Characterization of Zr-Nb-Fe(-Cr) precipitates in Zr-based alloys using density functional theory

Abstract

The presence of iron (and chromium) impurities in Zr-Nb based alloys leads to the formation of intermetallic Zr-Nb-Fe(-Cr) precipitates. These precipitates are known to affect the thermomechanical properties of Zr-Nb alloys. This study seeks to model these intermetallic precipitates from first principles using density functional theory (DFT) and ab initio Metropolis Monte Carlo (MC). A wide variety of Laves-type atomic structures were considered, composed of Zr, Nb, Fe and Cr in proportions close to experimental values. An exhaustive ensemble of elemental permutations was taken into account. The calculations show that hexagonal ternary alloys of equiatomic composition can be designated as a Zr(Nb, Fe)2 Laves phase. They also show that hexagonal precipitates found in Canadian Deuterium Uranium (CANDU) pressure tube material can be designated as Zr(Zr, Nb, Fe)2 and Zr(Zr, Nb, Fe, Cr)2 Laves phases. Ab initio MC simulations indicate that the lowest-energy Zr(Nb, Fe)2 phases considered in this work are stable under annealing at 1073 K. The DFT-optimized Zr(Zr, Nb, Fe, Cr)2 atomic positions are consistent with the plane d-spacings observed in TEM images.