Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 112

homogeneously throughout the gauge section until strain concentrates in
the burst region. However, this strain (i.e. the number of channels) is small
compared to the burst strain.
A main reason that anisotropic deformation is decreased relative to unir-
radiated material (Mahmood et al ., 2000) is that the high stresses needed to
reach the yield point activate alternate slip systems in irradiated Zircaloy.
The primary slip plane in unirradiated Zircaloy is the prism plane, the
so-called <1120 ̅ >(1010 ̅ ) system. As the applied stress becomes high, both
the pyramidal and basal planes can become active. Observations of prism
plane dislocation channels have been well documented (Adamson et al .,
1986; Bell, 1974; Adamson & Bell, 1986; Bourdiliau et al ., 2010 ), but obser-
vation of pyramidal and basal channels have also been reported (Bell,
1974 ; Fregonese et al ., 2000 ; Regnard et al ., 2001 ; Onimus et al ., 2004 , 2005 ;
Bourdiliau et al ., 2010). In fact the CEA group show with considerable data
and justifi cation that, for 350°C (623K) testing temperature, basal slip pre-
dominates, but that may yet prove to be a function of irradiation and test-
ing temperature, testing mode and impurity level (Bourdiliau et al ., 2010 ).
Dislocation channelling phenomena themselves and details about which
channelling planes predominate are important when modelling crack prop-
agation and material response to actual in-reactor loading patterns. Onimus
et al . (2005) have made good progress in modelling the phenomena for the
CEA conditions. The data is summarized in a ZIRAT 15 Annual Report
(Adamson et al ., 2010 ).
Specimen design plays a dual role, infl uencing ductility through both
geometry and stress state. The type of plane stress specimens shown in
Fig. 4.22 result in a 'classical' deformation band formation. The disloca-
tion channels can freely extend from surface to surface. In the plane strain
specimens of Fig. 4.25, the channels run into specimen regions where the
stress is signifi cantly lower before a free surface is reached, therefore pre-
venting formation of a well-developed deformation band. The latter case,
constrained plane strain, more realistically represents deformation in most
reactor component situations.

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Hardness of zirconium and Zircaloy

Knoop microhardness in a hot cell was used to determine hardness of zir-
conium and Zircaloy as a function of fl uence, purity and irradiation temper-
ature (Tucker & Adamson, 1984). The ranking of hardness was the same in
both unirradiated and irradiated materials. Hardness saturated with fl uence,
as it also does for tensile properties. Figure 4.26 gives some of the data. The
general trends are the same for all zirconium alloys.
The effects of post-irradiation annealing on mechanical properties
and radiation damage are given in an earlier section. In general both