Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 96

For a straightforward review of the relationship between irradiation-
induced microstructure and Zircaloy properties, see Adamson (2000). More
technical details and references can be found there.

4.3.2 Effects of irradiation on precipitates

Corrosion resistance in zirconium alloys is intimately related to the presence
of second phase particles (SPPs) formed in the zirconium matrix by delib-
erate additions of alloying elements. The precipitates are usually incoherent
crystalline intermetallic compounds, meaning that their physical structure
is unrelated to the Zr matrix in which they are imbedded. In as-fabricated
Zircaloy-4 the most common SPP is Zr(Fe,Cr) 2 , while in Zircaloy-2 they
are Zr(Fe,Cr) 2 and Zr 2 (Fe,Ni). For the ZrNb type alloys the most common
is

Nb (which is not an intermetallic) and for the ZrSnNbFe alloy types are
Zr(Nb,Fe) 2 and

β

Nb. Table 4.6 gives a more complete description, also indi-
cating some neutron irradiation effects.
At normal LW R temperatures (270-370°C, 543-643K) the SPPs change
under irradiation in a combination of two ways - amorphization and
dissolution.
Amorphization means that the original SPP crystalline structure is con-
verted to an amorphous structure. Amorphization is a complex process,
described in some detail by Griffi ths et al . ( 1987 ); Yang ( 1989 ); Motta ( 1997 );
Bajaj et al . ( 2002 ); and Taylor et al . ( 1999 ). It occurs when an intermetallic
compound accumulates enough irradiation-induced defects to cause it to
thermodynamically favour an amorphous rather than a crystalline structure.
The rate of amorphization depends on the relative rates of damage creation
and damage annealing in the SPP; therefore important parameters are neu-
tron fl ux, irradiation temperature and SPP chemistry. A critical tempera-
ture exists above which the annealing processes are fast enough to prevent
the damage accumulation of defects needed for transformation. For typical
reactor irradiations amorphization of both Zr(FeCr) 2 and Zr 2 (Fe,Ni) occurs
readily at temperatures near 100°C (373K) (although Fe is not related from
the SPPs into the Zr matrix, as discussed later). At typical (LWR) tem-
peratures (300°C, 573K) and neutron fl ux, Zr(Fe,Cr) 2 becomes amorphous
but Zr 2 (Fe,Ni) does not. Above about 330°C (603K) neither SPP becomes
amorphous.
The amorphization process begins at the outside surface of the SPP and
works its way inward with increasing fl uence. This is illustrated in Fig. 4.11
(Etoh & Shimada, 1993) where the SPP on the left has an amorphous rim
(dark area) and the one on the right, at higher fl uence, is fully amorphous.
There appears to be an incubation period prior to amorphization initiation,
with the incubation fl uence decreasing with temperature in the range 270-
330 ° C (543-603K).

β

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