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of the anisotropy and explain the partitioning of the PDs amongst the sinks.
(Holt, 1988 )
The key unique mechanistic feature of irradiation growth is the initiation
and growth of <c> component dislocations (either as irradiation-produced
loops or deformation-induced networks). Without them breakaway growth
does not occur. It also appears to be essential that diffusion of SIAs be
anisotropic and favour the directions in the basal plane. More details are
given in Holt
et al
. ( 1996 ), Woo ( 1988 ), Holt
et al
. ( 2000 ), Holt ( 2008 ) and
Christien and Barbu (2009).
Irradiation growth will occur to some extent in all zirconium alloys. To
minimize the effects of growth several actions are important:
minimize residual stresses,
plan for the effects of cold work,
minimize hydrogen absorption,
plan for the effects of temperature,
understand and control alloy chemistry,
control texture.
4.6.2 Irradiation creep
Creep is plastic deformation occurring as a constant volume process,
normally at low stresses below the yield stress. For materials in the irra-
diation fi eld of a nuclear reactor, the deformation occurs by the motion of
dislocations and irradiation-produced defects under the infl uence of stress.
Neutron irradiation produces large quantities of point defects - vacancies
and SIAs - which migrate to and collect at various sinks. Due to the anisot-
ropy of the zirconium crystal lattice, motion of both dislocations and SIAs is
anisotropic, preferring to occur parallel to the basal plane in the <a> direc-
tion of the lattice. Dislocations are sinks for both vacancies and SIAs, but
it is normally considered that an edge dislocation attracts SIAs more than
vacancies. Dislocations produced by deformation and by irradiation lie on
both basal and prism planes. Because of the diffusional anisotropy of SIAs,
they tend to be absorbed by dislocations lying on prism planes. The diffu-
sion of vacancies is isotropic, and they tend to be absorbed preferentially
by dislocations lying on basal planes. Similarly, SIAs tend to be absorbed at
grain boundaries oriented parallel to prism planes and vacancies on bound-
aries parallel to basal planes. Absorption of either vacancies or SIAs at dis-
locations of grain boundaries causes plastic strain: positive for SIAs and
negative for vacancies. If the absorptions occurred randomly and in a non-
biased fashion, the net strain would be zero; however in zirconium alloys the
built-in anisotropy results in separate positive and negative strains, and in
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