Dislocation loop formation during proton irradiation of zirconium

Abstract

Zirconium components in the nuclear reactor core undergo continuous neutron irradiation throughout its lifetime. Irradiation causes the displacement of atoms and the production of point defects, self-interstitial atoms and vacancies, which may further reconfigure into dislocation loops which are unique features of irradiation-damage. The evolution of dislocation loops in zirconium alloys are relevant to a macroscopic shape change known as irradiation-induced growth (IIG), characterized by a lengthening along prismatic crystallographic directions and a corresponding shrinkage along the basal pole direction. In neutron irradiated zirconium, prismatic a-loops of both vacancy and interstitial nature appear starting from a low radiation dose, and vacancy-type basal c-loops are later observed at higher doses correlating with an accelerated breakaway growth phenomenon. This work is part of a global collective effort to understand dislocation loop evolution and irradiation growth. We used proton-irradiation as a surrogate for neutron-irradiation to generate a variety of irradiation-damage microstructures, corresponding to a series of irradiation conditions, in both a commercial alloy, Zircaloy-4, and a nominally pure zirconium. We used state-of-the-art characterization techniques such as bright field scanning transmission electron microscopy (BF STEM) in tandem with X-ray diffraction (XRD) line profile analysis in an attempt to fully characterize dislocation loops in terms of their nature, size, distribution, and density. Pure zirconium specimens proton-irradiated to 4dpa at 350C had conveniently larger a-loops than usually found in their commercial alloy counterparts, allowing us to successfully use TEM techniques to characterize the nature of the loops. In this specimen, we observed the alignment of a-loops both parallel to the basal plane traces and parallel to the 10-10 directions. Loops in alignment with one another were of identical Burgers vectors, either all vacancy or all interstitial in nature. In an effort to validate and optimize the use of XRD line profile analysis for the study of irradiation microstructure, we measured the diffraction profile of Zircaloy-4 and pure Zr with a range of proton-irradiated damage microstructures using three slightly different experimental geometries, one of which was in the laboratory setting, and two were using synchrotron X-ray diffractometers. We found that diffraction line profile analysis produced dislocation density results that were reproducible within experimental error when comparing data from different diffraction geometries. Satellite peaks associated with irradiation-induced dislocation loops, however, varied in relative volume fraction from experiment to experiment, suggesting a significant grain-to-grain variation in the average nature of dislocation loops. Furthermore, using high energy synchrotron transmission X-ray diffraction, we performed a novel depth profiling experiment on proton-irradiated Zircaloy-4 and an alloy of Zr-0.1%Fe. We measured the ways in which dislocation loop density and strain broadening varied with respect to depth below the irradiation surface, and we found a saturation of irradiation-induced strain broadening occurring from a low dose 0.7dpa, above which a-loop nucleation was hindered and a-loops grew in size to maintain a steady-state strain. We discuss our results with respect to known phases of IIG to form an updated hypothesis of the mechanisms by which irradiation-damage evolves within zirconium alloys.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philipp Frankel (Supervisor) & Michael Preuss (Supervisor)