Uncovering the Nanoscale Distribution of Solutes in Zirconium Alloys

Abstract

Zirconium alloys are used as structural components and as fuel cladding in nuclear reactors due to their excellent mechanical properties, low neutron absorption cross section, and high corrosion resistance. However, zirconium alloys corrode quickly, their properties can degrade over time via exposure to radiation, and reactions with the reactor coolant produce hydrogen gas, which embrittles the cladding and reduces component lifetime. There is value in better understanding the microstructural features that contribute to the properties of these alloys and may trap the hydrogen. This thesis describes how advanced microscopy techniques were used to study the microstructure of Zr alloys, with a view towards designing alloys which last longer and are less susceptible to hydrogen embrittlement. Samples were primarily studied using atom probe tomography, a microscopy technique with near-atomic spatial resolution and equal sensitivity to all elements, including hydrogen and deuterium. Scanning electron microscopy, transmission electron microscopy, transmission Kikuchi diffraction and electron backscatter diffraction were used to support the APT measurements. In Optimised ZIRLO, electron microscopy and APT allowed for microanalysis of grain boundaries and several types of precipitates, including one never before reported. Electrochemical deuterium charging was used to induce hydride growth in Optimised ZIRLO, and cryogenic sample transfer protocols allowed study of the hydrides and their internal interfaces using APT. In Zircaloy-4, the composition of the precipitates and their interfaces have been characterized by APT for the first time, and microstructural evolution and solute segregation at grain boundaries after high-pressure torsion in Zircaloy-4 has been quantified. All these findings inform future research and applications of Zr alloys. These results contribute valuable information to enhance corrosion and hydrogen embrittlement resistance in the design of Zr alloys.