffusion
of vacancies can occur through the lattice (N-H) or via grain boundaries
(Coble creep). The diffusion of vacancies or the motion of atoms from one
grain boundary to another leads to a crystal strain which in turn contrib-
utes to the deformation of the grains and consequently the material. The
calculations of the steady-state fl ux of vacancies and the corresponding
steady-state creep rate lead to the following relationships for N-H and
Coble creep, respectively,

￿ ￿ ￿ ￿ ￿ ￿

σ

σ

σ

(a) σ (b)

3.4 (a) Schematic of N-H creep. Mass transport occurs through the
lattice. (b) Schematic of Coble creep. Mass transport occurs along grain
boundaries.