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.     

Creep behavior of oxide dispersion strengthened 8Cr-2WVTa and 8Cr-1W steels

Microstructures and creep behavior of two martensitic oxide dispersion strengthened (ODS) steels 8%Cr-2%W-0.2%V-0.1%Ta (J1) and 8%Cr-1%W (J2) with finely dispersed Y₂Ti₂O₇ have been investigated. Creep tests have been carried out at 670, 700 and 730 °C. Creep strength of J1 is stronger than that of any other ODS martensitic steels and the hoop strength of the ferritic ODS steel cladding. At the beginning of creep test, shrinkage was frequently observed for J1. This is one of the reasons for high creep strength of J1. The δ-ferrite, which is untransformed to austenite at hot isostatic press and hot rolling temperatures, was elongated along the rolling direction, and volume fraction of δ-ferrite in J1 is larger than J2. Although the elongated δ-ferrite affects the anisotropy of creep behavior, the extent of anisotropy in J1 is not so large as that of the ferritic ODS steel.     

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.     

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.     

Development of Oxide Dispersion Strengthened Steels for FBR Core Application, (II)

Previously manufactured oxide dispersion strengthened (ODS) ferritic steel cladding tubes had inferior internal creep rupture strength in the circumferential hoop direction. This unexpected feature of ODS cladding tubes was substantially ascribed to the needle-like grain structure aligned with the forming direction. In this study, the grain morphology was controlled by using the martensite transformation in ODS martensitic steels to produce an equi-axial grain structure. A major improvement in the strength anisotropy was successfully achieved. The most effective yttria addition was about 1 mass% in improving the strength of the ODS martensitic steels. A simple addition of titanium was particularly effective in increasing the strength level of the ODS martensitic steels to that of ODS ferritic steels.     

Formation of Y2O3 nanoclusters in nano-structured ferritic alloys: Modeling of precipitation kinetics and yield strength

The solubility product of Y₂O₃ in ferrite and the diffusion coefficient of yttrium in ferrite have been obtained by fitting a model based on the classical nucleation-growth-coarsening theory of precipitation, as adapted to an anisothermal heat treatment, to experimental small angle neutron scattering results of Y₂O₃ precipitate size distributions in a mechanically alloyed and consolidated Fe-15 at.%Cr-0.13 at.%Y-0.18 at.%O ferritic alloy. This precipitation model is coupled to a dispersed barrier model of structural hardening to predict the yield strength of the alloys as a function of heat treatment. The resulting model and thermodynamic/kinetic properties are then applied to better understand how the precipitation kinetics impact the yield stress in various anisothermal heat treatments, as compared to an isothermal heat treatment. The modeling results clearly indicate that the anisothermal heat treatments can be tailored to establish a higher density and a smaller size distribution of Y₂O₃ precipitates, which also increase the yield stress.     

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.     

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.     

HRTEM study of oxide nanoparticles in K3-ODS ferritic steel developed for radiation tolerance

Crystal and interfacial structures of oxide nanoparticles and radiation damage in 16Cr-4.5Al-0.3Ti-2W-0.37 Y₂O₃ ODS ferritic steel have been examined using high-resolution transmission electron microscopy (HRTEM) techniques. Oxide nanoparticles with a complex-oxide core and an amorphous shell were frequently observed. The crystal structure of complex-oxide core is identified to be mainly monoclinic Y₄Al₂O₉ (YAM) oxide compound. Orientation relationships between the oxide and the matrix are found to be dependent on the particle size. Large particles (>20 nm) tend to be incoherent and have a spherical shape, whereas small particles (<10 nm) tend to be coherent or semi-coherent and have a faceted interface. The observations of partially amorphous nanoparticles and multiple crystalline domains formed within a nanoparticle lead us to propose a three-stage mechanism to rationalize the formation of oxide nanoparticles containing core/shell structures in as-fabricated ODS steels. Effects of nanoparticle size and density on cavity formation induced by (Fe⁸⁺ + He⁺) dual-beam irradiation are briefly addressed.     

Microstructure and oxidation properties of 16Cr–5Al–ODS steel prepared by sol–gel and spark plasma sintering methods

The 16Cr–5Al oxide dispersion strengthened (ODS) ferritic steel was fabricated by sol–gel method in combination with hydrogen reduction, mechanical alloying (MA) and spark plasma sintering (SPS) techniques. The phase characterization, microstructure and oxidation resistance of the 16Cr–5Al–ODS steel were investigated in comparison with the Al free 16Cr–ODS steel. X-ray diffraction (XRD) patterns showed that the Al free and Al added 16Cr–ODS steels exhibited typical ferritic characteristic structure. The microstructure analysis investigated by transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS) revealed that Y–Ti–O complexes with particle size of 10–30 nm were formed in the Al free matrix and Y–Al–O complexes with particle size of 20–100 nm were formed in the Al contained high-Cr ODS steel matrix. These complexes are homogeneously distributed in the matrices. The fabricated 16Cr–5Al–ODS steel exhibited superior oxidation resistance compared with the Al free 16Cr–ODS steel and the commercial 304 stainless steel owing to the formation of continuous and dense Al₂O₃ film on the surface.     

Effect of thermo-mechanical treatments on the microstructure and mechanical properties of an ODS ferritic steel

The Fe-14Cr-2W-0.3Ti-0.3Y₂O₃ oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel was fabricated by mechanical alloying of a pre-alloyed, gas atomised powder with yttria nano-particles, followed by hot isostatic pressing and thermo-mechanical treatments (TMTs). Two kinds of TMT were applied: (i) hot pressing, or (ii) hot rolling, both followed by annealing in vacuum at 850 °C.The use of a thermo-mechanical treatment was found to yield strong improvement in the microstructure and mechanical properties of the ODS RAF steel. In particular, hot pressing leads to microstructure refinement, equiaxed grains without texture, and an improvement in Charpy impact properties, especially in terms of the upper shelf energy (about 4.5 J). Hot rolling leads to elongated grains in the rolling direction, with a grain size ratio of 6:1, higher tensile strength and reasonable ductility up to 750 °C, and better Charpy impact properties, especially in terms of the ductile-to-brittle transition temperature (about 55 °C).     

Tensile and Charpy impact properties of an ODS ferritic/martensitic steel 9Cr–1.8W–0.5Ti–0.35Y2O3

A 9Cr-ODS ferritic/martensitic steel with a composition of 9Cr–1.8W–0.5Ti–0.35Y₂O₃ was fabricated by mechanical alloying and hot isostatic pressing, followed by hot rolling. Tensile properties were measured at room temperature (23 °C) and 700 °C in the rolling direction (LT) and the transverse direction (TL). The ultimate tensile strength (UTS) of the as-rolled samples in both directions reached 990 MPa at 23 °C, and still maintained at 260 MPa at 700 °C. The tensile strength and elongation of the rolling direction was greater than that of the transverse direction. The Charpy impact was tested from −100 to 100 °C in the LT direction. The lower shelf energy (LSE) was more than 65% of the upper shelf energy (USE). The total absorbed energy was separated into the energies for crack initiation and propagation. The propagation energy was always higher than the initiation energy in the range of temperatures tested. The ductile-to-brittle transition temperature (DBTT) of the rolled 9Cr ODS evaluated by an absorbed energy curve was about 0 °C. However, the high LSE and the fracture surface that still contained dimples at lower shelf indicated good toughness of the as-rolled 9Cr ODS steels at temperature of −60 °C.     

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.     

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.     

High heat flux testing of 12-14Cr ODS ferritic steels

The thermal performance of Fe-(12-14)Cr-2W-0.3Ti-0.3Y₂O₃ ODS reduced activation ferritic steels, which are considered as candidate first wall materials for the future fusion power reactors and were manufactured by mechanical alloying in hydrogen and hot isostatic pressing, was assessed by high heat flux (HHF) testing with the electron beam JUDITH facility at the Forschungszentrum Jülich (FZJ), Germany. An analysis of the microhardness and microstructure of the specimens was done before and after HHF tests.In general, both materials present a ferritic (α-Fe, bcc) microstructure with a wide range of grain sizes from 100 to 500 nm up to a few micrometers. The coarse grains are almost dislocation-free, while the smaller ones are surrounded by tangles of dislocations. Oxide and carbide impurities (about a few hundreds nm in size) and a high density of Y-Ti-O nano-clusters, with a mean size of about 5 nm, are also present. The microhardness, density and tensile strength of the 14Cr material are slightly larger than those of the 12Cr material.HHF tests revealed that there is no difference in thermal performance, level of degradation and erosion behaviour of 12Cr and 14Cr ODS steels. The onset of melting of the materials occurs for an energy density between 1 and 1.5 MJ/m². Below this value only some kind of thermal etching takes place. This is a significant improvement compared to stainless steel, for which severe plastic deformation at the material surface was observed.     

Photoluminescence and XEL in Y2O3:Eu phosphor

Eu-activated Y₂O₃ phosphors were prepared by combustion synthesis and also by precipitation techniques. Photoluminescence and X-ray excited luminescence of prepared Y₂O₃:Eu phosphor, under two different techniques were compared and reported in this paper. Y₂O₃:Eu³⁺ phosphor were prepared by precipitation technique followed by annealing at 900 °C. It gives cubic nature of the particle that may be more favourable for high lumen output. X-ray excited luminescence of Y₂O₃:Eu³⁺ phosphors also reported in this paper.     

Determination of nitrogen partitioning coefficients in superduplex stainless steels by NRA using a nuclear microprobe

Superduplex stainless steels (SDSSs) combine the good mechanical behavior and the high corrosion resistance of the ferrite (α-Fe) and austenite (γ-Fe) phases. The SDSSs properties depend strongly on the partitioning of the elements that form the alloy. The ferrite is generally enriched in P, Si, Cr and Mo while the content of Ni, Mn, Cu and N in the austenite phase is higher. Nitrogen is known to be a strong austenite stabilizer and its presence increases the strength and the pitting corrosion resistance of the stainless steels. While the global nitrogen content in SDSSs can be readily determined using elemental analyzers, it cannot be measured at a microscopic scale.In this work, the nuclear microprobe of the Centro Nacional de Aceleradores (Sevilla) was used to obtain the quantitative distribution of nitrogen in SDSSs. A deuteron beam of 1.8 MeV was employed to determine the overall elemental concentration of the matrix by deuteron-induced X-ray emission, whereas the nitrogen partitioning coefficients were obtained by using the ¹⁴N(d, α₀)¹²C nuclear reaction. Mappings of this element show that the nitrogen ratio between the ferrite and austenite phases ranges from 0.3 to 0.6 in the analyzed samples.     

Thermal shock properties of 2D-SiCf/SiC composites

This paper dealt with the thermal shock properties of SiC_f/SiC composites reinforced with two dimensional SiC fabrics. SiC_f/SiC composites were fabricated by a liquid phase sintering process, using a commercial nano-size SiC powder and oxide additive materials. An Al₂O₃–Y₂O₃–SiO₂ powder mixture was used as a sintering additive for the consolidation of SiC matrix region. In this composite system, Tyranno SA SiC fabrics were also utilized as a reinforcing material. The thermal shock test for SiC_f/SiC composites was carried out at the elevated temperature. Both mechanical strength and microstructure of SiC_f/SiC composites were investigated by means of optical microscopy, SEM and three point bending test. SiC_f/SiC composites represented a dense morphology with a porosity of about 8.2% and a flexural strength of about 160 MPs. The characterization of SiC_f/SiC composites was greatly affected by the history of cyclic thermal shock. Especially, SiC_f/SiC composites represented a reduction of flexural strength at the thermal shock temperature difference higher than 800 °C.     

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.     

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.