A simple kinetic model of Zircaloy Zr(Fe,Cr)(2) precipitate amorphization during neutron irradiation

At neutron flux levels typical for Zircaloy fuel cladding in commercial power reactors, there is insufficient thermal energy below about 600 degreesK to maintain long-range order in hexagonal close packed (hcp) Zr(Fe,Cr), precipitates, and these Laves-phase intermetallics gradually become amorphous. The transformation is homogeneous with no change in composition at low temperatures, but above 500 degreesK, an amorphous zone containing only 10 at% Fe grows inward from the periphery as Fe moves outward to the adjacent alloy matrix. The shrinking central cores of Zr(Fe,Cr)(2) precipitates in Zircaloy-4 remain crystalline, while in Zircaloy-2 these precipitates quickly undergo partial transformation and the low-Fe amorphous front advances into a random mixture of amorphous and crystalline regions, each with the original composition. Above 600 degreesK, the Zr(Fe,Cr)(2) precipitates tend to retain both their hcp structure and original chemical composition. These observations suggest that a dynamic competition between kinetic excitation to an amorphous state and thermal recrystallization makes some fraction of the Fe atoms available for flux-assisted diffusion to the alloy matrix by displacing them from hcp lattice positions into metastable, probably interstitial, sites. With one set of kinetic constants, a simple analytic representation of these processes accurately predicts precipitate amorphization as a function of neutron flux, temperature, and time for either Zircaloy-2 or -4. By implication, over the composition range of interest, hcp Zr(Fe,Cr)(2) is most stable thermodynamically with about 33 at% Fe, typical of Zircaloy-2, but amorphous Zr(Fe,Cr)(2) has the smallest activation energy for recrystallization with the slightly higher Fe content typical of Zircaloy-4.