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The reactive elements (Ti, Al and Zr) have been widely used for the improvement of the strength or the oxide resistance of ODS steels. In the present work, we studied the effect of the reactive elements on the precipitation of oxide particles, especially focusing upon the appropriate amount of the oxide composing elements, by addition of the large amounts of Y2O3 and reactive elements. The mechanically alloyed powders are annealed in a wide temperature range from 773 to 1423 K and then analysed by various analysis methods (small-angle X-ray scattering and X-ray diffraction by synchrotron radiation X-ray, scanning transmission electron microscope combined with energy dispersion X-ray spectroscopy, high-resolution transmission microscopy and 3D-atom probe). Ti, Al and Zr enhanced the growth of oxide particles, opposite to the many studies of Ti- and Zr-added ODS steels, as a result of the addition of Y2O3 and these elements ten times larger than those for usual use. The nucleation and growth of oxide particles are discussed by simulating the critical radius of nucleation and the growth based on the LSW theory. Y4Zr3O12 is stable and easy to precipitate even at lower annealing temperature. All reactive elements enhance the growth of oxide particles because of their big molar volume as compared to that of Y2O3. [ABSTRACT FROM AUTHOR]

9CrODS steel, a candidate fission and fusion structural material, was subjected to hot-rolling with varying parameters of surface temperature and cooling rate just after hot-rolling. The deformation-induced dynamic ferrite transformation was confirmed at the rolling temperature 805 °C above Ar3 (780 °C). This transformation exhibits three characteristic features: transformation for extremely short duration (0.044 second), retaining carbon content equal to the original without long-distance carbon diffusion, and elongated coarse ferrite grains (10 μm). The massive transformation was proposed for the dynamic ferrite transformation from the hot-rolled austenite. The driving force for massive transformation was quantitatively estimated considering dislocations accumulated by hot-rolling. It was also shown that the oxide particles in 9CrODS steel play a critical role for dynamic ferrite transformation by suppressing the dynamic recrystallization at hot-rolling. [ABSTRACT FROM AUTHOR]

Abstract The high-temperature deformation mechanism of the FeCrAl-ODS ferritic steel was investigated at 1000 °C for the creep loading perpendicular to the elongated and aligned grains. The strain rate was varied in the range from the order of 10−2 to 10−7s−1. With decreasing strain rate from 10−2 to 10−5s−1, creep mechanism shifts from conventional dislocation creep pinned by oxide particles to grain boundary sliding (GBS) assisted concomitantly by diffusional creep. With further decreasing strain rate to 10−7s−1, deformation mechanism is drastically changed; group of three grains can move cooperatively, and cooperative GBS (CGBS) was originally recognized. The threshold stress for onset of CGBS was designated as σ t h I (C G B S). Rate limiting process of CGBS is dominated by dislocation movement over the oxide particles so as to relieve the stress accumulation due to CGBS. The σ t h I (C G B S) for CGBS corresponds to one third of the conventional threshold stress for dislocation creep. Highlights • Creep deformation mechanism was studied for FeCrAl-ODS ferritic steels. • At the strain rate of 10−7s−1, cooperative grain boundary sliding (GCGBS) was discovered. • CGBS is dominated by dislocation movement over the oxide particles. • The threshold stress for CGBS was found to be one third of the conventional threshold stress for dislocation creep. [ABSTRACT FROM AUTHOR]

The interaction between oxide particles and dislocations in a 9Cr-ODS ferritic steel is investigated by both static andin situTEM observation under dynamic straining conditions and room temperature. The measured obstacle strength () of the oxide particles was no greater than 0.80 and the average was 0.63. The dislocation loops around some coarsened particles were also observed. The calculated obstacle strength by a stress formula of the Orowan interaction is nearly equaled to the average experimental value. Not only cross-slip system but also the Orowan interaction should be considered as the main interaction mechanism between oxide particles and dislocation in 9CrODS ferritic steel. [ABSTRACT FROM AUTHOR]

In spent fuel nuclear reprocessing plant, nitric acid is the main medium used in PUREX method. The passive film compositions and corrosion resistance of 11 % Cr ferritic/martensitic and 9-15 % Cr oxide dispersion strengthened steels in different nitric acid concentrations were studied. The open circuit potential is shifted toward more noble potential as the concentrations increased from 1 M HNO to 9 M HNO in all the investigated alloy steels. The results of the potentiodynamic polarization plots also exhibited a shift in corrosion potential as the concentrations increased from 1 M to 9 M HNO This shift is undesirable because of risk of overshooting the potential beyond passive region and may result into transpassive corrosion. The X-ray photoelectron spectroscopy results of the passive film analysis are composed mostly of FeO, CrO, and YO, and the depth profile of Fe and Cr concentrations vary depending upon the nitric acid concentration. The surface morphology after the corrosion test does not show intergranular corrosion attack at the nitric acid concentrations studied. It is desirable that materials for use in nitric acid service are resistant to such corrosion induced degradation. [ABSTRACT FROM AUTHOR]

Abstract: The use of transient liquid-phase bonding of 9CrODS steels using Fe–3B–2Si–0.5C filler was investigated for bonding temperature of 1180°C and hold times of 0.5–4.0h. The sequential process, consisting of isothermal melting, solidification and homogenization, was confirmed for bonding the 9CrODS steel. The precipitation of chromium boride found in 19CrODS steel is avoided in 9CrODS steel due to the lower Cr content. Silicon tends to be slightly enriched inside the bonding zone. Agglomeration and coarsening of Y2O3 particles in 9CrODS steel lead to softening inside the bonding zone formed by incipient melting of the foil bonding alloy, and in a diffusion affected zone (DAZ) adjacent to the bonding zone. [Copyright &y& Elsevier]

Abstract: Tungsten is an effective element for solid-solution strengthening and is a ferrite former in 9CrODS ferritic steel, however excessive tungsten leads to Laves phase formation. Appropriate tungsten concentration in 9CrODS ferritic steel was found to be around 2mass%, which produces high temperature strength without Laves phase precipitation. The stress increment due to solid-solution strengthening by tungsten was characterized as relatively small, compared to enhanced ferrite formation, the so called residual α-ferrite. [Copyright &y& Elsevier]

Abstract: It is recognized that the high-temperature strength of 9Cr-ODS ferritic steels is maintained by the presence of a ferrite phase. In order to clarify the formation process of the ferrite phase, 9Cr-ODS ferritic steels containing various contents of Y2O3, i.e., 0mass%, 0.1mass%, 0.35mass%, and 0.7mass%, are prepared by means of mechanical alloying and hot-pressing. The ferrite phase is formed by the addition of 0.35mass% and 0.7mass% Y2O3; however, it is not formed for steels without and with 0.1mass% Y2O3 normalized for 1h at 1050°C. It is considered from the thermodynamic analyses that the pinning of the α–γ interface motion by the dispersed Y–Ti complex oxide particles can be attributed to the retention of the ferrite phase normalized at 1050°C, and this ferrite phase cannot be δ-ferrite that is in equilibrium with the γ-phase. [Copyright &y& Elsevier]

Abstract: Oxide dispersion strengthened (ODS) ferritic steels, which are the most promising candidate materials for advanced fast reactor fuel elements, have exceptional creep strength at 973K. The superior creep property of 9Cr-ODS ferritic steels is ascribed to the formation of a nonequilibrium phase, designated as the residual ferrite. The yield strength of the residual ferrite itself has been determined to be as high as 1360MPa at room temperature from nanoindentation measurements. The creep strength is also enhanced by minimizing the number of packet boundaries induced by the martensitic phase transformation. The creep strain occurs at a lower stress than that necessary for the deformation of the intragrain regions, which are strengthened by an interaction between nanosize oxide particles and dislocations; this occurs by sliding at weaker regions such as at the grain boundaries and packet boundaries. It is found that 9Cr-ODS ferritic steels behave as fiber composite materials comprising the harder residual ferrite and the softer tempered martensite. [Copyright &y& Elsevier]

Abstract: Strain-controlled low cycle fatigue tests were conduced for 9Cr–ODS and 12Cr–ODS steels at 873, 923, 973 and 1023K. The total strain ranges were controlled from 0.5% to 1.5% with strain rate of 0.1%/s. Corresponding plastic strain ranged from 0.01% to 1%. The ODS steels exhibit relatively low level of plastic strain and thus have longer fatigue life in the low total strain region compared to conventional ferritic steels such as Mod. 9Cr–1Mo steel. Neither noticeable cyclic hardening nor softening was observed in ODS steels, whilst Mod. 9Cr–1Mo steel shows an apparent cyclic softening at 873 and 923K. This is attributable to the excellent microstructure stability due to the presence of nano-sized oxide particles dispersed in ODS steels. [Copyright &y& Elsevier]

Abstract: Effects of varying oxygen concentration and final heat treatment on high-temperature strength and microstructure in 9Cr-oxide dispersion strengthened steel (9CrODS) were investigated. It was shown that appropriate control of excess oxygen concentration remarkably improves creep strength of 9CrODS. This creep strength improvement is ascribed to ultra-fine oxide particle dispersion in a part of the grains and increasing austenite (γ) to ferrite (α) diffusional transformation. Enhancement of austenite (γ) to ferrite (α) diffusional transformation leads to suppression of grain boundary sliding. Creep strength of 9CrODS can be remarkably improved by controlling excess oxygen concentration and performing a furnace-cooling heat treatment for austenite (γ) to ferrite (α) diffusional transformation. [Copyright &y& Elsevier]

Oxide-dispersion-strengthened (ODS) ferritic steel incorporating nanoscale oxide particles was produced by mechanical alloying, and the complex oxide particles, comprised of titanium and yttrium, characterized by high-resolution transmission electron microscopy (HRTEM) and characteristic X-ray nanoanalyses. The interface between the ferritic matrix and the major Ti-Y complex oxide was incoherent. From HRTEM and characteristic X-ray nano-analyses, plus the extremely limited literature data on ODS ferritic steels, it was determined that the most stable complex oxide was cubic Y2Ti2O7. which formed by a spontaneous reaction between nano-sized ceramics with a low solubility limit in the metal. [ABSTRACT FROM AUTHOR]

Considerable attention has been paid to the research of the electron tomography due to determine the three-dimensional (3D) structure of materials [1]. One of the electron tomography techniques, focused ion beam/scanning electron microscopy (FIB-SEM) imaging has advantages of high resolutions (10 nm), large area observation (μm order) and simultaneous energy dispersive x- ray microanalysis (EDS)/ electron backscatter diffraction (EBSD) analysis. The purpose of this study, three-dimensional EBSD analysis of ODS ferritic steel which carried out cold work using FIB-SEM equipment was conducted, and it aimed at analyzing the microstructure obtained there. The zone annealing tests were conducted for ferritic steel [2,3], whichwere produced through mechanical alloying and hot-extrusion. After zone annealing, specimens were mechanically polished with #400∼4000 emery paper, 1 µm diamond paste and alumina colloidal silica. The serial sectioning and the 3D-electron backscattering diffraction (3D-EBSD) analysis were carried out. We made the micro pillar (30 x 30 x 15 µm). The EBSD measurements were carried out in each layer after serial sectioning at a step size and milling depth was 80 nm with 30 slices. After EBSD analysis, the series of cross-sectional images were aligned according to arbitrarily specified areas and then stacked up to form a volume. Consequently, we obtained the 3D-IPF maps for ODS ferritic steel. In this specimen, the {111} and {001} grains are layered by turns. In addition, the volume fraction value of both plane are similar. The aspect ratio increases with specimen depth. The 3D-EBSD mapping is useful to analysis of the bulk material since this method obtain many microstructure information, such a shape, volume and orientation of the crystal, grain boundary. [ABSTRACT FROM PUBLISHER]

Abstract: The 15wt% CrODS steels (15CrODS steels) were manufactured as the high-temperature fusion reactor materials and their tensile properties at 1273K were correlated with the oxide dispersion strengthening before and after aging. Fine dispersion of the oxide particles at initial condition in 15CrODS steels gives significantly higher strength compared with commercial MA956. The strength decrease in 15CrODS steels after aging at 1473K for 100h is attributed to coarsening of oxide particles, thus weakening of dispersion strength. [Copyright &y& Elsevier]

Abstract: The microstructure and tensile properties at 973K in 9CrODS steels were investigated with respect to various fractions of the residual ferrite from zero to 47vol.%. The formation of the residual ferrite was discussed from a balance between a chemical driving force for α to γ reverse transformation and the oxide particle pinning force, while 0.04mass% carbon sample could contain an equilibrium δ-ferrite. With increasing volume fraction of the residual ferrite, strength at 973K increases but ductility decreases, which is attributed to the fact that the residual ferrite is harder than the tempered martensite. [Copyright &y& Elsevier]

Highlights: [•] Aluminum addition to ODS steel improves compatibility between U-Zr fuel and the ODS steels. [•] It increases the threshold temperature for reaction layer formation by 50K. [•] The compatibility improvement mechanism was discussed using general thermodynamic data. [ABSTRACT FROM AUTHOR]

Abstract: Microstructural observation was performed on a neutron-irradiated oxide dispersion strengthened (ODS) ferritic steel with emphasis on oxide behavior, including phase stability under irradiation at elevated temperature (∼973K). Transmission electron microscopy observation of the Y–Ti complex oxide particles showed they were fine (∼40nm) whereas the Ti-oxide particles were relatively coarse (∼300nm). Dispersion parameters of oxide particles, such as mean size and number density, changed due to irradiation. This implies recoil resolution of the oxide particles. When irradiated at 973K, some Y–Ti complex oxides survived and interacted with the dislocation structures, which delayed the dislocation recovery and stabilized the elongated grain structure. It is considered that oxide particles could be effective pinning points of dislocations in motion under irradiation to a dose of ∼100dpa. [Copyright &y& Elsevier]

Oxide dispersion strengthened (ODS) steels are being developing for application as advanced fast reactor cladding and fusion blanket materials, in order to allow increased operation temperature. Oxidation testing of ODS steel was conducted under a controlled dry air atmosphere to evaluate the high temperature oxidation behavior. This showed that 9Cr-ODS martensitic steels and 12Cr-ODS ferritic steels have superior high temperature oxidation resistance compared to 11 mass% Cr PNC-FMS and 17 mass% Cr ferritic stainless steel. This high temperature resistance is attributed to earlier formation of the protective α-Cr2O3 on the outer surface of ODS steels. [Copyright &y& Elsevier]

Highlights: [•] The passive film of F/M and ODS steels is evaluated in different electrolytic media. [•] The XPS analysis shows that the passive film has Fe2O3 and Cr2O3 along with Y2O3. [•] Low pitting corrosion resistance was observed in chloride media. [•] Presence of Cl− significantly reduces the pitting corrosion in both the steels. [•] The SEM micrographs shows severe pitting corrosion attack in chloride solutions. [ABSTRACT FROM AUTHOR]

Abstract: The effects of alloying elements and temperature on corrosion behavior in supercritical water (SCW) were investigated for oxide dispersion strengthened (ODS) steels to design alloy compositions for corrosion-resistant ODS ferritic steels. Corrosion tests of the ODS steels were carried out at 400, 500, and 600°C under 25MPa with 8ppm of dissolved oxygen. The addition of 4wt.%Al is effective to improve the corrosion resistance of 16Cr-ODS steel, suggesting that an adequate combination between Cr and Al content ranges (14–16)Cr and (3.5–4.5)Al. The addition of Ce or Zr to W-added ODS steel is expected to increase its corrosion resistance. At higher temperatures, the addition of 4wt.%Al to 16Cr-ODS steel induces better corrosion resistance rather than the temperature of 400°C. It is considered that for 16Cr-ODS steels with Al, the continuous alumina mainly suppresses the inward oxygen diffusion and the Fe–Cr spinel adds to the suppression of oxygen diffusion. [Copyright &y& Elsevier]

Abstract: Anisotropic fracture behavior of SOC-1 oxide dispersion strengthened (ODS) ferritic steel has been investigated for a hot-extruded bar by tensile tests and Charpy impact tests. These mechanical properties are better in the longitudinal direction than in the transverse directions against extrusion direction (ED). Fracture surface observations by scanning electron microscopy and auger electron spectroscopy indicated bundle-like morphology with existence of segregation/precipitation/inclusions along ED. Pole figures of the hot-extruded bar characterized using electron back scattering diffraction (EBSD) technique and X-ray diffraction exhibited 〈110〉 fiber texture formation along ED. The EBSD orientation map showed a complex bundle-like grain morphology which consists of elongated grains having a specific orientation 〈110〉// ED and relatively isotropic and small grains having other orientation. The results conclude that the combined effects of observed elongated grain morphology and these small grains with segregation/precipitation/inclusions along ED can explain the anisotropic fracture behavior of the hot-extruded ODS ferritic steel. [Copyright &y& Elsevier]

Abstract: The effect of aging on the ductile to brittle transition behavior of recrystallized high-Cr ODS ferritic steels were investigated using small-punch test technique. In order to reduce the anisotropy of microstructure in the as-received ODS steels, they were recrystallized after cold rolling. The ODS steel without Al hardly recrystallized at 1200°C for 90min, while the ODS steel with Al significantly recrystallized. Ductile–brittle transition curves were drawn for as received, recrystallized, and thermally aged (at 450°C for 1440h). Before aging, both materials had good impact properties with low ductile–brittle transition temperatures and high USEs. The ductile–brittle transition temperatures are not dependent of the grain size. The aging degraded their impact properties for ODS steel with Al. However, almost no degradation occurred for the ODS steel without Al. SEM observation showed an influence of the cold-rolling direction on the fracture behavior especially at low temperatures. [Copyright &y& Elsevier]

Abstract: A successful example of high-Cr oxide dispersion strengthened (ODS) steels development is introduced with showing key technologies to overcome the issues to meet material requirements for next generation nuclear systems as well as fusion blanket systems. Corrosion issue requires Cr concentration more than 14wt.%, but aging embrittlement issue requires it less than 16wt.%. An addition of 4wt.%Al is effective to improve corrosion resistance of 16wt.%Cr-ODS steel in supercritical water (SCW) and lead–bismuth eutectics (LBE), while it is detrimental to high-temperature strength. An addition of small amount of Zr or Hf results in a significant increase in creep strength at 973K in Al-added ODS steels. Feasibility of high-Cr ODS steel without Al addition is assessed for fusion application in terms of corrosion resistance in SCW. [Copyright &y& Elsevier]

Abstract: The phase and metal/oxide interface structure of the nanometer-scale particles in an Al-alloyed high-Cr oxide dispersion strengthened ferritic steel extruded at 1150°C and 1050°C were characterized by high-resolution transmission electron microscopy and diffraction contrast techniques, including weak beam electron microscopy. After extrusion at 1150°C, yttrium–aluminum–hexagonal (YAH, YAlO3) and yttrium–aluminum–perovskite (YAP, YAlO3) oxides (diameter ⩽10nm) constitute ∼55% and 38% of the particles, respectively; ∼78% of the particles (4.5–10nm in diameter), which include 40% YAH oxide and 38% YAP phase with misfit (translational) moiré fringe spacing of 2.15nm and 1.65nm, respectively, are semi-coherent with the matrix. After extrusion at 1050°C, almost all the particles are YAH phase, and ∼86.5% (diameter <4.5nm) are coherent with the matrix. The coherency of the oxides is size dependent. The crystallographic orientation correlations of the oxides and matrix were found. [ABSTRACT FROM AUTHOR]

Abstract: In order to examine irradiation effect on creep rupture strength of Oxide Dispersion Strengthened (ODS) steel claddings, the in-pile creep rupture test was conducted using Material Testing Rig with Temperature Control (MARICO)-2 in the experimental fast reactor JOYO. Fourteen creep rupture events were successfully detected by the temperature change in each capsule and the gamma-ray spectrometry of the cover gas. Time to creep ruptures of six ODS steel specimens were identified by means of Laser Resonance Ionization Mass Spectrometry (RIMS), and no irradiation effect on creep rupture strength was confirmed within the irradiation condition in the MARICO-2 test. [Copyright &y& Elsevier]

Abstract: The reduced activation ferritic steel (RAFS) has been selected as structural material for a variety of blanket systems for ITER test blanket modules (TBM). In the evaluation of integrated performance of ferritic steels as structural components of blanket systems, there are unique issues as well as common issues for each blanket system. One of the unique issues for each system is the compatibility of ferritic steels with the coolant materials. The corrosion rate of ferritic steels in hot water, super critical pressurized water (SCPW), humid air, Pb–17Li, lithium and Flibe at various temperatures is reviewed in this work. Efforts to improve corrosion resistance have been made, taking the alloy design into account. A dispersion of yttria was effective to improve corrosion resistance of a RAFS. The compatibilities of RAFSs with hot water, Pb–17Li, lithium and Flibe are considered to be good enough for the TBM applications. The liquid metal embrittlement (LME) is considered to be a critical issue for the utilization of RAFSs for the lithium systems. Several issues towards DEMO and beyond are shown from the compatibility point of view. [Copyright &y& Elsevier]

Abstract: Thermal helium desorption measurements were performed to investigate the difference in the helium trapping and accumulation behavior among a reduced activation ferritic (RAF) steel and oxide dispersion strengthening (ODS) steels after implantation of He+ ions at room temperature. Thermal helium desorption spectra (THDS) were obtained during annealing to 1200°C at a heating rate of 1°C/s. The THDS of the ODS steels are very similar to that of the RAF steel, except for the presence of the peak in the temperature range from 800 to 1000°C, where the α–γ transformation related helium desorption from the γ-phase is considered to occur in the 9Cr-ODS martensitic steels. The fraction of helium desorption becomes larger at higher temperatures, and this trend is increased with the amount of implanted helium. In the 9Cr-ODS steels, the fraction of helium desorption by bubble migration mechanism was smaller than that in the RAF steel. This suggests that the bubble formation was suppressed in the ODS steels. In the 12Cr-ODS steel, the fraction of helium desorption by bubble migration reached more than 90%, suggesting that the trapping capacity of martensite phase in the 9Cr-ODS steel is rather large. [Copyright &y& Elsevier]

Oxide dispersion strengthened (ODS) ferritic and martensitic steels are promising materials for fuel cladding in fast reactors and for structural materials in fusion reactors. ODS steels for fuel cladding in fast reactors have been developed by Japan Nuclear Cycle Development Institute (JNC). The first generation ODS steels had a serious problem from anisotropy in their mechanical properties. Recently, this problem has been solved by controlling grain shape as equiaxed grains. Three types of modified ODS steels were irradiated in the experimental fast reactor JOYO for investigation of the irradiation-induced microstructural changes. The dispersed oxide particles were very stable when irradiated. In addition, dislocation structure development was minor. The high resistance to irradiation-induced microstructural changes in the ODS steels was attributed to oxide particles which acted as sinks for point defects. [Copyright &y& Elsevier]

The effects of fast neutron irradiation on ring-tensile properties of the oxide dispersion strengthened (ODS) ferritic/martensitic steel claddings with improved microstructural isotropy from recrystallization and α–γ phase transformation were investigated. The samples were irradiated in the experimental fast reactor JOYO using the material irradiation rig at temperatures between 670 and 807 K to fast neutron fluences ranging from 5.0 × 1025 to 3.0 × 1026 n/m2 (E>0.1 MeV). The post-irradiation ring-tensile tests were carried out on samples with five sets of irradiation conditions. The experimental results showed that strengths were increased by 10% due to irradiation. Microstructural improvement considerably increased elongations for these samples and specimens retained respectable elongation after irradiation. The ring-tensile properties of these ODS steel claddings remain excellent within the fluence level achieved in this study. [Copyright &y& Elsevier]

Abstract: Ferritic–martensitic (FM) steels have been widely considered as structural materials for future fusion and fission systems due to their excellent swelling resistance and other properties under irradiation. NF616 is a newer generation FM steel that provides higher allowable operating temperatures, but there is limited information on the development of irradiated microstructures or associated mechanical properties in NF616. Studying the irradiation-induced defects as a function of dose could help to better understand the mechanisms of damage evolution. Proton irradiation at 773K, up to 1, 2 and 3dpa, was performed on NF616 to investigate damage evolution at different doses. Irradiation-induced defects, such as clusters, loops and dislocations, were investigated. [Copyright &y& Elsevier]