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.     

Diffuse reflectance and X-ray photoelectron spectroscopy of uranium in ZrO2 and Y2Ti2O7

Diffuse reflectance measurements were made over the wavenumber range of 4000-20,000 cm⁻¹ at room temperature on monoclinic and stabilised ZrO₂, together with Y₂Ti₂O₇ having the pyrochlore structure, all of which were doped with U and sintered in various atmospheres. X-ray photoelectron spectroscopy measurements were also carried out on selected samples. In monoclinic and stabilised zirconia, U exhibited valence states of +4 and/or +5, depending on the sintering atmosphere and the presence of appropriate charge compensators. Using both diffuse reflectance and X-ray photoelectron spectroscopy, U was also observed as mainly U⁴⁺ and/or U⁵⁺ in U-doped Y₂Ti₂O₇ sintered at 1400 °C in air or Ar, although a small amount of U⁶⁺ also appeared to be present in some U-doped Y₂Ti₂O₇ samples heated in air.     

Microstructures and mechanical properties of Fe-14Cr-3W-Ti-Y2O3 steel with 1 wt.% Cu addition fabricated by a new method

Effect of 1 wt.% copper addition on microstructures and mechanical properties of Fe-14Cr-3W-0.4Ti-0.4Y₂O₃ (wt.%) alloy has been investigated. Mechanically alloyed powders and pre-alloyed powders were blended and consolidated by hot extrusion. A bimodal grain structure with large grains (10-20 μm) and nanometer grains was formed. Through aging treatment, ε-Cu phase with face cubic centered lattice structure precipitated from the supersaturated solid solution. After aging for 6 h, the microhardness reached a peak value of HV326, which was attributed to precipitation of copper-rich phase. The alloy exhibited a high strength due to the strengthening of both copper-rich precipitates and Y-Ti-O nanoclusters, and an excellent ductility due to its bimodal structure.     

Development of oxides dispersion strengthened steels for high temperature nuclear reactor applications

By introducing a dispersion of nanosized yttrium oxides particles into a steel matrix, the upper temperature limit in mechanical creep strength can be enhanced in temperature by 100 K at least. Production routes for the production of a new class of oxides dispersion strengthened (ODS) steels are investigated within this work. Preliminary results obtained when doping pure iron matrix phase with two types of yttrium oxides (Y₂O₃) nanoparticles (commercial as well as laboratory fabricated nanopowder) are presented. The twofold purpose of this work is firstly to obtain a comparative analysis between the commercial and the laboratory fabricated Y₂O₃ nanopowder used to produce the doped iron, and secondly to demonstrate the feasibility of new production route by observing the nanostructure of the first test batches with pure iron. Observations are carried out with transmission electron microscopy (TEM) to determine the size distribution of the particles in the powder, while glow discharge optical emission spectroscopy (GDOES) and high resolution-scanning electron microscopy (HR-SEM) are used to analyze the chemical composition and the homogeneity of the produced doped iron. It is demonstrated, that even with small size particles nanopowder fabricated in the laboratory, the distribution is fairly homogeneous compared to the one obtained with a relatively large particles commercial nanopowder, confirming the feasibility of the new production route.     

Phase equilibria in the Li4SiO4Li2SiO3 region of the pseudobinary Li2OSiO2 system

The constitution of the Li₄SiO₄Li₂SiO₃ region of the pseudobinary L₂OSiO₂ system was investigated between 1000 and 1300°C by isothermal heat treatment, differential thermal analysis, ceramography and X-ray diffraction. The two boundary phases of the sub-system, Li₄SiO₄ and Li₂SiO₃, melt congruently at 1258°C and 1209°C, resp. An intermediate high-temperature phase, Li₆Si₂O₇, forms peritectically at 1030°C and decomposes eutectoidally at 1020°C. A eutectic sub-system exists between Li₄SiO₄ and Li₆Si₂O₇ with the eutectic temperature and composition of 1025°C and 38.3 mol% SiO₂, resp. Li₆Si₂O₇ can be quenched to room temperature in a metastable state and crystallizes in a tetragonal system with the lattice parameters a = 770.9 pm and c = 486.0 pm.     

Development and characterization of advanced 9Cr ferritic/martensitic steels for fission and fusion reactors

This paper presents the results on the physical metallurgy studies in 9Cr Oxide Dispersion Strengthened (ODS) and Reduced Activation Ferritic/Martensitic (RAFM) steels. Yttria strengthened ODS alloy was synthesized through several stages, like mechanical milling of alloy powders and yttria, canning and consolidation by hot extrusion. During characterization of the ODS alloy, it was observed that yttria particles possessed an affinity for Ti, a small amount of which was also helpful in refining the dispersoid particles containing mixed Y and Ti oxides. The particle size and their distribution in the ferrite matrix, were studied using Analytical and High Resolution Electron Microscopy at various stages. The results showed a distribution of Y₂O₃ particles predominantly in the size range of 5-20 nm. A Reduced Activation Ferritic/Martensitic steel has also been developed with the replacement of Mo and Nb by W and Ta with strict control on the tramp and trace elements (Mo, Nb, B, Cu, Ni, Al, Co, Ti). The transformation temperatures (A_c1, A_c3 and M_s) for this steel have been determined and the transformation behavior of the high temperature austenite phase has been studied. The complete phase domain diagram has been generated which is required for optimization of the processing and fabrication schedules for the steel.     

Heavy-ion irradiation effects on structures and acid dissolution of pyrochlores

The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi⁺ ions, for A₂Ti₂O₇ pyrochlores, in which A=Y, Sm, Gd and Lu, exhibits no significant effect of A-site ion mass or size. The room temperature dose for amorphization was found to be ∼0.18 dpa in each case. After irradiation with 2 MeV Au²⁺ ions glancing-incidence X-ray diffraction (XRD) revealed that each pyrochlore underwent an irradiation-induced structural transformation to fluorite in conjunction with amorphization. The effect of amorphization on the dissolution rates of fully dense pyrochlores, at 90°C and pH 2 (nitric acid) varied from a factor of 10 to 15 increase for Gd₂Ti₂O₇ to none for Y₂Ti₂O₇. Significant differences were observed in the A-site dissolution rates from the crystalline pyrochlores, indicating differences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy.     

Dual beam irradiation of nanostructured FeCrAl oxide dispersion strengthened steel

Nanostructured ferritic oxide dispersion strengthened (ODS) alloy is an ideal candidate for fission/fusion power plant materials, particularly in the use of a first-wall and blanket structure of a next generation reactor. These steels usually contain a high density of Y-Ti-O and Y-Al-O nanoparticles, high dislocation densities and fine grains. The material contains nanoparticles with an average diameter of 21 nm and was treated by several cold rolling procedures, which modify the dislocation density. Structural analysis with HRTEM shows that the chemical composition of the initial Y₂O₃ oxide is modified to perovskite YAlO₃ (YAP) and Y₂Al₅O₁₂ garnet (YAG). Irradiation of these alloys was performed with a dual beam irradiation of 2.5 MeV Fe⁺/31 dpa and 350 keV He⁺/18 appm/dpa. Irradiation causes atomic displacements resulting in vacancy and self-interstitial lattice defects and dislocation loops. Extended SRIM calculations for ODS steel indicate a clear spatial separation between the excess vacancy distribution close to the surface and the excess interstitials in deeper layers of the material surface. The helium atoms are supposed to accumulate mainly in the vacancies. Additionally to structural changes, the effect of the irradiation generated defects on the mechanical properties of the ODS is investigated by nanoindentation. A clear hardness increase in the irradiated area is observed, which reaches a maximum at a close surface region. This feature is attributed to synergistic effects between the displacement damage and He implantation resulting in He filled vacancies. Fine He cavities with diameters of a few nanometers were identified in TEM images.     

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.     

Swift heavy ion irradiation of pyrochlore oxides: Electronic energy loss threshold for latent track formation

Pyrochlore pellets with the Gd₂(Ti_2−xZr_x)O₇ stoichiometry (x = 0, 1 and 2) were irradiated with swift heavy ions in order to investigate the effects of electronic excitation and to determine the electronic stopping power threshold for track formation. XRD results showed that the electronic excitation induced by 870 MeV Xe and 780 MeV Kr ions leads to: (i) a crystalline-amorphous transition for Gd₂Ti₂O₇ and Gd₂TiZrO₇, (ii) a phase transition towards an anion-deficient fluorite structure (order-disorder transition) for Gd₂Zr₂O₇. Thus, zirconate pyrochlores present a better radiation resistance under swift heavy ion irradiation than titanate pyrochlores. Moreover results underline the existence of an electronic stopping power threshold around 13-14 keV/nm, below which phase transformations do not occur.     

Irradiation of synthetic garnet by heavy ions and α-decay of Cm

Garnet, A₃B₂X₃O₁₂, has a structure that can incorporate actinides. Hence, the susceptibility of the garnet structure to radiation damage has been investigated by comparing the results of self-radiation damage from α-decay of ²⁴⁴Cm and a 1 MeV Kr²⁺ ion irradiation. Gradual amorphization with increasing fluence was observed by X-ray diffraction analysis and in situ transmission electron microscopy. The critical dose, D_c, for an yttrium-aluminum garnet (Y₃Al₅O₁₂) doped with 3 wt.% ²⁴⁴Cm is calculated to be 0.4 displacements per atom (dpa). While the doses obtained by ion irradiation experiments of garnets with different compositions (Y_2.43Nd_0.57)(Al_4.43Si_0.44)O₁₂, (Ca_1.64Ce_0.41Nd_0.42La_0.18Pr_0.18Sm_0.14Gd_0.04)Zr_1.27Fe_3.71O₁₂, and (Ca_1.09Gd_1.23Ce_0.43)Sn_1.16Fe_3.84O₁₂, varied from 0.29 to 0.55 dpa at room temperature. The similarity in the amorphization dose at room temperature and critical temperature of the different garnet compositions suggest that the radiation response for the garnet structure is structurally constrained, rather than sensitive to composition, which is the case for the pyrochlore structure-type.     

Diffuse reflectance spectroscopy of Np and Pu in zirconia and pyrochlore-structured Y2Ti2O7

Diffuse reflectance spectroscopy (DRS) measurements were made over the wavenumber range of 4000–20,000 cm^?1 at room temperature on monoclinic and stabilized zirconia and pyrochlore-structured Y₂Ti₂O₇, which were doped with Np or Pu and sintered at ～1400 °C in various atmospheres. Np⁴⁺ was present in uncompensated monoclinic zirconia samples sintered in air, Ar or H₂/N₂. Np⁶⁺ was obtained in air/Ar-sintered monoclinic zirconia that also contained Y³⁺ as charge compensators, and Np⁴⁺ was formed on sintering in H₂/N₂. Np⁴⁺ was present in Y-stabilized zirconia after firing in air, Ar or H₂/N₂. Pu⁴⁺ was observed in air-fired Pu-doped monoclinic zirconia, and sintering in Ar or H₂/N₂ produced Pu³⁺ while only Pu⁴⁺ was identified in Y-stabilized zirconia sintered in air, Ar or H₂/N₂. Only Pu⁴⁺ and Np⁴⁺ were observed in Y₂Ti₂O₇ after sintering in air or argon, even when Ca was substituted for Y to try to encourage the formation of higher valence states of Pu and Np.     

Recent activities in the field of radiation measurement and nuclear data

The characteristics, that is, morphology, size distribution, alloy phase and microstructure of U₃Si and U₃Si₂ powders, solidified rapidly by a centrifugal atomization, were investigated. The atomized powders consist of spherical particles with a relatively narrow size distribution independent of the alloy composition. The particle size distribution can be controlled by adjusting the atomization parameters such as feeding rates of the melt and revolution speeds of the disk. The major phases of atomized U₃Si and U₃Si₂ powders are α-U and U₃Si₂ and U₃Si₂, respectively. The atomized U₃Si powder has a dendritic structure of very fine and non-faceted U₃Si₂ precipitates with less fibric and eutectic U₃Si₂ structure. The microstructure of U₃Si₂ powder shows a cellular structure with fine U₃Si₂ grains and finely dispersed silicon-rich precipitates. The time for complete peritectoid reaction of the atomized U₃Si particles and the resulting grain size are greatly reduced, due to the refinement of primary U₃Si₂ precipitates.     

Crystallization of lanthanum and yttrium aluminosilicate glasses

The crystallization behaviour of aluminosilicate glasses of lanthanum (LAS) and yttrium (YAS) containing 2–8 mol% of Ln₂O₃ (Ln = La or Y), 12–30 mol% of Al₂O₃, and 64–80 mol% of SiO₂ has been studied by DTA, XRD and SEM-EDX analysis. X-ray diffraction results indicate the presence of the mullite phase and La₂Si₂O₇ in the monoclinic high-temperature G form (group space P2₁/c) for the LAS glasses, and mullite y-Y₂Si₂O₇ in the monoclinic structure (group space C2/m) and a small amount of β-Y₂Si₂O₇ in the orthorhombic structure (space group Pna2) for the YAS. For both cases, very little tridymite phase is observed. The results also show that the values of T_g for YAS are higher than those for LAS glasses. The crystallization of LAS glasses is more difficult than that of YAS. For all samples, we observed only one kind of mullite (Al/Si = 3.14).     

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.     

Relaxation of strain during solid phase epitaxial growth of Ge+ ion implanted layers in silicon

Formation of Si_1−xGe_x-alloy layers by solid phase epitaxial growth (SPEG) of Ge⁺ ion implanted silicon has been studied. The ion implantations were performed with 40, 100, 150, 200 and 300 keV ⁷⁴Ge⁺ ions and various ion doses. The SPEG of the ion implanted layers was carried out in a conventional furnace at 850°C for 20 min under a flow of nitrogen gas. The Si_1−xGe_x-alloy layers were characterised by Rutherford backscattering spectrometry and transmission electron microscopy (TEM). For a given ion energy, a Si_1−xGe_x-alloy layer with no observable extended defects can be manufactured if the ion dose is below a critical value and strain-induced defects are formed in the alloy layer when the ion dose is equal to or above this value. The critical Ge⁺ ion dose increases with ion energy, while the critical maximum Ge concentration decreases. For ion energies ⩽150 keV, the defects observed in the alloy layers are mostly stacking faults parallel to the {1 1 1} planes. For higher ion energies, 200 keV and above, the majority of defects in the alloy layer are hairpin dislocations. In the whole ion energy range, the critical ion dose and the depth position of the nucleation site for the stacking faults obtained from the measurements are in good agreement with theoretical predictions. Extended defects are formed in the alloy layer during the SPEG when the regrowth of the crystalline/amorphous interface has reached the depth position in the crystal where the accumulated strain energy density is equal to the critical value of 235 mJ/m².     

Damage evolution in Au-implanted Ho2Ti2O7 titanate pyrochlore

Damage evolution at room temperature in Ho₂Ti₂O₇ single crystals is studied under 1 MeV Au²⁺ ion irradiation by Rutherford backscattering spectroscopy along the 〈0 0 1〉 direction. For a better determination of ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) were used to analyze ion channeling spectra. A disorder accumulation model, with contributions from the amorphous fraction and the crystalline disorder, is fit to the Ho damage accumulation data. The damage evolution behavior indicates that the relative disorder on the Ho sublattice follows a nonlinear dependence on dose and that defect-stimulated amorphization is the primary amorphization mechanism. Similar irradiation behavior previously was observed in Sm₂Ti₂O₇. A slower damage accumulation rate for Ho₂Ti₂O₇, as compared with damage evolution in Sm₂Ti₂O₇, is mainly attributed to a lower effective cross section for defect-stimulated amorphization.     

Phase identification,microstructural characterization,phase microanalyses and leaching performance evaluation of SYNROC-FA crystalline ceramic waste form

SYNROC-FA, a crystalline ceramic waste form designed to contain 50 wt% Amine process, uranium-rich, high-level, radioactive waste for ultimate deep-geologic disposal, has been characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Phase identification was carried out using XRD, electron diffraction in TEM, and backscattered electron imaging in SEM. Phase microanalyses were carried out using energy-dispersive X-ray analyzers (EDX) during SEM and TEM examinations. X-ray diffraction and grain microanalyses using EDX revealed the existence of a pyrochlore-structured phase CaU(Ti³⁺, Ti⁴⁺)₂O₇, perovskite (Ca, U)(Ti³⁺, Ti⁴⁺)O₃ and uraninite (U, Ca, Ti)O₂, while Ba-hollandite Ba(Al³⁺, Ti³⁺)₂Ti₅O₁₄ was identified using only XRD.The leaching resistance of SYNROC-FA was determined by carrying out a modified MCC-1 leach test in a simulated Canadian Shield groundwater at 90°C for 120 days. The normalized leach rate of Ba was $\text{6 × 10}{}^{\mathrm{-3}}\text{g · m}{}^{\mathrm{-2}}\text{· d}{}^{\mathrm{-1}}$ while the concentrations of U and other simulated fission products in the leachants were below the detection limits of inductively coupled plasma spectrometry and atomic absorption techniques. The leach rates of U and Ti were estimated to be less than $\text{6 × 10}{}^{\mathrm{-5}}$ and $\text{3 × 10}{}^{\mathrm{-5}}\text{g · m}{}^{\mathrm{-2}}$, respectively.     

The kinetics of U3Si formation in cast U3Si alloy

The growth of U₃Si in cast U-3.8 wt % Si alloy was measured by determining both the U₃Si₂/U₃Si interface movement, and the change in the total amount of the various phases. The activation energy controlling growth was found to be 50 ± 1 kcal/mole.     

Characterization of High Temperature Creep Properties in Recrystallized 12Cr-ODS Ferritic Steel Claddings

The high temperature strengthening mechanism of previously manufactured 12Cr-ODS ferritic steel claddings was clarified. In the recrystallized 12Cr-2W-0.3Ti-0.24Y₂O₃-ODS ferritic steel cladding, αY₂TiO₅ type complex oxide formation was responsible for the drastic reduction of oxide particle size and the resulting shortened distance between particles, which led to superior internal creep rupture strength at 973 K because of the high resistance to gliding dislocation. Internal creep deformation was considered to be controlled by the grain boundary sliding associated with grain morphology: the near Σ11, Σ and Σ19 coincidence boundaries with a (110) common axis.