The metallic elements Cr, Fe, Ni, Nb, and Mo have low solubilities in
zirconium, so that zirconium-based alloys made with these elements con
tain numerous small precipitates. Some of these undergo substantial ch
anges in composition, crystalline structure, and size during in-reacto
r exposure. Data on these phenomenon have been obtained using the scan
ning transmission electron microscope (STEM) to study precipitates bef
ore and after reactor exposure. Results on the following alloys are re
ported here: Zircaloy-2 (Zr-1.5Sn-0.15Fe-0.10Cr-0.05Ni), Zry-0.2Nb (Zi
rcaloy-2 with 0.2Nb), NoCr (Zr-1.5Sn-0.17Fe-0.17Ni), NSF-2 (Zr-1Nb-1Sn
-0.2Fe), and XLL (Zr-1.5Sn-0.3Nb-0.3Mo). The dissolution of Fe from va
rious precipitates is apparent from direct compositional measurements.
The dissolution of Cr can be inferred in Zry-0.2Nb from Cr: Nb gradie
nts in amorphous Cr-Fe-Zr-Nb precipitates and from the formation of in
tergranular, Cr,Fe-rich precipitates during postirradiation annealing.
Two types of phase change have been seen. First, crystalline phases c
an become amorphous, as with hexagonal Laves Zr(Fe,Cr)2 in Zircaloy-2
and cubic Fe(Zr,Nb)2 in NSF-2. In the second type, long-range order is
eliminated, as with Laves (Mo,Nb)2Zr precipitates in XLL which become
BCC. The precipitate size distributions can change in reactor. The gr
eatest changes were measured in small, Cr,Fe-rich precipitates.