Environmental Engineering Reference
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4.67 Schematics showing the irradiation growth process in a simplifi ed
manner (Holt, private communication).
Basic mechanism
The most simple view of the growth mechanism is that it is due to interstitial
<a> loops (with displacement vectors or Burgers vectors pointing in the
a-direction) lying on prism planes, and vacancy <c> loops (with displace-
ment vectors or Burgers vectors pointing in the c-direction) lying on basal
planes (Buckley 1961). Figure 4.67 gives a schematic illustration.
The simple vision is that planar arrays of vacancies cause shrinkage in the
direction normal to the plane, and planar arrays of interstitials cause expan-
sion. However, all vacancies and interstitials do not end up as loops. As dis-
cussed in earlier sections on irradiation creep, they can also be deposited at
grain boundaries, solute atoms and network dislocations (that is, dislocations
introduced by deformation rather than irradiation) having <a> or <c+a>
Burgers vectors (i.e., line dislocations having a net strain in the <a> or <c+a>
lattice directions). In the fi nal analysis, irradiation growth is due to aniso-
tropic deposition of vacancies and interstitials at these sinks and is strongly
infl uenced by the anisotropic diffusion of self interstitial atoms (SIAs) in the
basal plane, or more exactly, in the directions normal to the c-axis.
The mechanisms for irradiation growth parallel those for irradiation
creep, with the notable exception of any applied stress.
￿ ￿ ￿ ￿ ￿ ￿
In general, growth is now believed to occur when there is an anisotropic distri-
bution of sinks receiving a net fl ux of vacancies and this anisotropy is different
from that of the distribution of sinks receiving a net fl ux of the self intersti-
tial atoms (SIAs). The dilations associated with the addition of lattice planes
accompanying the precipitation of SIAs then do not cancel the contractions
associated with the removal of lattice planes associated with the condensation
of vacancies. For a complete understanding of irradiation growth one must
identify the possible sinks and explain their evolution, identify the source
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