Cr Oxide Dispersion Strengthened Steel Research Articles

Determination of the fracture toughness of a 9Cr ODS steel fuel cladding using the pin loading Tension test

A Pin Loading Tension (PLT) test was developed to evaluate the fracture behavior of thin-walled tubes. The fracture resistance curve (J-R curve) was determined from the load-CMOD curve using the single specimen technique based on the elastic unloading method following the procedure described in the ASTM E1820-22e1 standard [1]. However, the standard does not provide the necessary functions, geometric function f(a/W) and plastic factor ηpl, to apply the methodology to the PLT test. In this work, the geometric functions needed to evaluate the crack length from the unloading compliance, which allows to highlight effects not seen in the past and the elastic and plastic parts of the J-integral were determined using finite element analysis. Special care was taken to consider the effects of contact and friction between the test specimen and the loading device. The methodology was then applied to experimentally study the crack growth resistance of a 9Cr ODS (Oxide Dispersion Strengthened) steel tube, which is a candidate material for fuel claddings of future fast-neutron reactors.

Ultrafine-grained oxide-dispersion-strengthened 9Cr steel with exceptional strength and thermal stability

As important structural materials for sodium-cooled fast reactors and fusion reactors, oxide-dispersion-strengthened (ODS) steels of the ferritic/martensitic (F/M) type containing 8–12 wt% Cr have been extensively studied. The F/M matrixes in these ODS steels, however, are often microcrystalline (between ∼ 1 and 10 μm). Decreasing the grain size to an ultrafine scale (0.1 – 1 μm) should increase both the strength and the radiation tolerance. Here, by means of mechanical alloying and high-pressure/high-temperature consolidation techniques, we prepared novel ultrafine-grained (UFG) hafnium-doped 9Cr ODS steels. The UFG 9Cr ODS steels are composed of matrix(es) with an average grain size of ∼ 0.17 μm and high-density (5 × 1022 m−3) hafnium-enriched oxide nanoprecipitates with an average particle size of ∼ 6 nm. No significant grain growth is observed in UFG 9Cr ODS steel annealed at 1000 °C. In contrast, the grain of previous UFG T91 alloys is stable below 500 °C. The extremely high thermal stability is caused by the combined kinetic and thermodynamic stabilization effects, i.e., the grain growth is kinetically hindered by the high-density oxide nanoprecipitates and thermodynamically hindered by the lowered grain boundary energy resulting from the segregation of elemental Mo at grain boundaries. The yield strength of UFG 9Cr ODS steel is 2449 MPa, approx. twice that of previously reported microcrystalline 9Cr ODS steels and UFG T91 alloys. The high yield strength is mainly caused by dislocation and grain boundary strengthening effects. The strategies found in this study should help design UFG F/M ODS steels with enhanced structural stability and mechanical property for nuclear reactor applications.

Method for Evaluating Irradiation Effects on Flow Stress in Fe-9%Cr ODS Using TEM In Situ Cantilevers

Transmission electron microscopic (TEM) in situ mechanical testing has become a widely utilized tool for simultaneously measuring mechanical properties and understanding fundamental deformation mechanisms in irradiated and nuclear materials. Although tensile and compression specimen geometries are among the most common, opportunities remain for investigating alternative geometries that could provide unique insights into the plasticity of irradiated materials. This work demonstrates a new TEM in situ cantilever beam configuration. Cantilevers are produced from as-received and proton-irradiated (1 dpa, 500°C) Fe-9%Cr oxide dispersion-strengthened steel. Flow stress is measured using a TEM in situ depth-sensing mechanical testing holder. A 200-MPa increase in flow stress is measured due to irradiation. Size effects arise when the intrinsic (i.e., microstructural) size approaches the extrinsic (i.e., external dimensions) size and can be described using a power law relationship as a function of the material microstructure and cantilever dimensions.

Characterization of nano-sized particles in 14%Cr oxide dispersion strengthened (ODS) steel using classical and frontier microscopy methods

Oxide dispersion strengthened (ODS) steels exhibit superior mechanical properties and irradiation resistance due to nano-sized oxides, highly dispersed in the metallic matrix. The mechanical properties are affected by the structure, composition, size and density of the nano-sized oxides. Despite numerous reports on the characterization of these oxides, ambiguity regarding their composition, crystallographic structure and orientation relationship with the matrix remains. In the present study, characterization of the crystallographic structure of oxide particles existing in 14%Cr ODS steel was performed using classical and novel transmission electron microscopy (TEM) methods. 3D dispersion, density and composition of these oxides were evaluated by atom probe tomography (APT). Three populations of particles were detected: highly dispersed, 3–20 nm Fe(Cr,Ti,Y)O particles with spinel structure; 50–150 nm YTiO3 and large (100–200 nm) particles identified as cubic TiC.The spinel-type particles displayed Bain and Kurdjumov-Sachs orientation relationships (OR) with the ferritic matrix. Applying electron diffraction tomography, the YTiO3 structure was attributed to the GdFeO3 (distorted perovskite)-type and its lattice parameters were refined as a = 5.46 Å, b = 7.66 Å and c = 5.28 Å. Orientation relationship of the YTiO3 particles and Fe matrix were determined as [110]Fe//[210]oxide and (110)Fe//(002)oxide.

Mechanical properties evaluation and plastic instabilities of Fe-9%Cr ODS steels

The Fe-9%Cr oxide-dispersion-strengthened (ODS) steels were fabricated by mechanical alloying, hot isostatic pressing and subsequent different thermomechanical treatments. The microstructure of the Fe-9%Cr ODS steels was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). The SEM analysis revealed that the matrixes of quenched Fe-9%Cr ODS steel and tempered Fe-9%Cr ODS steel are martensitic and ferritic, respectively. Large-sized particles can be found distributed along the grain boundaries by SEM analysis, while the TEM analysis showed that a large number of fine nano-sized Y-Ti-O particles distributed in the grains. The room-temperature tensile properties of Fe-9%Cr ODS steels treated at different conditions were measured and the uniform elongations (UEs) were evaluated. The UEs of the researched ODS steels are less than 10% probably due to the decohesion phenomenon caused by the large-sized (200–500 nm) particles distributed along the grain boundaries. Furthermore, Charpy impact tests were done to investigate the impact properties of the tempered Fe-9%Cr ODS steel from −180°C to 200°C. The ductile-brittle transition temperature (DBTT) of tempered Fe-9%Cr ODS steel is about −60°C as judged by the energy-temperature curve. These results are important for the mechanical processing and component fabrication of ODS steels.

Si-containing 9Cr ODS steel designed for high temperature application in lead-cooled fast reactor

The oxide-dispersion strengthened (ODS) ferritic steels have been considered as promising candidate structural materials for liquid lead-bismuth eutectic (LBE) cooled reactors, and the corrosion of the ODS steel in liquid LBE presents a critical challenge. In order to improve the corrosion resistance of the ODS steel, a type of Y-Si-O strengthened ODS steel was designed and fabricated. The grain structure and dispersed nanoparticles were carefully characterized by the scanning electron microscope (SEM), the transmission electron microscope (TEM), the high-resolution transmission electron microscope (HRTEM) and the atomic probe tomography (APT) methods. Y2Si2O7 and Y2SiO5 nanoparticles both with monoclinic cubic crystal structure were found to be dominated in the matrix. And the number density and average diameter of the distributed nanoparticles were counted to be 1.8 × 1024 m−3 and ∼6 nm, respectively. The three dimensions APT reconstruction revealed that the as-received ODS alloy contained a high number density of nanometer scale yttrium-, silicon-, vanadium-, and oxygen-enriched nano-particles Furthermore, the high temperature property and corrosion resistance of the ODS steel were evaluated preliminarily. The miniature specimen tensile tests performed at 550 °C showed satisfactory high tensile strength of the ODS steel. The corrosion resistance was significantly improved by the addition of silicon according to the test of 1000 h exposure to liquid LBE.

Long-term thermal-aging stability of oxide-dispersion-strengthened ferritic steels at 753 K

Oxide-dispersion-strengthened (ODS) ferritic steels are promising candidates for structural applications in the future nuclear reactors. The higher chromium contents of ODS ferritic steels, the better the corrosion resistance, which can meet the harsh corrosion environment of the advanced reactors. However, increasing the Cr content may also lead to the brittleness of the ODS steels when serving at high temperatures. The ODS ferritic steels with different Cr contents (12, 16 and 18 wt% Cr, respectively) were fabricated by mechanical alloying, hot isostatic pressing and forging. Mechanical properties and microstructure evolution of the ODS ferritic steels after aging at 753 K for 2000 h were investigated. It is found that both Vickers hardness and yield strength of 18%Cr ODS ferritic steel were strongly increased and the impact energy was decreased after aging at 753 K. In order to explore the reasons for changes in the mechanical properties, the fracture surfaces were characterized by scanning electron microscopy, and microstructures after aging were observed by transmission electron microscopy. The impact fracture of 18%Cr ODS ferritic steel belongs to quasi-cleavage facture, which is consistent with its very low impact energy. The grain size and dispersed oxide particles of different ODS steels are very stable. M23C6 carbide and M2C carbide were found in 12%Cr ODS steel and 16%Cr ODS steels, respectively.

An in situ SANS study of nanoparticles formation in 9Cr ODS steel powders

The characteristics of strengthening nanoparticles have a major influence on the mechanical property and irradiation resistance of oxide dispersion strengthened (ODS) steels. In situ small angle neutron scattering (SANS) experiments were performed to determine the evolution of nanoparticles in 9Cr ODS steel powders varied with temperature. Initial-added Y2O3 was completely dissolved into the steel matrix after mechanical alloying. Nanoparticles emerge after annealing at 600°C for 10min and coarsen slightly after 12h. The radius of nanoparticles increases from 0.73nm at 600°C to 1.63nm at 1000°C, while the volume fraction decreases with increasing annealing temperature.

Investigation of the relationships between mechanical properties and microstructure in a Fe-9%Cr ODS steel

Ferritic-martensitic Oxide Dispersion Strengthened (ODS) steels are potential materials for fuel pin cladding in Sodium Fast Reactor (SFR) and their optimisation is essential for future industrial applications. In this paper, a feasibility study concerning the generation of tensile specimens using a quenching dilatometer is presented. The ODS steel investigated contains 9%Cr and exhibits a phase transformation between ferrite and austenite around 870 °C. The purpose was to generate different microstructures and to evaluate their tensile properties. Specimens were machined from a cladding tube and underwent controlled heat treatments inside the dilatometer. The microstructures were observed using Electron Backscatter Diffraction (EBSD) and tensile tests were performed at room temperature and at 650 °C. Results show that a tempered martensitic structure is the optimum state for tensile loading at room temperature. At 650 °C, the strengthening mechanisms that are involved differ and the microstructures exhibit more similar yield strengths. It also appeared that decarburisation during heat treatment in the dilatometer induces a decrease in the mechanical properties and heterogeneities in the dual-phase microstructure. This has been addressed by proposing a treatment with a much shorter time in the austenitic domain. Thereafter, the relaxation of macroscopic residual stresses inside the tube during the heat treatment was evaluated. They appear to decrease linearly with increasing temperature and the phase transformation has a limited effect on the relaxation.

Synchrotron study on load partitioning between ferrite/martensite and nanoparticles of a 9Cr ODS steel

Oxide dispersion strengthened (ODS) steels exhibit exceptional radiation resistance and high-temperature creep strength when compared to traditional ferritic and ferritic/martensitic (F/M) steels. Their excellent mechanical properties result from very fine nanoparticles dispersed within the matrix. In this work, we applied a high-energy synchrotron radiation X-ray to study the deformation process of a 9Cr ODS steel. The load partitioning between the ferrite/martensite and the nanoparticles was observed during sample yielding. During plastic deformation, the nanoparticles experienced a dramatic loading process, and the internal stress on the nanoparticles increased to a maximum value of 3.7GPa, which was much higher than the maximum applied stress (∼986MPa). After necking, the loading capacity of the nanoparticles was significantly decreased due to a debonding of the particles from the matrix, as indicated by a decline in lattice strain/internal stress. Due to the load partitioning, the ferrite/martensite slightly relaxed during early yielding, and slowly strained until failure. This study develops a better understanding of loading behavior for various phases in the ODS F/M steel.

Hot deformation behavior and processing map of a 9Cr ferritic/martensitic ODS steel

The hot deformation behavior of 9Cr oxide-dispersion-strengthened (ODS) steel fabricated through the process of mechanical alloying and hot isostatic pressing (HIP) as investigated through hot compression deformation tests on the Gleeble-1500D simulator in the temperature range of 1050–1200°C and strain rate range of 0.001s−1–1s−1. The relationship between the rheological stress and the strain rate was also studied. The activation energy and the stress and material parameters of the hyperbolic-sine equation were resolved according to the data obtained. The processing map was also proposed. The results show that the flow stress decreases as the temperature increases, and that decreasing of the strain rate of the 9Cr ODS steel results in a positive strain rate sensitivity. It is clear that dynamic recrystallization is influenced by both temperature and strain rate. The results of this study may provide a good reference for the selection of hot working parameters for 9Cr ODS steel. The optimum processing domains are at 1200°C with a strain rate of 1s−1 and in the range of 1080–1100°C with a strain rate between 0.018s−1 and 0.05s−1.

The corrosion resistance and passive film compositions of 12% Cr and 15% Cr oxide dispersion strengthened steels in nitric acid media

The passive film compositions and corrosion resistance of 12% Cr and 15% Cr oxide dispersion strengthened (ODS) steels in 3M HNO3 to 9M HNO3 was evaluated. The potentiodynamic polarization plots exhibited a shift in corrosion potential, and higher passive current density with increasing HNO3 concentrations. In 12% Cr ODS steel, high corrosion rate was observed in all HNO3 concentrations. However, low corrosion rate observed in 15% Cr ODS steel was attributed to Al2O3 enrichment. The TEM analysis indicated a complex Y2Hf2O7 formation in 15% Cr ODS steel, and this may have a role in suppressing intergranular corrosion attack.

The influence of thermomechanical processing on the microstructure and mechanical properties of 13.5Cr ODS steels

Reduced activation oxide dispersion strengthened (ODS) ferritic steels are the most promising structural materials for future fusion power plants and innovative fission nuclear reactors. The structure and mechanical properties of fine-grained 13.5% Cr ODS ferritic steels, processed by hot cross rolling to large strains of 70% and subsequent annealing have been investigated. The microstructure after hot cross rolling is partially recrystallized. The texture of the ODS ferritic steel after rolling and annealing has a sharp rotated cube ({100} 〈110〉) texture component. The γ-fiber component (〈111〉||normal direction) typical of recrystallized rolled ferritic steels is absent. Similar values of tensile strength are obtained for rolled samples regardless of the sample orientation in comparison with hot isostatically pressed samples whereas a substantial enhancement in total elongation (22%) is present at 500°C. Cross-rolled ODS steels represent an increase in upper shelf energy to 2J compared with hot isostatically pressed samples. The relatively poor toughness properties, compared to 9Cr ODS RAFM steel EUROFER ODS, can be attributed mainly to the absence of the γ-fiber texture component, bimodal grain size distribution and high amounts of interstitial impurities (O, N).

Corrosion Resistance of 9% Cr Oxide Dispersion-Strengthened Steel in Different Electrolytic Media

The corrosion resistance and passive film compositions of 9% Cr oxide dispersion-strengthened steel in different acidified and chloride media was evaluated. The results presented below show that with increasing concentrations of 1 M to 11.5 M nitric acid (HNO3), the open-circuit potential reveals a more noble potential, while in chloride-containing media, a less noble potential was observed. The potentiodynamic polarization plots exhibited higher breakdown/transpassive potentials in acidic media with 0.5 M sulfuric acid (H2SO4) and 1 M to 11.5 M HNO3, and there was a shift in the corrosion potential toward the transpassive region as the HNO3 concentration increased from 1 M to 11.5 M. However, no intergranular corrosion attack was observed at the HNO3 concentrations studied. In acidic-chloride media (0.5 M H2SO4 with 0.1 M and 0.5 M sodium chloride [NaCl]) and in chloride only media (0.1 M NaCl and 0.5 M NaCl) poor pitting corrosion resistance was attributable to the microstructural inhomogeneity and inclusions in the steel. The x-ray photoelectron spectroscopy analysis indicated that the passive film has iron(III) oxide (Fe2O3) and chromium(III) oxide (Cr2O3), along with yttrium oxide (Y2O3). The scanning electron microscopy (SEM) examinations showed that the pits on the specimen were hemispherical and formed lace-like patterns. The corrosion properties affected by the dispersed oxide are also discussed in the paper.

The passive film characterization and anodic polarization behavior of 11% Cr ferritic/martensitic and 15% Cr oxide dispersion strengthened steels in different electrolytic solutions

The ferritic/martensitic (F/M) and oxide dispersion strengthened (ODS) steels are the most promising candidate materials for future nuclear power plants. In the present work, the passive film compositions and its correlation with anodic polarization behavior of 11% Cr F/M and 15% Cr ODS steels was examined in different electrolytic solutions of borate buffer, acidic, acidified chloride and in chloride solution. The X-ray photoelectron spectroscopy analysis of the passive film reveals the existence of the layers of Cr2O3 and Fe2O3 and in 15% Cr ODS steel along with Y2O3. The open circuit potential and potentiodynamic anodic polarization measurements show that pitting corrosion resistance is strongly dependent on electrolytic solutions and passive film compositions. The ODS steel indicated increased breakdown potential and wider passive range as compared to F/M steel in chloride containing media. In a non-chloride environment, this difference is not prominent. However, low pitting corrosion resistance of both steels in chloride solutions was attributable to Cl− and microstructural inhomogeneity. The SEM micrographs of surface morphology of corrosion attack showed severe pitting corrosion attack in chloride solutions. The anodic polarization behavior of F/M and ODS steels in different electrolytic solutions was studied for this work in relation to passive film compositions.

Response of nanoclusters in a 9Cr ODS steel to 1 dpa, 525 °C proton irradiation

Ferritic–martensitic (F/M) alloys are expected to play an important role as cladding or structural components in Generation IV and other advanced nuclear systems operating in the temperature range 350–700 °C and to doses up to 200 displacements per atom (dpa). Oxide dispersion strengthened (ODS) F/M steels have been developed to operate at higher temperatures than traditional F/M steels. These steels contain nanometer-sized Y–Ti–O nanoclusters for additional strengthening. A proton irradiation to 1 dpa at 525 °C has been performed on a 9Cr ODS steel to determine the nanocluster stability at low dose. The evolution of the nanocluster population and the composition at the nanocluster–matrix interface were studied using electron microscopy and atom probe tomography. The data from this study are contrasted to those from a previous study on heavy-ion irradiated 9Cr ODS steel.

Corrosion Studies of Candidate Materials for European HPLWR

The High Performance Light Water Reactor (HPLWR) design is one of the concepts chosen for Generation IV reactors; however, the material requirements for HPLWR offer challenges because of the extreme operating temperatures and pressures. Consequently, general corrosion rates were studied in water at 300 to 650°C at supercritical pressure using weight gain measurements. Oxide thicknesses were determined from cross-section samples. The compositions of the oxide layers were analyzed using scanning electron microscopy in conjuction with energy dispersive spectroscopy. The surface layers of selected samples were analyzed also by X-ray diffraction. The test matrix included ten materials from four alloy classes: ferritic/martensitic steels, oxide dispersion strengthened (ODS) steels, austenitic stainless steels, and nickel-base alloys. A high oxidation resistance was seen in Ni-base alloy 625, austenitic stainless steels with high Cr content (>18 wt% Cr), and an ODS steel containing 20% Cr at all applied test temperatures (300 to 650°C). The oxidation rates of austenitic stainless steels with lower Cr content, 15 to 18%, increase considerably at temperatures >500°C. The oxidation rates of 9% Cr ODS steels were moderate or high at all temperatures. Ferritic/martensitic steels showed high oxidation rates at all temperatures.

Creep Behavior and Microstructure of 8Cr Oxide Dispersion Strengthened Martensitic Steel

In international thermonuclear experimental reactor (ITER), reduced activation ferritic/martensitic steels will be used for plasma-facing materials. However, it is necessary to raise the temperature of operation in order to elevate efficiency of electric power generation by using the material which is more excellent in strength at elevated temperature. Oxide dispersion strengthened (ODS) steels are promising candidate for high temperature materials of a nuclear fusion reactor. There are many reports that ODS steels show very high creep strength, but there are few reports on creep deformation mechanism. In this work, creep deformation behavior of 8 wt% Cr ODS steel was investigated. This ODS steel had high density of fine dispersed Y2Ti2O7 particles and -ferrite grains elongated along the hot-rolling direction. The creep curve showed a low creep strain rate until specimen ruptured. Vickers hardness of the gauge part of specimens in interrupted creep tests decreased with increasing the loading time. However, that of the grip part did not change significantly. Accordingly, although dynamic recovery occurred in the ODS steel, it had not affected the creep deformation rate.

Oxidation of 9Cr oxide dispersion strengthened steel exposed in supercritical water

The oxidation behavior of a 9 at.% Cr oxide dispersion strengthened (ODS) ferritic/martensitic (F/M) steel exposed to supercritical water at different application temperatures was examined. For comparison, two non-ODS F/M steels HCM12A (∼12 at.% Cr) and NF616 (∼9 at.% Cr) were also examined. The oxidized samples were characterized using gravimetry, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction, electron back-scatter diffraction and transmission electron microscopy/selected area diffraction. A lower weight gain was consistently observed in 9Cr ODS steel at 500 °C and 600 °C. During exposure, the formation of an internal oxidation layer in the 9Cr ODS steel played a key role in establishing the oxidation behavior. In the 600 °C exposure, grain boundary diffusion of cations may no longer proceed dominantly as it did in the 500 °C exposure. Volume diffusion was likely accelerated and the bulk grains became a more important path for element migration, and the benefit from a small quantity of yttrium (0.28 wt%) in the steel became limited. However, because of the fine grain size of the 9Cr ODS steel, Cr quickly segregated to the ferritic grain boundary region in the internal oxidation layer and especially to the internal oxidation layer/base steel interface, resulting in the formation of Cr-enriched spinel ribbons and a Cr-enriched continuous spinel layer. The microstructure that developed slows down the further diffusion of both cations and anions at 600 °C.

Particle size effects in mechanically alloyed 9Cr ODS steel powder

The superior creep properties of 9Cr ODS (Oxide Dispersion Strengthened) steels result from small oxide particles dispersed in the matrix. Thus many investigations have studied the creep properties by focusing on the oxide particle distribution, but these results are not sufficient to clarify the creep mechanism and to develop material processing procedures for mass production. Potential microstructural features affecting the creep properties should be studied. The 9Cr ODS steel has many potential microstructural features, for example, PPB (Prior Particle Boundary) pores, precipitates, and two matrix phases. In particular, PPB pores link up and develop into large creep cavities. Therefore, particle size classification of mechanically alloying (MA) produced powders was performed to control PPB. Contrary to our expectations, the particle size classification did not only affect PPB creep cavities, but also other microstructural features. MA powder particle size effects on microstructural features and the creep properties were studied.