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Oxide dispersion-strengthened (ODS) steels are main candidates for structural applications in future fusion reactors. Understanding their irradiation-induced behaviour is a key in building optimised components with enhanced radiation resistance. In this work, the stability of an ODS Fe12Cr steel was investigated by transmission electron microscopy after single- (Fe) and simultaneous triple-ion-beam irradiation (Fe, He and H) at room temperature to doses of 4.4 and 10 dpa. The irradiations were accomplished at the JANNUS-Saclay facility. Results after single-ion-beam irradiation were also compared with those from a reference Fe12Cr steel produced following the same route. Analyses focused on determining the irradiation-induced loop size and density in the ODS and reference materials, investigating the grain boundary microchemistry and studying the evolution of the secondary phases present. These experiments show that the Y-rich nanoparticles present in the ODS steel are quite stable under these irradiation conditions although evolution of larger Cr-rich carbides could be taking place. Loop sizes are smaller for the ODS steel than for the reference material and appear to increase with dose. Cr segregates at some of the grain boundaries, though this segregation also occurs in the absence of irradiation. [ABSTRACT FROM AUTHOR]

Abstract: Atom-probe tomography was employed to characterise specimens containing the oxidised part of a grain boundary from a 304 stainless steel coupon specimen exposed to simulated PWR primary water. A TEM foil containing part of the same oxidised grain boundary was also extracted and characterised for comparison. Surface and grain boundary oxide compositions were identified and Ni enrichment was observed around the oxides. The data provides novel information on the role of the minor impurities and the formation of early-stage oxides. These studies were conducted as part of a broader investigation of the mechanisms underlying stress corrosion cracking in stainless steels. [Copyright &y& Elsevier]

Abstract: Understanding the behaviour of oxide-dispersion strengthened (ODS) ferritic martensitic steels under irradiation is of prime importance in the design of future fusion reactors. Although changes in grain boundary chemistry during irradiation can significantly affect fracture strength, little is known on the behaviour of grain boundaries in ODS steels. Here, the effect of heavy-ion implantation at 500°C on grain boundary chemistry in a model ODS Fe–12wt.% Cr alloy was investigated using atom-probe tomography (APT) and analytical scanning-transmission electron microscopy ((S)TEM) techniques. While chromium and carbon segregation at grain boundaries is found in annealed alloys before irradiation, the three-dimensional APT reconstructions and TEM observations after irradiation reveal a complex distribution of Cr segregation and depletion at grain boundaries of varying character. [Copyright &y& Elsevier]

The geometry and three-dimensional (3D) morphology of the ceria particles synthesized by spray pyrolysis (SP) from two different precursors—cerium acetate hydrate and cerium nitrate hydrate (CeA and CeN ceria particles)—were characterized by transmission electron microscopy and electron tomography. Results were compared with surface area measurements, confirming that the surface area of CeA ceria particles is twice as large as that of CeN ceria particles. This result was supported by 3D microstructural observations, which have revealed that CeA ceria particles contain open pores (connected to surfaces) and closed pores (embedded in particles), while CeN ceria particles only contained closed pores. This experimental result suggests that the type of porosity is controlled by the precursors and could be related to their melting temperature during the heating process in SP. [ABSTRACT FROM AUTHOR]

Fe-Nb-B alloys prepared by ball milling can undergo a complex microstructural evolution during milling. In order to overcome the limitations imposed by traditional X-ray bulk analysis, a comprehensive multi-technique approach was devised to systematically characterize samples with the required resolution. A combination of in situ FIB (focused ion beam) lift-out and high-resolution ATEM (analytical transmission electron microscopy) has allowed the characterization of the phase evolution during milling. In particular, boron inclusions, not detected by X-ray diffraction, have been found to remain undissolved in the Fe matrix. [ABSTRACT FROM AUTHOR]

Abstract: The preparation of samples containing stress corrosion crack tips for 3D atom-probe tomography and transmission electron microscopy is of ultimate importance for understanding the mechanisms controlling crack propagation. In this paper, it will be shown that a focused ion beam machine equipped with an in situ micromanipulator is an ideal tool to systematically prepare such demanding samples. The methodology is described and discussed in detail, and several results are presented to demonstrate the potential of the technique. [Copyright &y& Elsevier]

The crystallization process, both at the initial and subsequent stages, of amorphous Al88-RE4-Ni8 alloys (RE = Y, Sm and Ce) has been studied. Additionally, the consequences of adding 1 at.% Cu replacing Ni or Al were studied. The stability of the amorphous structure in melt spun ribbons was thermally studied by differential scanning calorimetry, with Ce alloys being the most stable. The effect of Cu to reduce the nanocrystal size during primary crystallization was analyzed by transmission electron microscopy. This latter technique and x-ray diffraction showed the formation of intermetallic phases at higher temperatures. A clear difference was observed for the Ce alloy, with a simpler sequence involving the presence of Al3Ni and Al11Ce3. However, for the Y and Sm alloys, a more complex evolution involving metastable ternary phases before Al19RE5Ni3 appears, takes place. The shape of the intermetallics changes from equiaxial in the Ce alloys to elongate for Y and Sm, with longer particles for Sm and, in general, when Cu is added to the alloy. [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]

• The mitigating effect of ICs on SCC of Alloy 600 after cold working still existed. • Oxidation in the matrix surrounding ICs was supposed to be the rate-limiting process of intergranular oxidation. • Cr in the ICs diffused out to facilitate the formation of Cr-rich oxide. • Cracks propagated along the porous oxide-matrix interfaces under the micro-mechanical testing. • Curved oxide-matrix interfaces increased the total crack length and decreased the effective stress. Intergranular oxidation occurred in a cold-worked Alloy 600 after exposure to pressurized water reactor primary water. Micromechanical testing was conducted to study the process of intergranular cracking under the external loading. High-resolution analytical electron microscopy was conducted to characterize the microstructure and chemistry of the oxidized grain boundaries before and after the micromechanical testing. The matrix around the intergranular carbides was preferentially oxidized and Cr in the intergranular carbides diffused out to facilitate the formation of Cr-rich oxide. Cracks preferentially propagated along the oxide-matrix interfaces during the micromechanical tests. Intergranular carbides are assumed to increase cracking resistance after cold-working. [ABSTRACT FROM AUTHOR]

High-resolution characterization was used to understand the mechanisms controlling stress corrosion cracking (SCC) in Alloy 600 exposed to simulated PWR primary water conditions. Three potential active crack tips obtained from different types of grain boundaries were studied and compared. The results suggest that the dominant mechanism controlling SCC propagation is intergranular internal oxidation. The applied stress, pre-existent residual strain, the accumulation of defects around the crack tip, the formation of a Fe-Cr-depleted zone, and a porous intergranular oxide are acknowledged as necessary precursors to SCC. Based on the results obtained in this study, a model of SCC propagation is proposed. [ABSTRACT FROM AUTHOR]

Novel atom probe tomography (APT) data of an intergranular stress corrosion crack tip has been acquired. Using APT for stress corrosion cracking research, very small, localized features and their distribution around the crack tip can be studied in 3D. This work details the development of a technique for the preparation of atom probe needles. Initial characterization via analytical transmission electron microscopy provides with a complementary analysis and accurately locates features that can be correlated with the reconstructed APT data. Ni enrichment and intergranular oxidation ahead of the crack tip have been studied with APT in 3D and with near-atomic resolution. [ABSTRACT FROM AUTHOR]

Understanding the behavior of oxide dispersion strengthened (ODS) ferritic/martensitic steels under irradiation is vital in the design of advanced fusion reactors. In this work, a simultaneous triple ion implanted ODS Fe12Cr steel was investigated by transmission electron microscopy in order to determine the effect of irradiation on the grain and dislocation structures, oxide nanoparticles and other secondary phases present in the steel. The ODS steel was irradiated at RT with Fe 8+ , He + and H + at the JANNUS-Saclay facility to a damage of 4.4 dpa. Results show that ODS nanoparticles appear very stable under these irradiation conditions. [ABSTRACT FROM AUTHOR]

• DIGM ahead of the crack tips of three alloys were examined. • DIGM occurred ahead of SCC crack tips is the result of Cr, Fe, and Ni diffusion driven by PIO. • DIGM length and width are the result of competing elemental lateral and in-depth diffusion and PIO. • Both temperature and material composition could affect the DIGM. • A tortuous migrated GB could retard the crack propagation. Three austenitic alloys with different Ni content were stress corrosion cracking (SCC) tested in simulated pressurized water reactor (PWR) primary water at 320 and 360 °C. Diffusion-induced grain boundary migration (DIGM) associated with preferential intergranular oxidation (PIO) is observed ahead of all SCC crack tips. The occurrence of DIGM is revealed to be driven by PIO-induced diffusion of Cr, Fe, and Ni. The extent of DIGM is controlled by the lateral and in-depth elemental diffusion and PIO, with these processes interconnected. Temperature and alloy composition are revealed to affect the extent of DIGM by affecting these processes. [ABSTRACT FROM AUTHOR]

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-ZrO 2 is observed to transform into cubic phase, stabilised by the strain induced by irradiation-induced 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 irradiation-induced 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 irradiation-induced processes, including grain boundary migration, phase transformations and texture evolution. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The microstructure of the surface oxide film formed on an Fe-9Cr Ferritic-Martensitic (F-M) steel after exposure to deaerated high-temperature steam at 600°C for 100 h has been analyzed in detail by advanced characterization techniques. The surface oxide film has been revealed to have a triplex structure, including an outer oxide layer, an inner oxide layer, and an internal oxide layer. Although the outer and inner oxide layers are continuous, the internal oxide layer has been proved to consist of interconnected metallic and chromite phases, which is a typical feature of internal oxidation. The formation mechanisms of each layer have been discussed, finding that, contrary to what the available space model suggests, an external oxidation is not the controlling oxidation mechanism of F-M steels in high-temperature steam. The higher resolution used in this study confirms that the controlling mechanism is internal oxidation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

It is well-known that steels always oxidize faster in the environments containing water vapour than in dry oxygen. Due to the difficulties in obtaining necessary experimental scale of observations, the mechanisms responsible for the steam-accelerated oxidation are still unclear. Through a combination of multiscale characterization techniques, the surface oxide film formed on an Fe-17Cr-9Ni stainless steel after exposure to high-temperature steam has been studied in detail. The characterization results obtained in this study reveal that the inner oxide layer actually consists of two phases Fe-Ni austenite and FeCr 2 O 4 oxide, which formed due to internal oxidation. The classic internal oxidation model underestimates the thickness of the inner oxide layer by one order of magnitude. This difference can be explained by the existence of fast diffusion channels in the inner oxide layer. This study provides direct evidence of a high density of nanopores in the oxide phase of the internal oxide layer, which can act as fast-diffusion channels if interconnected, and proposes their mechanisms of formation, a consequence of water dissociation-induced protons promoting the formation, migration, and clustering of both cation and anion vacancies. 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]

It is often assumed that internal oxidation cannot occur at temperatures below 400 °C. However, in the present work, internal oxidation was observed in a 20% cold-worked Fe-17Cr-12Ni stainless steel (SS) after exposure to simulated primary water of a pressurized water reactor at 340 °C and not in a similarly tested sample without prior cold-work. The formation of discrete Cr-oxide precipitates and the role of cold-work are discussed. The internal oxidation model is also proposed as a plausible stress corrosion cracking mechanism of Fe-17Cr-12Ni SS at that temperature. Unlabelled Image [ABSTRACT FROM AUTHOR]

Highlights • Surface oxide film on 316 L SS has triplex structure and the mechanisms are discussed. • Mechanisms controlling the surface oxidation and crack tip oxidation are similar. • Dislocation and stress can enhance oxidation. • The formation of Ni-rich zone ahead of crack tip and in the surface oxide is controlled by selective oxidation. Abstract Oxidation and stress corrosion cracking (SCC) of 316L stainless steel were studied in simulated pressurized water reactor primary water. Surface, crack flank, and crack tip oxides were analyzed and compared by high-resolution characterization, including oxidation state mapping. All oxides were found to have a triplex structure, although of different dimensions and composition, revealing the effects of local water chemistry and applied stress. The higher oxidation rate at the crack tip could be explained due to the existence of a higher dislocation density, higher level of stress and cation unavailability from the environment. The implications to SCC mechanisms are discussed. [ABSTRACT FROM AUTHOR]

Abstract Stress corrosion cracking of Alloy 600 (A600) and 316 stainless steel (316SS) exposed to simulated pressurized water reactor primary water at temperatures of 320–360 °C has been investigated and compared. Intergranular oxidation developed ahead of all crack tips prepared from these two alloys. High-resolution characterization reveals that there are two main rate-controlling mechanisms contributing to crack propagation: a diffusion-based and a mechanical deformation-based. The different temperature dependence of crack growth rate (CGR) in A600 and 316SS can be explained after confirming that the two rate-controlling mechanisms exhibit different "weights" in the two alloys. The diffusion-based mechanism plays a dominant role in accelerating the CGR of A600, while the mechanical deformation-based mechanism is responsible for the observed CGR decrease of 316SS at higher temperatures. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Graphical abstract Chemical composition and crystallography of a migrated grain boundary. Highlights • DIGM was observed ahead of all examined SCC crack tips. • The extent of DIGM varied from boundary to boundary. • Different extent of DIGM leads to different retardation effect on SCC growth. • A method is proposed to quantify the role of DIGM on SCC crack growth. Abstract Crack tips prepared from Ni-based alloys after stress corrosion cracking testing in simulated pressurized water reactor primary water have been studied by high-resolution characterization. Diffusion-induced grain boundary migration (DIGM) was observed in all the cracks and its potential role on the crack propagation is discussed in detail. The extent of DIGM observed changed with grain boundary type and alloy composition, which could lead to different extent of retardation effects on the crack propagation. A method is proposed to quantify the role of DIGM on crack propagation based on the results obtained in this work. [ABSTRACT FROM AUTHOR]

Abstract The mitigating effect introduced by intergranular Cr carbides on the stress corrosion cracking propagation of a cold-worked Alloy 600 has been firstly examined through high-resolution 3-dimensional (3D) sequential sectioning. High-resolution transmission electron microscope (TEM) and transmission Kikuchi diffraction (TKD) are used to reveal the underlying mechanisms contributing to the mitigating effect. Previously reported mechanisms contributing to the increased stress corrosion cracking resistance are evaluated and discussed. A new mechanism based on grain boundary migration inhibition and crack path deviation is proposed. Graphical abstract Image 1 Highlights • Mitigating effect of intergranular carbides on SCC was observed in 3D. • Cold woking-induced preferential deformation around intergranular carbides was observed via TKD. • Previously reported mechanisms contributing to the mitigating effect were evaluated and discussed. • A new mechanism based on grain boundary migration inhibition and crack path deviation was proposed. [ABSTRACT FROM AUTHOR]

Model FeCrAl alloys of Fe-10%Cr-5%Al, Fe-12%Cr-4.5%Al, Fe-15%Cr-4%Al, and Fe-18%Cr-3%Al (in wt %) were irradiated with 1 MeV Kr ++ ions in-situ with transmission electron microscopy to a dose of 2.5 displacements per atom (dpa) at 320 °C. In all cases, the microstructural damage consisted of dislocation loops with ½〈111〉 and 〈100〉 Burgers vectors. The proportion of ½〈111〉 dislocation loops varied from ∼50% in the Fe-10%Cr-5%Al model alloy and the Fe-18Cr%-3%Al model alloy to a peak of ∼80% in the model Fe-15%Cr-4.5%Al alloy. The dislocation loop volume density increased with dose for all alloys and showed signs of approaching an upper limit. The total loop populations at 2.5 dpa had a slight (and possibly insignificant) decline as the chromium content was increased from 10 to 15 wt %, but the Fe-18%Cr-3%Al alloy had a dislocation loop population ∼50% smaller than the other model alloys. The largest dislocation loops in each alloy had image sizes of close to 20 nm in the micrographs, and the median diameters for all alloys ranged from 6 to 8 nm. Nature analysis by the inside-outside method indicated most dislocation loops were interstitial type. [ABSTRACT FROM AUTHOR]

In situ ion irradiation in a transmission electron microscope was used to investigate the effects of temperature on radiation-induced bubble lattice formation in Cu by low energy (12 keV) helium ions. Bubble lattices were observed to form between − 100 and 100 °C, but at 200 °C lattice formation was impeded by continued growth and agglomeration of bubbles. Both nucleation of bubbles, and to a lesser extent bubble lattice formation, are observed at lower fluences as temperature increases, which we suggest is due to increased point defect mobility. Previous work on point defect concentrations in irradiated copper is considered when interpreting these results. [ABSTRACT FROM AUTHOR]

Complementary analytical transmission electron microscopy (TEM) and transmission Kikuchi diffraction (TKD) were used to study the influence of temperature on the crack growth rate (CGR) in SUS316 stainless steels. An Arrhenius-type temperature dependence of the CGR has been observed between 250 °C and 320 °C. However, stress corrosion cracking (SCC) CGRs were found to decrease between 320 °C and 360 °C, which cannot be explained in terms of a single operating mechanism. High-resolution characterization has produced direct evidence that the SCC CGR in SUS316 is subjected to, at least, two rate-controlling processes: thermally activated diffusion and mechanical response to external stress and internal strain. While diffusion of metallic and non-metallic species at the crack flanks are enhanced at higher temperatures (350 °C and 360 °C), mechanical response-based mechanisms appear to dominate at lower temperatures (320 °C and 340 °C). Higher strain concentrations and dislocation densities around the crack tip were found at low temperature, potentially leading to accelerated crack growth and a peak in the CGR at ∼320 °C. It is suggested that phenomena occurring near the crack tip can be potentially very different at high and low temperatures. [ABSTRACT FROM AUTHOR]

The microstructure and microchemistry of the oxide scales formed on Fe-21Cr-32Ni and Fe-17Cr-9Ni steels after exposure to deaerated high-temperature high-pressure steam at 600 °C for 1500 h have been analysed and compared by several advanced characterization techniques. By comparing the oxide scales formed at different-stages of exposure, it is shown that Fe-21Cr-32Ni steel was internally oxidized at the early-stage, and then an external oxide scale was developed together with an inner chromia band under the internal oxidation zone. In comparison, Fe-17Cr-9Ni steel was internally oxidized together with an external Fe-rich oxide scale during the entire experimental period. The thicknesses of the internal oxidation zone of Fe-21Cr-32Ni and Fe-17Cr-9Ni steels were ∼7 and ∼70 µm, respectively. Further characterisation revealed that the internal oxidation zone contained (Cr, Fe, (Ni)) 3 O 4 and nanoscale nickel networks, together with numerous nano-pores. The effects of these structures on mass transfer and reaction product formation were discussed, in connection with the alloy composition and the formation of the chromia layer. [Display omitted] [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]