Understanding irradiation-induced nanoprecipitation in zirconium alloys using parallel TEM and APT

A Harte,R Prasath Babu,C.A Hirst,T.L Martin,P.A.J Bagot,M.P Moody,P Frankel,J Romero,L Hallstadius,E.C Darby,M Preuss

doi:10.1016/j.jnucmat.2018.08.033

Abstract

We investigate nano-scale irradiation-induced precipitation in a ZrSnFeCrNi-alloy (Zircaloy-2) by combining atom probe tomography (APT) for chemical detail with scanning transmission electron microscopy (STEM) and high resolution energy dispersive X-ray (EDX) spectroscopy for wider context and complimentary and correlative TEM diffraction techniques for crystallographic relationships. We find that Fe and Cr-rich nano-rods precipitate in Zircaloy-2 following proton irradiation at 350 °C to a low dose of ∼2 dpa. The long-axis of the nano-rods are aligned in a direction 12–15° from the Zr matrix 〈0001〉, align in the basal plane and are of width 1.5–5 nm. Smaller rods are of APT-determined composition Zr4(Fe0.67Cr0.33), tending towards Zr3(Fe0.69Cr0.31) as the rod volume increases to > ∼400 nm3, in agreement with STEM-EDX determination of composition resembling that of Zr3Fe with Cr replacing some of the Fe. The Fe/Cr ratio has been shown to increase with distance from the nearest partially-dissolved Zr(Fe,Cr)2 phase particle. The nucleation of nano rods has implications for macroscopic irradiation-induced deformation phenomena, irradiation-induced hardening and the evolution of dislocation loops and other defects.

Highlights

Zr alloys are used as the cladding and structural components of nuclear reactor cores due to their low average neutron absorption cross section and their retention of mechanical properties and corrosion resistance at operating temperatures [1]
Better corrosion resistance has been correlated to reduced irradiation-induced growth strain [11] and second phase particles (SPPs) and other chemical effects are known to play a role in the type, density and spatial relationship of dislocations that form as a result of irradiation [12,13,14,15]
Type II defects are distinct from the Type I in that they are visible when imaged parallel to the g = 0002 systematic row (Figure 2b), which the dislocation loops are not

Summary

Introduction

Zr alloys are used as the cladding and structural components of nuclear reactor cores due to their low average neutron absorption cross section and their retention of mechanical properties and corrosion resistance at operating temperatures [1]. Better corrosion resistance has been correlated to reduced irradiation-induced growth strain [11] and SPPs and other chemical effects are known to play a role in the type, density and spatial relationship of dislocations that form as a result of irradiation [12,13,14,15]. SPPs in the Zircaloys are known to undergo irradiation-induced dissolution processes at intermediate to high neutron irradiation temperatures (280-450 °C), depleting preferentially in Fe and in Cr and Ni in the Zr(Fe,Cr) and Zr2(Fe,Ni) SPPs, respectively [6,16,17,18,19,20,21,22,23,11]. It is important to study the results of this dissolution and what it means for the microstructure-related performance properties

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Conclusion