Investigating the Effect of Zirconium Oxide Microstructure on Corrosion Performance: A Comparison between Neutron, Proton, and Nonirradiated Oxides

Displacive transformations have been widely reported in metals, alloys and ceramics, but rarely reported to be important in the aqueous corrosion of alloys. We report here our analysis of the formation of the hexagonal-ZrO suboxide during the aqueous corrosion of α-Zr alloys and propose this to be a paraequilibrium displacive transformation with the rate controlled by oxygen diffusion. Two orientation relationships were identified between α-Zr and hexagonal-ZrO, ( 0002, with the first one more commonly observed. No specific orientation relationships between either hexagonal-ZrO and monoclinic-ZrO2 or α-Zr and monoclinic-ZrO2 were identified, which suggests that the formation of often-reported bulk oxide texture during aqueous corrosion is not related directly to the texture of the metallic substrate. These results provide a guideline for understanding the mechanisms of crystallographic evolution during oxide growth on commercial zirconium alloys, and also demonstrate the capability of transmission Kikuchi diffraction to investigate orientation relationships in nano-scale materials.

Section: Bulk Oxide Texture Analysissupporting

confidence: 90%

Section: Oxide Phase Distribution and Grain Morphologymentioning

confidence: 99%

Mechanism of the α-Zr to hexagonal-ZrO transformation and its impact on the corrosion performance of nuclear Zr alloys

Liu

Karamched

et al. 2019

Section: Damage To the Oxidesupporting

confidence: 55%

“…Similar observation is seen in proton irradiated Zircaloy-2 [65]. Garner et al [65] studied neutron irradiated Zircaloy-2, revealed no extra stabilisation of the tetragonal grains which was attributed to the faster oxide growth and low total irradiation damage. This is not contradictory to our findings since Zr-1.0Nb has slower oxide growth than the Zircaloy-2 samples, so that longer total damage time in our Zr-1.0Nb gives enough time to introduce more tetragonal phase.…”

Section: Damage To the Oxidesupporting

confidence: 55%

Effect of neutron and ion irradiation on the metal matrix and oxide corrosion layer on Zr-1.0Nb cladding alloys

Garner

Frankel

et al. 2019

Self Cite

A detailed study has been carried out on recrystallised Zr-1.0Nb alloys corroded and irradiated under different conditions, including ex-autoclave and ex-reactor samples. After 540 days in reactor and damage around 5 dpa, the neutron irradiated sample shows no serious evidence for radiation enhanced corrosion and is still in pre-transition stage. The good corrosion resistance of the neutron irradiated Zr-1.0Nb can be related to the higher volume fraction of tetragonal phase and fewer interconnected nano porosity/ cracks in the oxide. This indicates less tetragonal to monoclinic transition, leads to more protective oxide in the neutron irradiated sample, containing little evidence for short circuit paths for the penetration of oxygen or water towards the metaloxide interface. These observations on a sample with a slow overall oxidation rate are consistent with the hypothesis that interconnected porosity can lead to early transitions and rapid oxidation. Tetragonal oxide can be either stabilised by irradiation, or stabilised by local release of impurity species from SPPs such as dissolution of Fe from Zr-Nb-Fe precipitates or radiation introduced precipitates (RIPs) which is likely to be small β-Nb clusters. The oxide consists of well-aligned columnar-equiaxed microstructure in the autoclave sample while a more complex oxide grain structure was observed in the neutron-irradiated sample. As oxide continues to grow, there are more and loops, dissolved Fe and RIPs in the metal matrix, however, the corrosion rate is low enough for the tetragonal oxide to stabilise and suboxide + Zr(Osat) phases exist for protectiveness, so there is no enhanced corrosion after radiation. In situ ion irradiation in the TEM revealed no visible defect clusters or voids in the oxide, suggesting that radiation damage to the metal matrix rather than oxide may have a stronger effect on corrosion mechanisms after neutron irradiation, however, cascade damages are not visible in this case. Neutron irradiation also seems to have little effect on promoting fast oxidation or dissolution of β-Nb precipitates into the surrounding oxide or metal during irradiation. These results are discussed in the light of the current mechanisms for corrosion of nuclear fuel cladding alloys.

“…In this study, the zirconium oxides grown by autoclave corrosion were observed to have a high resistance to irradiation-induced amorphisation up to a fluence of 1.9 × 10 16 ions.cm -2 at 10 12 ions.cm -2 .s -1 , but a gradual evolution of microstructure in the bulk oxide can be seen in long term in-situ irradiation experiments as shown in Figure 3. Strong diffraction contrast can be seen in the pre-irradiation sample, Figure 3 (a), arising from the characteristic nanocrystalline grain structure in a corrosion scale grown under simulated reactor conditions, and the complex SAD patterns are also characteristic of the textured zirconium oxide layer [69,70]. The diffraction contrast in the bright-field micrographs gradually changed under irradiation and we observed fewer but brighter spots in the diffraction patterns.…”

Section: In-situ Observation Of Irradiation-induced Phase Transformationmentioning

confidence: 76%

In-situ TEM study of irradiation-induced damage mechanisms in monoclinic-ZrO2

Liu

Mir

et al. 2020

We have investigated the microstructural and crystallographic evolution of nanocrystalline zirconia under heavy ion irradiation using in-situ transmission electron microscopy (TEM) and have studied the atomic configurations of defect clusters using aberration-corrected scanning transmission electron microscopy (STEM). Under heavy ion irradiation the monoclinic-ZrO2 is observed to transform into cubic phase, stabilised by the strain induced by irradiationinduced defect clusters. We suggest that the monoclinic-to-cubic transformation is martensitic in nature with an orientation relationship identified to be (100)m∥(100)c and [001]m∥[001]c. By increasing the damage dose, both the formation of voids and irradiationinduced grain growth were observed. A model for the formation of voids is proposed, taking defect interactions into consideration. The study has also demonstrated that high resolution orientation mapping by transmission Kikuchi diffraction (TKD) combined with in-situ irradiation in a TEM is a powerful method to probe the mechanisms controlling irradiationinduced processes, including grain boundary migration, phase transformations and texture evolution.