Abstract
We have used a range of advanced microscopy techniques to study the microstructure, the
nanoscale chemistry and the porosity in a range of zirconium alloys at different stages of oxidation.
Samples from both autoclave and in-reactor conditions were available to compare, including
ZIRLOTM, Zr-1.0Nb and Zr-2.5Nb samples with different heat-treatments. (Scanning) Transmission
Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD)1 and automated crystal
orientation mapping with TEM 2,3 were used to study the grain structure and phase distribution.
Significant differences in grain morphology were observed between samples oxidised in the
autoclave and in-reactor samples, with shorter, less well-aligned monoclinic grains and more
tetragonal grains seen in the neutron irradiated samples. A combination of Energy Dispersion X-ray
(EDX) mapping in STEM and Atom Probe Tomography (APT) analysis of SPPs can reveal the main and
the minor element distributions respectively. Neutron irradiation seems to have little effect on
promoting fast oxidation or dissolution of β-Nb precipitates, but encourages dissolution of Fe from
Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of
Nb in β-Nb SPPs in the oxide reveal the fully oxidised Nb5+ state in the SPPs deep into the oxide, but
Nb2+ in the crystalline SPPs near the metal-oxide interface. EELS analysis and automated crystal
orientation mapping with TEM have also revealed Widmanstatten-type suboxide layers in some
samples with the hexagonal ZrO structure predicted by ab initio modelling4. The combined thickness
of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the
estimated instantaneous oxidation rate, suggesting that the presence of this oxygen rich zone is part
of the protective oxide that is rate limiting in the key in the transport processes involved in
oxidation5. Porosity in the oxide has a major influence on the overall rate of oxidation, and there is
much more porosity in the rapidly oxidising annealed Zr-1.0Nb alloy than found in either the
recrystallised alloy or the similar alloy exposed to neutron irradiation.
nanoscale chemistry and the porosity in a range of zirconium alloys at different stages of oxidation.
Samples from both autoclave and in-reactor conditions were available to compare, including
ZIRLOTM, Zr-1.0Nb and Zr-2.5Nb samples with different heat-treatments. (Scanning) Transmission
Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD)1 and automated crystal
orientation mapping with TEM 2,3 were used to study the grain structure and phase distribution.
Significant differences in grain morphology were observed between samples oxidised in the
autoclave and in-reactor samples, with shorter, less well-aligned monoclinic grains and more
tetragonal grains seen in the neutron irradiated samples. A combination of Energy Dispersion X-ray
(EDX) mapping in STEM and Atom Probe Tomography (APT) analysis of SPPs can reveal the main and
the minor element distributions respectively. Neutron irradiation seems to have little effect on
promoting fast oxidation or dissolution of β-Nb precipitates, but encourages dissolution of Fe from
Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of
Nb in β-Nb SPPs in the oxide reveal the fully oxidised Nb5+ state in the SPPs deep into the oxide, but
Nb2+ in the crystalline SPPs near the metal-oxide interface. EELS analysis and automated crystal
orientation mapping with TEM have also revealed Widmanstatten-type suboxide layers in some
samples with the hexagonal ZrO structure predicted by ab initio modelling4. The combined thickness
of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the
estimated instantaneous oxidation rate, suggesting that the presence of this oxygen rich zone is part
of the protective oxide that is rate limiting in the key in the transport processes involved in
oxidation5. Porosity in the oxide has a major influence on the overall rate of oxidation, and there is
much more porosity in the rapidly oxidising annealed Zr-1.0Nb alloy than found in either the
recrystallised alloy or the similar alloy exposed to neutron irradiation.
| Original language | English |
|---|---|
| Title of host publication | 18th International Symposium on Zirconium in the Nuclear Industry |
| Publisher | ASTM International |
| Publication status | Published - 17 Aug 2016 |