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Secondary Ion Mass Spectrometry is an important technique for the study of the composition of a wide range of materials because of the exceptionally high sensitivity that allows the study of trace elements and the ability to distinguish isotopes that can be used as markers for reactions and transport processes. However, when studying nuclear materials, it is often necessary to analyse highly radioactive samples, and only rather few SIMS facilities are available in active environments. In this paper, we present a methodology using focussed ion beam milling to prepare samples from radioactive specimens that are sufficiently large to undertake SIMS mapping experiments over microstructurally significant regions, but with overall activities small enough to be readily transported and analysed by a SIMS instrument in a normal laboratory environment. Radioactive samples prepared using this methodology can also be used for correlative SIMS analysis with other analytical microscopies. SIMS results showing the distributions of deuterium in oxides on in‐reactor corroded zirconium alloys are presented to demonstrate the potential of this sample preparation technique. [ABSTRACT FROM AUTHOR]

Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal-oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys. [ABSTRACT FROM AUTHOR]

Multi-scale characterisation using TEM, EELS and 3D FIB reconstruction was carried out on both pre-transition and post-transition oxides grown in high-temperature water. 3-D characterisation of cracks using high resolution FIB sequential sectioning gave a quantitative measure of crack density over time. There is no sudden burst of cracking occurring at the kinetic transition. On the nanometre scale, Fresnel contrast was used to reveal porosity of 1 - 3 nm in size in the oxide, which develops during growth from isolated pores to a connected network. Quantification of oxygen concentration using STEM-EELS has identified a suboxide, ZrO, layer tens of nanometres thick at the metal - oxide interface. [ABSTRACT FROM AUTHOR]

Abstract: Zirconium alloys have been long used for fuel cladding and other structural components in water-cooled nuclear reactors, but waterside corrosion is a primary limitation on both high fuel burn-up and extended fuel cycle operation. Understanding the processes that occur at the metal/oxide interface is crucial for a full mechanistic description of the oxidation process. In this paper we show that reliable quantification of the oxygen content at the metal/oxide interface can be obtained by Electron Energy Loss Spectrometry (EELS) if enough care is taken over both the preparation of Transmission Electron Microscopy (TEM) samples and the methodology for quantification of the EELS data. We have reviewed the accuracy of theoretically calculated inelastic partial scattering cross-sections and effective inelastic mean-free-paths for oxygen and zirconium in oxidized Zr-alloy samples. After careful recalibration against a ZrO2 powder standard, systematic differences in the local oxygen profile across the interface in different zirconium alloys were found. The presence of a sub-stoichiometric oxide layer (a suboxide) was detected under conditions of slow oxide growth but not where growth was more rapid. This difference could arise from the different corrosion resistances of the alloys or, more likely, as a result of the transition in oxidation behaviour, which refers to a sharp increase in the oxidation rate when the oxide is a few microns thick. [Copyright &y& Elsevier]

• Valence mapping of oxidized second phase particle oxidation in zirconium alloys has been successfully achieved by EELS. • Valence change has revealed that Cr and Zr oxidize preferentially, while Fe remains metallic for longer. • Cr oxidation is complete prior to transition. • Fe only oxidises when in full contact with the environment (via water in cracks or nanopores) after transition. The evolution of Cr and Fe within the oxide formed on Zircaloy-4 in high-temperature pure water and a high-pH aqueous environment has been followed in detail by electron energy loss spectroscopy (EELS). The oxidation states of Cr, Fe and Zr have been determined as a function of exposure time and location in the oxide film. Much Cr and all the Fe diffuse out of the original Laves phase and precipitate within cracks or on the outer oxide surface. Cr oxidation is complete prior to transition; Fe oxidation occurs mainly after transition. The different hypotheses concerning the mechanisms by which the alloying elements influence oxidation and hydrogen pickup are examined in the light of this information. The mechanisms which agree with these observations include: doping of the bulk ZrO 2 by low levels of alliovalent cations; doping ZrO 2 grain boundaries with alliovalent cations; injection of holes into near-surface oxide by islands of Fe 2 O 3 ; reaction of water with islands of metallic Fe (and Ni) on surfaces of internal cracks around transition; control of transition by mechanical properties [ABSTRACT FROM AUTHOR]

Oxidation of metals by water generates hydrogen which can enter the solid causing serious degradation of its mechanical properties and may also influence the corrosion rate. The present work focuses on hydrogen pickup during the corrosion of zirconium alloys in an aqueous environment. Transmission electron microscopy using Fresnel imaging on three different samples of oxidized Zr has been used to study the type, distribution, concentration and connectivity of nano-porosity as a function of depth through the oxide layer. Extensive interconnected nano-pipes are found in the non-protective outer part of the oxide, while in the protective barrier layer closer to the metal-oxide interface, continuous nano-pipes turn into individual nano-pores. Ab initio calculations show that molecular hydrogen is formed spontaneously by the reaction of water with oxygen vacancies in zirconium oxide. Molecular dynamics simulations reveal that these H 2 molecules can diffuse rapidly through nano-pores and nano-pipes as small as 0.5 nm in the oxide layer. Calculations demonstrate that molecular hydrogen dissociates spontaneously on surfaces of suboxides found experimentally at the metal-oxide interface. Oxygen vacancies in ZrO enable the ingress and diffusion of H atoms with an energy barrier of approximately 65 kJ/mol. Further diffusion of hydrogen through oxygen-saturated α -Zr metal is fast, leading to the formation of thermodynamically stable zirconium hydrides. Thus, formation and diffusion of molecular hydrogen through nano-pores in the bulk oxide and ingress of H atoms via suboxides is a possible mechanism of hydrogen pickup in any metal or alloy covered by an oxide scale that contains nano-porosity. Image 1 [ABSTRACT FROM AUTHOR]

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010. [ABSTRACT FROM PUBLISHER]

Hydrogen diffusion through the oxide grown on Zr alloys by aqueous corrosion processes plays a critical role in determining the rate of hydrogen pickup (HPU) which can result in embrittlement of fuel cladding and limit the burnup of the nuclear fuel it encapsulates. Mapping the hydrogen/deuterium distributions in these oxide layers, especially in the barrier layer close to the metal/oxide interface, is a powerful way to understand the mechanism of both oxidation and hydrogen pickup. Here we have characterised by high-resolution SIMS analysis the deuterium distribution in oxide layers on a series of Zr alloys, including autoclave-oxidised Zircaloy-4, Zr-1Nb and Zr-2.5Nb alloys, and in-flux and out-of-flux corroded Zr-2.5Nb samples. Pre-transition Zircaloy-4 samples show a high deuterium trapping ratio in the oxide and a higher diffusion coefficient than in oxides on the Nb-containing samples. Neutron irradiation increases the deuterium diffusion coefficient, the deuterium concentration in the oxide and the pickup fraction in Zr-2.5 Nb samples. Comparative NanoSIMS and EDX/SEM analysis demonstrates that the deuterium is not preferentially trapped at second phase particles in the oxides on any of the alloys studied, but there is direct evidence for trapping at the surfaces of small oxide cracks especially in Zircaloy-4 samples. The high resolution mapping of these hot-spots in 3D can provide unique information on the mechanisms of hydrogen uptake, and suggests that the development of interconnected porosity in the oxide may be the critical rate-determining mechanism that controls HPU in the aqueous corrosion of zirconium alloys in water-cooled reactors. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The oxidation behavior of zirconium alloy in high-temperature steam plays a dominant role in the operation of nuclear reactors under accident conditions. The present work investigated the effect of Sn on the oxidation behavior of zirconium alloys in high-temperature steam at 1000 °C. The results of scanning electron microscopy (SEM), metallographic microscopy and synchrotron X-ray diffraction (S-XRD) analyses show that there are three layers in the oxidized sample, including prior β-Zr, α-Zr(O) and ZrO 2. High resolution analyses conducted in these layers by transmission electron microscopy (TEM) show that Sn atoms segregate around the oxide-metal (O-M interface and Zr-Sn intermetallic precipitates only present in the oxide film. Clear atomic arrangements of Zr 5 Sn 3 were obtained by TEM, resulting in accurate identification of the Zr-Sn intermetallic phase. Also, TEM energy dispersive spectroscopy (EDS) maps show that Zr-Sn particles invariably accompany the segregation or precipitation of Nb. However, no clear precipitation sequence was observed, which suggests that a co-precipitation mechanism of Sn and Nb is in operation. As oxidation proceeds, nanovoids next to the Zr-Sn intermetallic particles were observed accompanied by local areas deficient in oxygen, identified as ZrO. Combining the interdiffusion trend obtained by Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations, a mechanism for the transformation from the Zr-Sn intermetallic precipitates to nanovoids was proposed. This proposed mechanism may shed new light into the role of Sn in the oxidation resistance of zirconium alloy under LOCA conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

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) α − Z r ∥ (1 ¯ 011) h − Z r O and [ 2 ¯ 110 ] α − Z r ∥ [ 10 1 ¯ 2 ] h − Z r O or (0002) α − Z r ∥ (22 4 ¯ 1 ¯) h − Z r O and [ 2 ¯ 110 ] α − Z r ∥ [ 1 1 ¯ 01 ] h − Z r O , with the first one more commonly observed. No specific orientation relationships between either hexagonal-ZrO and monoclinic-ZrO 2 or α-Zr and monoclinic-ZrO 2 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. Image 1 [ABSTRACT FROM AUTHOR]

The development of oxide texture and microstructure formed on two zirconium alloys with differing Sn contents (Zr–1Nb–1Sn–0.1Fe, i.e. ZIRLO™ and Zr–1.0Nb–0.1Fe) has been investigated using transmission Kikuchi diffraction (TKD) in the scanning electron microscope (SEM) and automated crystal orientation mapping in the transmission electron microscope (TEM). Bulk texture measurements were also performed using electron backscatter diffraction (EBSD) in order to quantify and compare the oxide macrotexture development. The Sn-free alloy showed significantly improved corrosion performance by delay of the transition region and reduced levels of hydrogen pickup. The macroscopic texture and grain misorientation analysis of the oxide films showed that the improved corrosion performance and reduced hydrogen pick up can be correlated with increased oxide texture strength, the improved oxide grain alignment resulting in longer, more protective columnar grain growth. A lower tetragonal phase fraction is also observed in the Sn-free alloy. This results in less transformation to the stable monoclinic phase during oxide growth, which leads to reduced cracking and interconnected porosity and also to the formation of larger, well-aligned monoclinic grains. It is concluded that the Zr–1.0Nb–0.1Fe alloy is more resistant to hydrogen pickup due the formation of a denser oxide with a larger columnar grain structure. [ABSTRACT FROM AUTHOR]

Here we report a methodology combining TEM, STEM, Transmission-EBSD and EELS to analyse the structural and chemical properties of the metal–oxide interface of corroded Zr alloys in unprecedented detail. TEM, STEM and diffraction results revealed the complexity of the distribution of suboxide grains at the metal–oxide interface. EELS provided accurate quantitative analysis of the oxygen concentration across the interface, identifying the existence of local regions of stoichiometric ZrO and Zr 3 O 2 with varying thickness. Transmission-EBSD confirmed that the suboxide grains can be indexed with the hexagonal ZrO structure predicted with ab initio by Nicholls et al. (2014) . The t-EBSD analysis has also allowed for the mapping of a relatively large region of the metal–oxide interface, revealing the location and size distribution of the suboxide grains. [ABSTRACT FROM AUTHOR]

Fretting corrosion of as received and shot peened zirconium alloys exposed to high temperature high pressure water is tested. High resolution characterization techniques are applied to study the microstructure and composition of the worn surface. The results show that shot peening increases the wear resistance of the material, changing the wear mode from abrasive wear plus delamination to abrasive wear. Mechanical wear promotes corrosion of zirconium alloy by forming a thick dual-layer structure oxide scale in the debris buildup area. While the effects of corrosion on the fretting wear depend on the integrity of the oxide scale. • Fretting corrosion of as received and shot peened zirconium alloy is investigated. • Shot peening increases the corrosion and fretting corrosion resistance of zirconium alloy. • Fretting wear accelerates material corrosion by oxide film peeling off and cracking. • The integrity of the oxide scale determines the effects of corrosion on the fretting wear. [ABSTRACT FROM AUTHOR]

• Ground surface has higher oxidation resistance than mechanically polished surface. • A fine-grained layer favors formation of dense oxide film with high Cr/Fe ratio. • Highly network Cr-bearing δ-ferrite improves the oxidation resistance. • Thermally activation enhanced self-diffusion of Cr in ferrite inhibits oxidation. • The ferrite-affected oxidation zone expands with the increase in temperature. The properties of the oxide films grown on 308 L SS cladding with various surface treatments and temperatures in simulated PWR primary water are studied. The thickness of the inner oxide layer increases with the decrease of the surface roughness. The high oxidation resistance of the ground surface is due to the thicker fine-grained layer with high dislocations density and subgrain boundaries in the near-surface. The ferrite-affected oxidation zone (FAOZ) expands with the increase in temperature, resulting from the thermally activated self-diffusion of Cr in ferrite. The effects of surface states, ferrite and temperature on oxidation performance of 308 L SS cladding are discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Zr alloys were oxidised for various times in an autoclave to simulate PWR conditions. [•] Isotopic tracers 18O and 2H were added to reveal active oxidation sites by NanoSIMS analysis. [•] Hydrides were present in all samples, even those with short oxidation times. [•] Porosity mediated transitions between corrosion regimes occur at critical oxide thicknesses. [Copyright &y& Elsevier]

• Oxidation and SCC of 308L SS cladding with or without hydrogen are studied. • Phase boundary (PB) has lower oxidation resistance than austenite and δ-ferrite. • Hydrogen-induced PB oxidation and preferential grain boundary oxidation. • Charged hydrogen decreases the oxidation resistance of δ-ferrite and austenite. • Charged hydrogen increases the SCC susceptibility of 308L SS cladding. Oxide film properties and stress corrosion cracking (SCC) behaviour are investigated in the 308L stainless steel cladding layer with and without being H-charged in a deaerated high temperature water. Phase boundary (PB) has lower oxidation resistance than austenite and δ-ferrite matrix. A compact and high Cr content-bearing inner oxide layer improves the oxidation resistance of the δ-ferrite phase without H charging. H-induced PB penetration oxidation and grain boundary (GB) preferential oxidation are observed in H-charged specimen due to the PB and GB act as H trapping sites. Charged H decreases the oxidation resistance of δ-ferrite and austenite. δ-Ferrite acts as a path for H diffusing and H flux facilitates mass transport, resulting in the poor oxidation resistance of δ-ferrite. The mechanism underlying the effect of H on the oxidation behaviours of δ-ferrite and austenitic phases, as well as SCC performance, are discussed. [ABSTRACT FROM AUTHOR]

• Fresnel imaging in the TEM has been used to identify different types of porosity in the protective oxide near the "barrier layer", around 0˜200 nm from the metal-oxide interface. • More disorganised oxide grains and nano-porosity network is found by TEM and TKD in rapidly oxidising samples. • NanoSIMS analysis suggests that the transition event enables the propagation or trapping of deuterium while the pre-transition sample is a good barrier to deuterium penetration. • APT data provides strong direct evidence for D segregation in the "barrier layer" at oxide grain boundaries and nano porosity. • The nanopores, if interconnected, can be the dominant pathway for hydrogen transport to the metal matrix. We have used (S)TEM, TKD, NanoSIMS and APT to study nano-porosity in the oxide grown on deuterated Zr-1.0Nb and Zr-2.5Nb alloys. A detailed analysis of "barrier layer" nano-porosity by TEM and TKD has revealed that the oxide grain structure is much more disorganised and the nano-porosity network better developed in the rapidly oxidising post-transition alloy. Direct observations of the trapped deuterium (D) distributions from NanoSIMS analysis also shows much more penetration of the oxide layer post-transition. APT analysis shows that there is Fe and D segregation to some of the oxide grain boundaries with occasional evidence of porosity containing trapped D and H. We conclude that interconnected porosity would offer a dominant pathway for the transport of hydrogenic species to the metal substrate during the aqueous corrosion of zirconium alloys in service. [ABSTRACT FROM AUTHOR]