From solid solution to cluster formation of Fe and Cr in $\alpha$-Zr

TL;DR

This study combines experimental and computational methods to investigate how Fe and Cr atoms cluster in Zr alloys, revealing their preferred sites, effects on lattice structure, and implications for corrosion and irradiation damage.

Contribution

It provides new insights into the non-equilibrium solubility and clustering behavior of Fe and Cr in Zr alloys, including the discovery of a previously unmodelled Fe site.

Findings

Cr occupies interstitial and substitutional sites in Zr.

Fe favors interstitial sites and a low-symmetry site.

Clusters with vacancies increase solubility of Fe and Cr.

Abstract

To understand the mechanisms by which Fe and Cr additions increase the corrosion rate of irradiated Zr alloys, a combination of experimental (atom probe tomography, x-ray diffraction and thermoelectric power measurements) and modelling (density functional theory) techniques are employed to investigate the non-equilibrium solubility and clustering of Fe and Cr in binary Zr alloys. Cr occupies both interstitial and substitutional sites in the {\alpha}-Zr lattice, Fe favours interstitial sites, and a low-symmetry site that was not previously modelled is found to be the most favourable for Fe. Lattice expansion as a function of alloying concentration (in the dilute regime) is strongly anisotropic for Fe additions, expanding the $c$-axis while contracting the $a$-axis. Defect clusters are observed at higher solution concentrations, which induce a smaller amount of lattice strain compared to the dilute defects. In the presence of a Zr vacancy, all two-atom clusters are more soluble than individual point defects and as many as four Fe or three Cr atoms could be accommodated in a single Zr vacancy. The Zr vacancy is critical for the increased solubility of defect clusters, the implications for irradiation induced microstructure changes in Zr alloys are discussed.