a b s t r a c tTwo varieties of Zircaloy-2, with different second phase particle (SPP) size distributions and different corrosion resistance, were oxidized in a steam autoclave. Transmission electron microscopy was used for investigation of the fine-scale lateral cracks present in the oxide scales. Crack quantification was performed and the number of cracks was correlated with the number of SPPs. A mechanism for crack formation is presented, in which the driving force is the local tensile stresses in the oxide close to the oxide/ metal interface, and the initiation sites are un-oxidized SPPs located within this stress field.
h i g h l i g h t sWe have conducted an atom probe tomography study on a Zircaloy-2 fuel cladding material that has been subjected to 9 annual cycles of in-reactor exposure.The results show that large numbers of nanosized Fe and Cr rich particles have precipitated along parallel planes in the matrix. Fe and Sn are seen to segregate to ring-shaped features that are interpreted to be c-loops. A sub-oxide of approximate composition ZrO is found at the metal-oxide interface.
a b s t r a c tAn atom probe tomography study of the microstructure of a Zircaloy-2 material subjected to 9 annual cycles of BWR exposure has been conducted. Upon dissolution of secondary phase particles, Fe and Cr are seen to reprecipitate in large numbers of clusters and particles of 1-5 nm sizes throughout the Zr metal matrix. Fe and Sn were observed to segregate to ring-shaped features in the metal that are interpreted to be
While the evolution of irradiation-induced dislocation loops is well correlated with irradiation-induced growth phenomena, the effect of alloying elements on this evolution remains elusive, especially at low fluences. To develop a more mechanistic understanding of the role of Fe on loop formation, state-of-the-art techniques have been used to study a proton-irradiated Zr-0.1Fe alloy and proton-and neutronirradiated Zircaloy-2. The two alloys have been irradiated with 2 MeV protons up to 7 dpa at 350 °C and Zircaloy-2 up to 14.7 x10 25 n m-2 , ~24 dpa, in a BWR at ~300 °C. Baseline TEM characterisation showed that the Zr 3 Fe secondary phase particles in the binary system are larger and fewer in number than the Zr(Fe, Cr) 2 and Zr 2 (Fe, Ni) particles in Zircaloy-2. Analysis of the irradiated binary alloy revealed only limited dissolution of Ze 3 Fe suggesting little dispersion of Fe into the matrix while at the same time a higher a-loop density is observed in comparison to that in Zircaloy-2 at equivalent proton dose levels. It was also found that the redistribution of Fe during irradiation leads to the formation of Fe nanoclusters. A delay in the onset of c-loop nucleation in proton-irradiated Zircaloy-2 compared to the binary alloy was observed. The effect of Fe redistributed from secondary phase particles, due to dissolution, on the density and morphology of a-and c-loops is described. The implication this may have on irradiation-induced growth of Zr fuel cladding is also discussed.
Automated crystal orientation mapping in the transmission electron microscope has been used to simultaneously map the phase, orientation and grain morphology of oxides formed on Zircaloy-2 after 3 and 6 cycles in a BWR reactor in unprecedented detail. For comparison, a region of a pre-oxidised autoclave-formed oxide was also proton irradiated at the Dalton Cumbrian Facility. The proton irradiation was observed to cause additional stabilisation of the tetragonal phase, attributed to the stabilising effect of irradiation-induced defects in the oxide. In the reactor-formed oxides, no extra stabilisation of the tetragonal grains was observed under neutron irradiation, as indicated by the similar tetragonal phase fraction and transformation twin boundary distributions between the non-irradiated and reactor-formed oxides. It is suggested that the damage rate is too low in the newly formed oxide to cause significant stabilisation of the tetragonal phase. This technique also reveals the oxide formed under reactor conditions has a more heterogeneous microstructure and the growth of well-oriented columnar monoclinic grains is significantly reduced when compared to a non-irradiated oxide. High angle annular dark field scanning transmission electron microscopy (HAADF STEM) also revealed the development of extensive networks of intergranular porosity and eventually grain decohesion in the
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