Nanoscale characterization and formation mechanism of nanoclusters in an ODS steel elaborated by reactive-inspired ball-milling and annealing

Reactive-inspired ball-milling is proposed as a new production route for oxide dispersion strengthened (ODS) steels. So a Fe-14Cr-2W-1Ti-0.8Y-0.2O (wt.%) ODS steel is elaborated by ball-milling of FeCrWTi and YFe₃ plus Fe₂O₃ powders instead of Y₂O₃ and then by annealing at 800 °C for 5 min. Characterizations by Electron Probe MicroAnalysis and Atom Probe Tomography (APT) are performed after milling and after annealing. For the very first time, nanoclusters are observed after ball-milling by APT. Those nanoclusters are enriched in titanium, yttrium and oxygen and their mean radius is 0.8 nm. With annealing, the mean radius rises up to 1.4 nm and the number density as well as the enrichment factor in O, Ti and Y increase. So a new formation mechanism of nanoclusters is observed in those conditions of synthesis: ball-milling initiates the nanoclusters nucleation and during annealing, nucleation continues, accompanied by a slight growth of nanoclusters. Thus reactive-inspired ball-milling appears as a promising route for synthesizing ODS steels with a fine and dense dispersion of oxides.     

Correlation between chemical composition and size of very small oxide particles in the MA957 ODS ferritic alloy

ODS (oxide dispersion strengthened) alloys have superior creep properties. As it is well known, these excellent creep properties result from very fine oxide particles dispersed with the matrix. However, there is no common understanding about the nature of the very small oxide particles. Two hypotheses arise from the literature, 1: non-stoichiometric Y-, Ti-, O-enriched clusters and 2: stoichiometric Y₂Ti₂O₇. In this work, both chemically extracted residue method and extraction replica method were applied to the commercial ODS ferritic alloy, MA957. These samples were then observed using XRD (X-ray diffractometry) and FEG-STEM (field emission gun-scanning transmission electron microscopy) with EDS (energy dispersive X-ray spectrometer). From the results, it was concluded that the composition of small particles is related to the particle size. They exhibit at least two types of phase, 1: non-stoichiometric Y-, Ti-, O-enriched clusters from ∼2 to ∼15 nm (Y/Ti < 1) and 2: stoichiometric Y₂Ti₂O₇ from ∼15 to ∼35 nm. Based on the result, it is suggested that the appropriate increase of titanium content compared to yttrium content in oxide particles by modifying the chemical compositions of ODS alloys could be an effective way to obtain a finer dispersion of oxide particles.     

Ab initio simulation of yttrium oxide nanocluster formation on fcc Fe lattice

Using results of density functional theory (DFT) calculations the first attempt towards the understanding of Y₂O₃ particles formation in oxide dispersed strengthened (ODS) ferritic-martensitic steels was performed. The present work includes modeling of single defects (O impurity atom, Fe vacancy and Y substitute atom), interaction between substituted Y atoms, Y-Fe vacancy pairs and oxygen impurity atoms in the iron matrix. The calculations have showed the repulsive interaction between the two Y substitute atoms at any separation distances that might mean that the oxygen atoms or O atoms with vacancies are required to form binding between atoms in the yttrium oxide nanoclusters.     

Analysis of hardening limits of oxide dispersion strengthened steel

Oxide dispersion strengthened (ODS) ferritic/martensitic (F/M) steels are promising materials for high temperature applications. The hardening limits from room temperature to 1000 °C of one of such steel, ODS EUROFER97, together with the impact of the material production steps, are investigated at a microstructural level by coupling hardness, tensile tests and transmission microscopy, including in situ heating experiments. The oxides, ytttria and complex yttrium titanium oxides, reinforce the material by forming more or less stable obstacles to dislocations, and by promoting grain refinement by pinning grain boundaries. It appears that part of the yttrium titanium oxides particles dissolves from about 600 °C while pure yttria particles are stable at least to 1000 °C in the steel. The concurrent roles of the oxides and the dislocation structure in the hardening are rationalized using the dispersion barrier hardening model. It appears that hardening due to dislocations can overcome the one due to oxides but is more sensitive to temperature than the one due to oxides, and that the main limiting factor is the thermal stability of the oxides.     

Dosimetric properties of UV irradiated calcium co-doped borate glass-ceramic

The thermoluminescence (TL) response of Dy and Li doped 20CaB₄O₇-80CaB₂O₄ (wt%) glass-ceramic irradiated with ultraviolet (UV) radiation was studied. In order to act as TL activator ions, the Dy and Li ions were included in the matrix during the melting process to increase its TL efficiency. A single crystalline CaB₂O₄ phase was present in the glass-ceramic as determined by X-ray diffraction (XRD). The glass-ceramic 20CaB₄O₇-80CaB₂O₄:Dy,Li wt% (named 20CBO7:Dy,Li) is a newly prepared TL material. Its thermoluminescent dosimetric characteristics have shown a linear response under UV radiation exposure and a good TL signal reproducibility, thus proving to be a promising material for using as an ultraviolet radiation dosimeter.     

Addition of Fe2O3 as oxygen carrier for preparation of nanometer-sized oxide strengthened steels

Nano-structured ferritic alloys, which are prepared almost exclusively via the mechanical alloying of Y₂O₃, have recently attracted much attention. Our preliminary results show that the usage of Fe₂O₃ as oxygen source leads to better control of powder properties than Y₂O₃ and a high density of nanometer-sized oxide particles can be formed by atomic mixing of Y, Ti and O. This may provide a new route with reduced costs and improved reproducibility for industrial production of nanometer-sized oxide strengthened steels.     

Radiation damage effects in the uranium-bearing δ-phase oxide Y6U1O12

Ion irradiation damage effects in delta (δ) Y₆U₁O₁₂ were characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Experimental results revealed no amorphization transformation occurs in Kr-ion irradiated Y₆U₁O₁₂ to a maximum displacement damage dose of ∼50 displacements per atom at cryogenic temperature. Density functional theory calculations indicate that δ-Y₆U₁O₁₂ possesses a relatively low cation antisite formation energy, which may help to explain the observed resistance of δ-Y₆U₁O₁₂ to irradiation-induced amorphization of δ-Y₆U₁O₁₂.     

Microstructural investigation of the stability under irradiation of oxide dispersion strengthened ferritic steels

Some fuel pin cladding made from a ferritic steel reinforced by titanium and yttrium oxides were irradiated in the French experimental reactor Phénix. Microstructural examination of this alloy indicates that oxides undergo dissolution under irradiation. This irradiation shows the influence of dose and, in a smaller part, of temperature. In order to better understand the mechanisms of dissolution, three ferritic steels reinforced by Y₂O₃ or MgO were irradiated with different charged particles. Inelastic interactions induced by 1 MeV He ion irradiation do not lead to any modification, neither in their chemical composition, nor in their spatial and size distribution. In contrast, isolated Frenkel pairs created by electron irradiation lead to significant oxide dissolution with a radius decrease proportional to the dose. Moreover, the comparison between irradiation with ions (displacements cascades) and electrons (Frenkel pairs only) shows the importance of free point defects in the dissolution phenomena.     

First-principle calculations on color center in Y-Al-O system

Based on a serial of ab initio calculations, we studied various F-type color centers in Y-Al-O system material: α-Al₂O₃, Y₂O₃, Y₃Al₅O₁₂ (YAG), YAlO₃ (YAP). The local atomic structures and formation energy of various color centers are calculated and discussed among the above four oxides. The stability of various color centers in Y-Al-O material is analysed. Our results show that the neutral F-center needs the largest formation energy while, +2 charged F²⁺-center needs the lowest formation energy in each of studied oxides. Same type of color center in α-Al₂O₃ needs the lowest formation energy, while needs the largest energy in YAG. And more, in all four oxides, the F²⁺-center is relatively stable than other two species.     

Microstructural evolution of Y2O3 and MgAl2O4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing

Different ODS EUROFER steels reinforced with Y₂O₃ and MgAl₂O₄ were elaborated by mechanical milling and hot isostatic pressing. Good compromise between strength and ductility could be obtained but the impact properties remain low (especially for the Y₂O₃ ODS steel). The materials were structurally characterized at each step of the elaboration. During milling, the martensite laths of the steel are transformed into nano-metric ferritic grains and the Y₂O₃ oxides dissolve (but not the MgAl₂O₄ spinels). After the HIP, all the ODS steels remain ferritic with micrometric grains, surrounded by nano-metric grains for the Y₂O₃ ODS steels. The mechanisms in the Y₂O₃ ODS steels are complex: the Y₂O₃ oxides re-precipitate as nano-Y₂O₃ particles that impede a complete austenitization during the HIP. The quenchability of the ODS steels is modified by the milling process, the oxide nature and the oxide content. Eventually, the advantages and drawbacks of each oxide type are discussed.     

Ion irradiation damage effects in δ-phase Y6W1O12

Polycrystalline Y₆W₁O₁₂ samples were irradiated with 280 keV Kr²⁺ ions to fluences up to 2 × 10²⁰ ions/m² at cryogenic temperature (100 K). Ion irradiation damage effects in these samples were examined using grazing incidence X-ray diffraction (GIXRD) and cross-sectional transmission electron microscopy (TEM). The pristine Y₆W₁O₁₂ possesses rhombohedral symmetry (structure known as the δ-phase), which is closely related to cubic fluorite structure. GIXRD and TEM observations revealed that the irradiated Y₆W₁O₁₂ experiences an ordered rhombohedral to disordered cubic fluorite transformation by a displacement damage dose of ∼12 displacements per atom (dpa). At the highest experimental dose of ∼50 dpa, the uppermost irradiated region was found to be partially amorphous while the buried damage region was found to contain the same fluorite structure as observed at lower dose.     

10 MeV Au ion irradiation effects in an MgO-HfO2 ceramic-ceramic (CERCER) composite

Room temperature ion irradiation damage studies were performed on a ceramic composite intended to emulate a dispersion nuclear fuel. The composite is composed of 90-mole% MgO and 10-mole% HfO₂. The as-synthesized composite was found to consist of Mg₂Hf₅O₁₂ (and some residual HfO₂) particles embedded in an MgO matrix. X-ray diffraction revealed that nearly all of the initial HfO₂ reacted with some MgO to form Mg₂Hf₅O₁₂. Ion irradiations were performed using 10 MeV Au³⁺ ions at room temperature over a fluence range of 5 × 10¹⁶-5 × 10²⁰ Au/m². Irradiated samples were characterized using both grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM), the latter using both selected-area electron diffraction (SAED) and micro-diffraction (μD) on samples prepared in cross-sectional geometry. Both GIXRD and TEM electron diffraction measurements on a specimen irradiated to a fluence of 5 × 10²⁰ Au/cm², revealed that the initial rhombohedral Mg₂Hf₅O₁₂ phase was transformed into a cubic-Mg₂Hf₅O₁₂ phase. Finally, it is important to note that at the highest ion fluence used in this investigation (5 × 10²⁰ Au/m²), both the MgO matrix and the Mg₂Hf₅O₁₂ second phase remained crystalline.     

Structure and elastic property of nanosized complex oxide particles in ferritic/martensitic alloy: An electron energy-loss spectroscopy study

The structure and elastic property of nanosized complex oxide particles in a ferritic/martensitic alloy containing titanium and silicon were studied by transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The nanosized complex Y-Si-O particles were found in the matrix of the alloy in addition to Y-Ti-O, and the size of Y-Si-O is smaller than that of Y-Ti-O particles. The formation of Y_2.16Si_1.76O₇ and Y_2.15Ti_1.95O₇ were further confirmed by O K, Si L_2,3 and Ti L_2,3 edges, respectively. The bulk modulus of Y_2.16Si_1.76O₇ was shown to be lower than that of Y_2.15Ti_1.95O₇, which implies that the nanosized Y_2.16Si_1.76O₇ particles would provide more effective dislocation pinning at elevated temperatures.     

Synthesis of zirconia sphere particles based on gelation of sodium alginate

Zirconia sphere particles were synthesized through the gelation process of Na-alginate, and cermet (ZrO₂-Mo) pellets were fabricated under several conditions. In this process, a zirconia slurry was prepared by mixing oxide powders (ZrO₂, Y₂O₃, Er₂O₃, CeO₂), distilled water and Na-alginate, and subsequently dropped into CaCl₂ solution. As a result, zirconia sphere particles coated with a gelled film were synthesized. The slurry density (zirconia content in slurry) of 30-64 wt.% and Na-alginate concentration of a few% were good for gelation for up to 10 wt.% CaCl₂ solution. Sphere particles with smaller diameter were obtained by dropping slurry with a mechanical vibration. The prolongation of the ball milling time for mixture of oxide powders was effective to increase the sintered density of zirconia sphere particles, especially for higher CeO₂ concentration. The dense cermet pellets were fabricated for max. 50% volume ratio of zirconia phase for Mo matrix using zirconia particles covered with Mo powder by a rotating granulation method.     

Characterization of phases in ‘crud’ from boiling-water reactors by transmission electron microscopy

This paper reports phases identified in samples of crud (activated corrosion products) from two commercial boiling-water reactors using transmission and analytical electron microscopy and selected-area electron diffraction. Franklinite (ZnFe₂O₄) was observed in both samples. Hematite (α-Fe₂O₃), crystalline silica (SiO₂), a fine-grained mixture of iron oxides probably including magnetite (Fe₃O₄), hematite (α-Fe₂O₃), and goethite (α-FeOOH), and an unidentified high-Ba, high-S phase were observed in one of the samples. Willemite (Zn₂SiO₄), amorphous silica, and an unidentified iron-chromium phase were observed in the other. Chloride-bearing phases were found in both samples, and are assumed to represent sample contaminants. Because of the small sample volumes and numbers of particles studied and the possibility of contamination, it is not clear whether the differences between the phases observed in the two crud samples represent actual differences in the assemblages formed in the reactors.     

Recovery of neutron-irradiation-induced defects of Al2O3, Y2O3, and yttrium-aluminum garnet

SiC fiber-reinforced SiC matrix composites (SiC_f/SiC) are considered as one of the candidates for blanket materials in future fusion reactors and as an advanced fuel cladding material for next-generation fission reactors. Generally, the densification of SiC needs sintering additives and oxides such as Al₂O₃, Y₂O₃, and yttrium-aluminum garnet (YAG, Y₃Al₅O₁₂), which are frequently added to SiC. However, the effects of neutron irradiation on sintering additives are still unclear. In this study, we performed the neutron irradiation of Al₂O₃, Y₂O₃, and YAG at fluences up to 2.0–2.5 × 10²⁴ n/m² (E > 0.1 MeV) at 60–90 °C. The isochronal recovery of the macroscopic volume of Al₂O₃ against annealing temperature showed smooth and continuous shrinkage at a temperature of up to 1200 °C, and the volume slightly increased above that temperature. In contrast, the volume of Y₂O₃ showed quick shrinkage at the low temperature range, and slower and smooth recovery was observed up to ～1100 °C. In the case of YAG, the recovery of volume occurred in a step-wise manner at 600–750 °C, and continuous shrinkage occurred at temperatures lower and higher than that temperature range. The activation energies for the macroscopic volume recoveries of three oxides were obtained from the Arrhenius plots of the rate coefficients. Two-stage recovery was observed for Al₂O₃, whereas more complicated recovery processes were suggested for Y₂O₃ and YAG.     

Fabrication of particle dispersed inert matrix fuel based on liquid phase sintered SiC

In the present work, liquid phase sintered SiC (LPS-SiC) was proposed as an inert matrix for the particle dispersed inert matrix fuel (IMF). The fuel particles containing plutonium and minor actinides were substituted with pure yttria stabilized zirconia beads. The LPS-SiC matrix was produced from the initial mixtures prepared using submicron sized α-SiC powder and oxide additives Al₂O₃, Y₂O₃ in the amount of 10 wt.% with the molar ratio 1Y₂O₃/1Al₂O₃. Powder mixtures were sintered using two sintering methods; namely conventional high temperature sintering and novel spark plasma sintering at different temperatures depending on the method applied in order to obtain dense samples. The phase reaction products were identified using X-ray diffraction (XRD) and microstructures were investigated using light microscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) techniques. The influence of powder mixing methods, sintering temperatures, pressures applied and holding time on the density of the obtained pellets was investigated. The samples sintered by slow conventional sintering show lower relative density and more pronounced interaction between the fuel particles and matrix in comparison with those obtained with the fast spark plasma sintering method.     

Spark plasma sintering of tungsten-yttrium oxide composites from chemically synthesized nanopowders and microstructural characterization

Nano-crystalline W-1%Y₂O₃ (wt.%) powder was produced by a modified solution chemical reaction of ammonium paratungstate (APT) and yttrium nitrate. The precursor powder was found to consist of particles of bimodal morphology i.e. large APT-like particles up to 20 μm and rectangular yttrium containing ultrafine plates. After thermal processing tungsten crystals were evolved from W-O-Y plate like particles. spark plasma sintering (SPS) was used to consolidate the powder at 1100 and 1200 °C for different holding times in order to optimize the sintering conditions to yield high density but with reduced grain growth. Dispersion of yttrium oxide enhanced the sinterability of W powder with respect to lanthanum oxide. W-1%Y₂O₃ composites with sub-micron grain size showed improved density and mechanical properties as compared to W-La₂O₃ composites. Sample sintered in two steps showed improved density, due to longer holding time at lower temperature (900 °C) and less grain growth due to shorter holding time at higher temperature i.e. 1 min at 1100 °C.     

Glass matrices for immobilizing nuclear waste containing molybdenum and phosphorus

Vitrification has been selected in France as the process for immobilizing high-level waste arising from spent fuel reprocessing. Some high-level solutions generated by reprocessing legacy fuel contain high molybdenum concentrations. Molybdenum is known to be sparingly soluble in conventional borosilicate glass, and work is in progress to find suitable glass formulations for such waste. The results of a basic study to identify borosilicate glasses composition zones of potential interest are discussed. A vast composition range was investigated by defining a fine mesh. The limits considered to delimit the range of the study were intentionally extended to identify formulations such as SiO₂-B₂O₃-Al₂O₃-Na₂O-P₂O₅ that are of interest for vitrifying molybdenum-rich waste. Observation of more than 50 tested mixtures revealed two composition zones of potential interest. One forms a homogeneous glass after melting at 1300 °C and rapid cooling; the other vitreous material comprises unconnected microbeads uniformly dispersed in a borosilicate glass.     

Comparative study and imaging by PhotoElectroChemical techniques of oxide films thermally grown on zirconium and Zircaloy-4

The oxidation of iron and chromium that are present as impurities in zirconium metal or as alloying elements in Zircaloy-4 was investigated with PhotoElectroChemical techniques (PEC), highlighting the chemical nature, the size and the lateral distribution of Fe and Cr-containing phases in thin zirconia scales formed during the oxidation of pure zirconium and Zircaloy-4 at 470 °C in oxygen. In the case of zirconium, iron and chromium impurities led to the formation of oxides distributed in a homogeneous way in the zirconia scale, while in the case of Zircaloy-4 these elements, present in the form of intermetallic particles in the substrate, led to the formation of localised haematite Fe₂O₃, rhomboedric solid solution (Fe_xCr_1−x)₂O₃ and chromia Cr₂O₃ phases. These phases were accurately studied via the measurement of their respective band-gap (Fe₂O₃: 2.2 eV, (Fe_xCr_1−x)₂O₃: 2.6 eV and Cr₂O₃: 3.0 eV). It is concluded that PEC techniques represent a sensitive and powerful way to locally analyse the various semiconductor phases in the oxide scale at a micron scale.