Creep deformation of materials in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 28

(b)

(a)

(d)

(c)

1.75 μ m

3.6 (a) Denuded zones in Mg-0.5Zr alloy, 38 (b) grain boundary grooves
in copper, 42 (c) dislocation cross slip in Al 43 (d) slip-bands in a Ti alloy. 17

elongation of the copper grains was ascribed to diffusional creep deforma-
tion. Recently McNee et al . 42 carried out detailed experiments on OFHC
grade copper in the diffusional creep regime. A surface scratch technique
was employed to establish the operation of a diffusional creep mechanism.
In addition to measurements of the surface scratch displacements, grain
boundary grooves were identifi ed and subsequently quantifi ed through
an atomic force microscopy (AFM). Grooves, as shown in Fig. 3.6b, were
formed predominantly on boundaries transverse to the applied stress. Grain
boundary grooves were thus suggested as a microstructural feature char-
acteristic of diffusional creep. While denuded zones, elongated grains and
grain boundary grooves are essentially features developed due to N-H or
Coble creep, the features associated with H-D and S-N creep are different.
Dislocations cross slipping 43 as shown in Fig. 3.6c and slip-bands sheared by
grain boundaries, 17 Fig. 3.6d, are suggested to be evidence of H-D and S-N
creep, respectively.

￿ ￿ ￿ ￿ ￿ ￿

3.3.2 The n = 2 regime: grain boundary sliding

Grain boundary sliding as a mechanism of creep is usually observed at tem-
peratures higher than 0.4T M . GBS is typically a response of grain boundaries