Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 109

4.4.2 Deformation
In order for signifi cant strain to occur, dislocations must overcome the
obstacles to their motion - the irradiation-induced <a> loops. At low
stresses this may happen by the process of dislocation climb, which means
irradiation-induced point defects (PDs) diffuse to the dislocation and allow
it to move around the obstacle. This is an important creep process, to be
covered further in Section 4.6. At high stress or high strain rates, as in a
power excursion, or at all practical strain rates out-of-reactor, the disloca-
tions can actually interact with the <a> loop defects and remove them from
the microstructure. In effect this creates a localized soft area, where addi-
tional deformation tends to concentrate: this process is called dislocation
channelling. The physical process is illustrated in Fig. 4.21. The long straight
￿ ￿ ￿ ￿ ￿ ￿
4.21 Dislocation channels in zirconium alloys: (a) zirconium,
showing a channel with no radiation damage; (b) Zircaloy-4, showing
channels along traces of prism planes (101 ̅ 0). (Source: Reprinted,
with permission, from Adamson et al . (1986) and Cheadle et al .
(1974), copyright ASTM International, 100 Barr Harbor Drive, West
Conshohocken, PA 19428.)