A Simple Kinetic Model of Zircaloy Zr(Fe,Cr)₂Precipitate Amorphization During Neutron Irradiation

Abstract

At neutron flux levels typical for Zircaloy fiel cladding in commercial power reactors, there is insuffkient thermal energy below about 600°K to maintain long-range order in hexagonal close packed (hcp) Zr(Fe,Cr)2 precipitates, and these Laves-phase intermetallics gradually become amorphous. The transformation is homogeneous with no change in composition at low temperatures, but above 500°& an amorphous zone containing only 10 atO/O 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°K, 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 diffixsion 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 arnorphization 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.

Laves phases: a review of their functional and structural applications and an improved fundamental understanding of stability and properties

“…Owing to the alloyed Fe, Cr and Nb, these Zr alloys contain ZrFe 2 , Zr(Fe,Cr) 2 or Zr(Fe,Nb) 2 Laves phase precipitates, the presence and behavior of which strongly affect the materials properties. Therefore, detailed studies have been performed on structure and precipitation of the Laves phases [987][988][989][990][991][992][993] and on the irradiation behavior [994][995][996][997][998][999][1000][1001][1002]. It has long been known that irradiation by neutrons and bombardment by protons or various types of ions can result in amorphization of the Laves phase particles as was summarized in a review by Yan et al [1002] (see Fig.…”

Amorphization of Laves‐Phase Precipitates in Zircaloy‐4 by Neutron Irradiation

“…Using Equation 5, we estimated that @ =9.6x10-22 cm2 n-' with a rather subjective analysis of time-dependent data in Reference 8 [18]. Note that sequential application of Equation 8 requires the simultaneous use of Equation 5 to provide 'A' for the next interval.…”

“…The model postulates a homogeneous amorphous fraction ci at the core/zone interface whose rate of change is the difference between the temperature-inde~endent rate of amomhization by the neutron flux $&d the rate o{thermal recrystalliza~on [18]: - With assistance from the neutron flux, excited Fe atoms diffuse through the Fe-depleted amorphous zone from the core and accumulate in the matrix adjacent to the interface. Since Fe transport is proportionti to the amorphous-zone growth rate, and the concentration gradient across the zone is small relative to the steep gradients at each interface,…”

“…ht a previous paper [18], we inmtiuced a quantitative model for the amorphization of Zr(Fe,Cr)z precipitates in Zircaloy-2 and Zkcaloy-4 using data from Reference 8. In this paper, we validate the model for Zircaloy-4 over wider ranges of flux and temperature, extend its predictions to the total amorphization of spherical precipitates, and improve its ability to predict transient behavior.…”