This work was performed as part of an EPSRC Leadership Fellowship [EP/I005420/1] for the study of irradiation damage in Zr alloys, and is supported heavily by industrial contributors and especially by Westinghouse, Studsvik and Rolls-Royce plc. for the investigation of mechanisms relating to irradiation-induced growth (IIG). This thesis is an analysis of the microchemical and microstructural evolution of Zircaloy-2 under both proton and neutron irradiation. Comparisons are made between the effects of the different irradiative species through the use of scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). The work takes advantage of advances in EDS capability with large solid angles of collection 0.7 srad coupled with an aberration-corrected FEI Titan ChemiSTEMTM with a high brightness X-FEG electron source.2 MeV proton irradiation experiments were performed to doses of 2.3, 4.7 and 7.0 displacements per atom (dpa) at a dose rate of ~6.7 x10-6 dpa s-1 and at 350 °C. Electropolished TEM foils from Zircaloy-2 cladding and channel components of a BWR were supplied by Westinghouse in the fluence range 8.7 to 14.7 x1025 n m-2 ~14.5 to 24.5 dpa. Comparisons have been made in relation to SPP chemical composition, grain boundary chemistry, dislocation density, correlations between dislocation evolution and microchemical segregations and the nature of irradiation-induced precipitates. Proton irradiation-induced dissolution was observed for both Zr(Fe,Cr)2 and Zr2(Fe,Ni) SPPs, the depletion of Fe was preferentially from the edge region in the former SPP and from throughout the whole SPP in the latter. While no proton-induced amorphisation was observed for the Zr(Fe,Cr)2, the compositional changes in all SPPs agreed well with the reports of other authors. All grain boundaries display Fe and Ni segregation prior to irradiation, which disperses into the matrix after both proton and neutron irradiation, while Sn segregates to the boundary. Sn and the light transition elements Fe, Cr and Ni have shown contrasting behaviour in the matrix also. After irradiation by both protons and neutrons, a-component dislocation loops (a-loops) align parallel to the basal plane and Fe, Cr and Ni segregate to the a-loop positions. Sn, conversely, segregates to between a-loop positions parallel to the basal trace. The threshold dose in c-component dislocation loop (c-loop) nucleation under proton irradiation (~4.5 dpa) is shown as similar to that due to neutron irradiation (~5 dpa). We observe that a-loop density decreases at the onset of c-loop nucleation and that the position of c-loops are in alignment with the a-loops but that they are anticorrelated in position along the basal trace. We therefore propose that chemical ordering promotes the alignment of a-loops, which then provides the conditions necessary for c-loop nucleation. Nanoprecipitation is evident in the matrix after both proton and neutron irradiation. After proton irradiation to ~2 dpa, parallel atom probe tomography and STEM-EDS investigations have shown the nano-rods to be of composition Zr4(Fe0.67Cr0.33), tending towards Zr3(Fe0.69Cr0.31) as the rod volume increases. The rods are higher in density than the a-loops by a factor of ~3 and so are likely to be a significant influence on mechanical properties and IIG phenomena.

<p>Zr-based materials have found their main application in the nuclear field having high corrosion resistance and low neutron absorption cross-section. The oxide layer that is formed on the surface of these alloys is meant to be the barrier between the metal and the corrosive environment. The deterioration of this protective layer limits the lifetime of these alloys. A better understanding of the transport phenomena, which take place in the oxide layer during oxidation, could be beneficial for the development of more resistant alloys.</p><p>In the present study, oxygen and hydrogen transport through the zirconia layer during oxidation of Zr-based materials at temperatures around 400C have been investigated using the isotope-monitoring techniques Gas Phase Analysis and Secondary Ion Mass Spectrometry. The processes, which take place at oxide/gas and oxide/metal interface, in the bulk oxide and metal, have to be considered in the investigation of the mechanism of hydration and oxidation. Inward transport of oxygen and hydrogen species can be influenced by modification of the surface properties. We found that CO molecules adsorbed on Zr surface can block the surface reaction centers for H₂ dissociation, and as a result, hydrogen uptake in Zr is reduced. On the other hand, coating the Zr surface with Pt, resulted in increased oxygen dissociation rate at the oxide/gas interface. This generated enhanced oxygen transport towards the oxide/metal interface and formation of thicker oxides. Our results show that at temperatures relevant for the nuclear industry, oxygen dissociation efficiency decreases in the order: Pt > Zr₂Fe > Zr₂Ni > ZrCr₂ ≥ Zircaloy-2.</p><p>Porosity development in the oxide scales generates easy diffusion pathways for molecules across the oxide layer during oxidation. A novel method for evaluation of the gas diffusion, gas concentration and effective pore size of oxide scales is presented in this study. Effective pore sizes in the nanometer range were found for pretransition oxides on Zircaloy-2.</p><p>A mechanism for densification of oxide scales by obtaining a better balance between inward oxygen and outward metal transport is suggested. Outward Zr transport can be influenced by the presence of hydrogen in the oxide/metal substrate. Inward oxygen transport can be promoted by oxygen dissociating elements such as Fe-containing second phase particles. The results suggest furthermore that a proper choice of the second-phase particle composition and size distribution can lead to the formation of dense oxides, which are characterized by low oxygen and hydrogen uptake rates during oxidation.</p>

Zr-based materials have found their main application in the nuclear field having high corrosion resistance and low neutron absorption cross-section. The oxide layer that is formed on the surface of these alloys is meant to be the barrier between the metal and the corrosive environment. The deterioration of this protective layer limits the lifetime of these alloys. A better understanding of the transport phenomena, which take place in the oxide layer during oxidation, could be beneficial for the development of more resistant alloys. In the present study, oxygen and hydrogen transport through the zirconia layer during oxidation of Zr-based materials at temperatures around 400C have been investigated using the isotope-monitoring techniques Gas Phase Analysis and Secondary Ion Mass Spectrometry. The processes, which take place at oxide/gas and oxide/metal interface, in the bulk oxide and metal, have to be considered in the investigation of the mechanism of hydration and oxidation. Inward transport of oxygen and hydrogen species can be influenced by modification of the surface properties. We found that CO molecules adsorbed on Zr surface can block the surface reaction centers for H2 dissociation, and as a result, hydrogen uptake in Zr is reduced. On the other hand, coating the Zr surface with Pt, resulted in increased oxygen dissociation rate at the oxide/gas interface. This generated enhanced oxygen transport towards the oxide/metal interface and formation of thicker oxides. Our results show that at temperatures relevant for the nuclear industry, oxygen dissociation efficiency decreases in the order: Pt &gt; Zr2Fe &gt; Zr2Ni &gt; ZrCr2 ≥ Zircaloy-2. Porosity development in the oxide scales generates easy diffusion pathways for molecules across the oxide layer during oxidation. A novel method for evaluation of the gas diffusion, gas concentration and effective pore size of oxide scales is presented in this study. Effective pore sizes in the nanometer range were found for pretransition oxides on Zircaloy-2. A mechanism for densification of oxide scales by obtaining a better balance between inward oxygen and outward metal transport is suggested. Outward Zr transport can be influenced by the presence of hydrogen in the oxide/metal substrate. Inward oxygen transport can be promoted by oxygen dissociating elements such as Fe-containing second phase particles. The results suggest furthermore that a proper choice of the second-phase particle composition and size distribution can lead to the formation of dense oxides, which are characterized by low oxygen and hydrogen uptake rates during oxidation.