ODS Steel Research Articles

Review of Solid-State Consolidation Processing Techniques of ODS Steels (Hot Extrusion, Hot Isostatic Pressing, Spark Plasma Sintering, and Stir Friction Consolidation): Resulting Microstructures and Mechanical Properties

Review of Solid-State Consolidation Processing Techniques of ODS Steels (Hot Extrusion, Hot Isostatic Pressing, Spark Plasma Sintering, and Stir Friction Consolidation): Resulting Microstructures and Mechanical Properties

On the theory of the nucleation of gas bubbles at grain boundaries and incoherent inclusions

On the base of the critical analysis of two-dimensional models of the nucleation of gas filled bubbles at grain boundaries of helium-implanted specimens under the action of tensile stresses, a new model is developed within the framework of the Reiss theory of homogeneous nucleation in binary systems. This approach considers that gas bubbles are formed as a result of agglomeration in a binary system of vacancies and gas atoms at grain boundaries, avoiding significant simplifications of previous models based on the classical nucleation theory for single-component (unary) systems. The new model is extended to consider the nucleation of Xe bubbles at grain boundaries in UO2 under irradiation conditions and can be used for numerical analysis of experimental observations after the foreseen implementation in a fuel performance code. A similar approach can be applied to the nucleation and growth of gas bubbles on incoherent inclusions, such as those observed in irradiated ODS steels.

Structural evolution and mechanical properties of 9Cr ferritic/martensitic ODS steels developed by a direct powder forging route

ABSTRACT A nominal composition of mechanically alloyed Fe-9Cr-2W-0.2Ti-0.12C-0.35Y2O3 steel powders was consolidated via a direct powder forging route. The powder-forged samples exhibited an equiaxed grain structure with the formation of Y-Ti-O (Y2Ti2O7 or YTiO3) complex oxides and the absence of prior particle boundaries. The powder-forged samples were further heat treated, i.e. cooled at three different rates, i.e. furnace cooling, air cooling, and water quenching, followed by tempering for the air-cooled and water-quenched samples. The tensile strength at room temperature for water-quenched and normalised samples was comparable, whereas the ductility for the normalised samples was higher, while at high temperature, a good combination of strength and ductility was achieved in the tempered air-cooled samples among all the heat-treated samples. The mechanical properties achieved in the present study were at par and superior in some cases to those reported in literature. The absence of prior particle boundaries network, almost full density and equiaxed grains indicated that the powder forging route can emerge as an alternative consolidation technique to HIP and hot extrusion for the consolidation of powder metallurgy oxide dispersion strengthened steels for nuclear applications.

Precipitation kinetics of ferritic / martensitic oxide dispersion strengthened steels: Influence of the matrix phase transformation

ODS steels are candidate materials for the future generation of nuclear power plants. Ferritic / Martensitic (F/M) ODS steels display better formability thanks to high temperature austenitic transformation. The precipitation kinetics of a F/M Fe‑9Cr ODS steel during powder consolidation up to 1100 °C has been characterized by in‑situ Small Angle X-ray Scattering (SAXS). The influence of the matrix phase transformation has been established, showing an increase of the growth rate of the nano‑oxides in austenite, leading to nano‑oxides ∼ 2x larger in diameter than in Fe‑14Cr ferritic ODS grades at the end of the thermal treatment. These results are further supported by local atom probe tomography (APT) performed across grains showing contrasted microstructure and composition. Anomalous SAXS as well as comparison between APT and SAXS provide evidence that the nano‑oxides stabilize with a Y2Ti2O7 or Y2TiO5 stoichiometry around 1100 °C.

Investigation of nano-clusters evolved with Ti and Zr in ferritic ODS steel and their effect on microstructure and mechanical properties

In the present study, four nominal compositions Fe-15Cr-2W-0.3Y2O3-x (x = 0, 0.3Ti, 0.3Zr, 0.3Ti-0.3Zr) of ODS steel have been consolidated via spark-plasma-sintering of mechanically alloyed powders. All the samples were characterized via SEM, XRD, EBSD and TEM analysis and tested for hardness and compressive strength at room as well as elevated temperatures. Further, strength contributions of temperature dependent strengthening mechanisms were determined and correlated with the experimental values of compressive yield strength. Results suggest that contrary to Ti, addition of Zr was found with the precipitation of dense and much finer nano-clusters (∼3 nm) and observed to be more effective in enhancing the hardness and compressive strength at room as well as high temperatures. Composition containing both Ti and Zr solutes resulted in ultrafine grains of nearly uniform sizes and exhibited considerably enhanced hardness as well as compressive strength at all temperatures. However, it favoured the formation of Y-Zr-O nano-clusters instead of Y-Ti-O due to exhibiting lower formation enthalpy as determined from their reaction energies. As per the theoretically evaluated strength contributions, Orowan strengthening was found prevailed at high temperatures and it had a major contribution in compressive yield strength at room temperature as well. The study suggests the prevailing effect of formation of Y-Zr-O nano-clusters over Y-Ti-O nano-clusters in terms of obstacles to grain boundaries and mechanical properties which may provide an insight for the development of advanced nuclear materials.

Corrosion Behavior of Ferritic 12Cr ODS and Martensitic X46Cr13 Steels in Nitric Acid and Sodium Chloride Solutions.

This paper presents corrosion resistance results of a 12Cr ferritic ODS steel (Fe-12Cr-2W-0.5Zr-0.3Y2O3) fabricated via a powder metallurgy route as a prospective applicant for fuel cladding materials. In a spent nuclear fuel reprocessing facility, nitric acid serves as the primary solvent in the PUREX method. Therefore, fundamental immersion and electrochemical tests were conducted in various nitric acid solutions to evaluate corrosion degradation behavior. Additionally, polarization tests were also performed in 0.61 M of sodium chloride solutions (seawater-like atmosphere) as a more general, all-purpose procedure that produces valid comparisons for most metal alloys. For comparison, martensitic X46Cr13 steel was also examined under the same conditions. In general, the corrosion resistance of the 12Cr ODS steel was better than its martensitic counterpart despite a lower nominal chromium content. Potentiodynamic polarization plots exhibited a lower corrosion current and higher breakdown potentials in chloride solution in the case of the ODS steel. It was found that the corrosion rate during immersion tests was exceptionally high in diluted (0.1-3 M) boiling nitric acid media, followed by its sharp decrease in more concentrated solutions (>4 M). The results of the polarization plots also exhibited a shift toward more noble corrosion potential as the concentrations increased from 1 M to 4 M of HNO3. The results on corrosion resistance were supported by LSCM and SEM observations of surface topology and corrosion products.

Modern Methods of Creation and Application of Powder Ferritic/Martensitic ODS Steels

Modern Methods of Creation and Application of Powder Ferritic/Martensitic ODS Steels

Comparison of the corrosion behaviour of 10Cr and 17Cr Al added ODS steel in simulated reprocessing nitric acid medium

It is imperative to comprehend the corrosion behavior of different Cr containing Al-added ODS steel reactor clad materials in nuclear reprocessing simulating conditions, as high dissolution of clad material is not desirable. In this present work, the effect of varying HNO3 concentrations (3, 6, 11.5 M HNO3) and with oxidizing (Cr6+, Ce3+) and non-oxidizing (Fe3+) ions in 6 M HNO3 on 10Cr (10Cr-5Al-0.5Ti-0.5Y2O3-Fe) and 17Cr (16.78Cr-4.46Al-0.5Ti-0.45Y2O3-Fe) Al-ODS steels are investigated. The potentiodynamic anodic polarization studies revealed an ennoblement of corrosion potential by elevated cathodic current resulting from increased oxidizing conditions in HNO3 by concentration and oxidizing ions. Through various electrochemical techniques like open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), the 17Cr Al-ODS steel exhibited improved corrosion resistance with lower Ecorr, increased passivation range (Epass range), lower corrosion current density (icorr) and increased charge transfer resistance (Rct). The SEM studies revealed variation in corrosion morphology as shallow corrosion cavities and deeper corrosion pits in 17Cr Al-ODS and 10Cr Al-ODS steel, respectively, at high oxidizing environment. The enhanced corrosion resistance of 17Cr Al-ODS is associated to the improved passivation with the high Cr level in the steel compared to 10Cr-ODS steel, which is corroborated by XPS analysis. New knowledge on nitric acid corrosion mechanism on the Cr in Al-ODS steels and the influences of oxidizing ions in reprocessing environments are described.

Effect of yttria content on microstructural evolution, mechanical properties and temperature dependent strengthening mechanisms in 9Cr-oxide dispersion strengthened (ODS) steel developed by hot powder forging

9Cr-oxide dispersion strengthened (ODS) ferritic/martensitic steel with varying yttria (Y2O3) contents (0, 0.35, 0.5, and 1 wt%) were prepared by mechanical alloying followed by hot powder forging. The powder forged samples were subsequently normalized and tempered by a standard heat treatment. The microstructure consisted of a dual phase structure of tempered martensite and residual ferrite, along with fine distribution of carbides (MC/M23C6). The grain size was strongly influenced by the yttria contents. Tensile tests were conducted at room temperature, 300°C, 500°C, and 700°C for all the compositions. A trend of increasing tensile strength and decreasing total elongation was observed with increase in yttria contents at all the temperatures. Contrary to the results in literature, superior ductility and a comparable tensile strength of ODS steels with the higher yttria contents (≥0.5 wt%) at room as well as high temperatures indicate the benefits of the powder forging process. As per the quantitative estimation of yield strength at room as well as elevated temperature via strengthening mechanisms, root mean square method (σrms) is proven more reliable than linear superposition method (σls). Additionally, strength from grain boundaries, dislocation density, and Orowan was found major at room temperature while dispersoids seemed to be prevailed at high temperature.

Impact of electron beam welding on the microstructure of PM2000 ODS steel

This work investigated the impact of electron beam (EB) welding, using various traverse speeds of the electron beam (1200–2200 mm/min), on the microstructure of PM2000 ODS steel which was characterized using analytical electron microscopy and microhardness. The different heat inputs during welding explored in this study (72–39 J/mm), related to the traverse speed used, correlated with small differences in the widths of the fusion zone. Only minor differences in local microstructures and hardness of the fusion zones were detected amongst the four butt welds produced. The fusion zone is characterized by large columnar grains in the ferritic matrix, together with a coarsened Y-Al-O particle distribution and a reduction in its particle number density as compared to the base material. There is also a sharp border between the fusion zone and base material, with the joint showing little evidence of a heat-affected zone. These changes in microstructure during welding led to a hardness reduction of ∼30 % in the fusion zone of the welds with respect to the base material.

Secondary phases characterization by SANS and XAS of an ODS ferritic steel after thermal aging at 873 K

An ODS steel with nominal composition Fe–14Cr–2W–0.4Ti-0.3Y2O3 (wt.%) was produced by mechanical alloying and compacted by hot isostatic pressing (HIP) followed by hot cross rolling (HCR). To check the effects of thermal aging at relevant temperatures of operation in fusion power plants, the alloy was thermally aged at 873 K for 2000 h. In this work, small-angle neutron scattering (SANS) and X-ray absorption spectroscopy (XAS) techniques are used for the advanced characterization of secondary phases and the oxide nanoparticle dispersion. SANS results show that the oxide nanoparticles remain stable after the thermal aging treatment. Composition of the oxide nanoparticles was identified as Y2TiO5 or Y2Ti2O7 by SANS, while non-stoichiometry was found by XAS analysis. Laves phase precipitation after the thermal aging treatment is further confirmed by SANS, from the magnetic anisotropic contribution to the scattering intensity associated to this metallic phase, and by XANES.

The Effects of Irradiation on the Improvement in Oxidation Behavior of MX-ODS Steel in Liquid Pb.

Lead-cooled fast reactors exhibit strong inherent safety performance and good economic features, while material degradation due to corrosion and irradiation is still challenging. Oxide dispersion-strengthened steels are one of the promising candidates for fuel cladding materials. The effects of both irradiation and corrosion on ODS steel need to be further studied. In this work, MX-ODS steel was irradiated by Fe ions at 500 °C up to 46 dpa. Later, the as-received specimen and the irradiated specimen were used to conduct corrosion tests in oxygen-saturated Pb at 550 °C for 1 h. In the as-received specimen, discontinuous oxides penetrated by Pb and Pb in contact with steel matrix were observed, demonstrating unsatisfactory corrosion resistance of the material. However, in the irradiated specimen after corrosion experiment, a protective oxide layer formed and prevented Pb attack. The oxidation behavior differences between the two specimens can be attributed to the defects produced by irradiation and the structural discrepancy in oxides caused by the formation process. A possible mechanism of irradiation on the corrosion is discussed. In the as-received specimen, Fe atoms loss led to voids in the oxides, and lead penetrated the oxides through these voids. In the irradiated specimen, defects left by previous irradiation helped to form a more uniform oxide layer. The adhesive outer magnetite oxide and the Fe ions generated from where grain boundary oxidation developed retarded the presence of voids and made the oxide layer protective.

Manufacturing ODS Steels from GARS Powders by Friction Consolidation and Extrusion

Manufacturing ODS Steels from GARS Powders by Friction Consolidation and Extrusion

Effect of annealing temperature on the evolution of microstructure, texture, and mechanical properties of hot-rolled 12Cr-ODS steel

To achieve high thermal efficiency, today's nuclear reactor structures are exposed to higher temperatures and pressures, which requires the use of high-strength steels with specific properties, such as ODS steels. There is a need to clarify the evolution of the microstructure and properties of steels at elevated temperatures. This study systematically investigates the evolution of microstructure, texture, and mechanical property variations of 12Cr-ODS steels after hot-rolling and subsequent annealing at 1000 °C and 1200 °C. The investigation utilizes optical microscopy, electron backscatter diffraction technique, and mechanical property measurements. The microstructure of hot-rolled samples shows a layered alternating distribution, which is distinguished by a notable presence of the α-fiber texture. Following annealing at 1000 °C, the martensite grains are smaller with a reduced hardness, while maintaining a strong α-fiber texture. After annealing at 1200 °C, there is a rapid increase in the growth of martensite grains, a significant rise in hardness, a reduction in the α-fiber texture characteristics, and an improvement in the γ-fiber texture characteristics. Moreover, the maximum intensity of the α-fiber texture diminishes as the annealing temperature increases. The mechanical properties of the samples deteriorated after annealing at 1200 °C, which can be attributed to the coarse martensite grains and the texture components containing the {001} cleavage plane dominating the occurrence of brittle cleavage fracture. The 0.1Y sample after annealing at 1000 °C exhibits an excellent combination of strength (1458 MPa) and ductility (20.3%), which is owing to the unique heterogeneous grain structure and the evolution of favorable texture.

Microstructure and mechanical properties of thin-walled ODS ferritic steel tubes fabricated by a near-net forming method

This research successfully applied a new near-net forming process for producing thin-walled ODS ferritic steel tubes. The process involves hot isostatic pressing (HIP), hot extrusion (HE), and Pilger-rolling (PR). The tubes produced have a length of 2800 mm and a thickness of ∼1 mm. The microstructural analysis shows that the thin-walled tubes have partially elongated grains and a γ-fiber {111} texture. The precipitated nano-oxide particles in the thin-walled tubes are Y4Zr3O12 and Y2(Zr0.6Ti0.4)2O7, with an average size of 7.21 nm and a number density of 4.3 × 1023 m−3. The tubes have an ultimate tensile strength (UTS) of 908.3 MPa at ambient temperature and 186.2 MPa at 800 °C along the hoop direction. These values are comparable to those reported for ODS steel tubes produced by conventional methods, including powder hot extrusion, drilling machining, and Pilger-rolling. Based on the results, this near-net forming method effectively produces high-performance thin-walled ODS steel tubes. In addition, compared to conventional methods, the manufacturing process has a high material utilization rate of over 90%, which is a significant improvement.

Corrosion resistance of 9–12Cr ODS steels in pressurized water for fusion application

The blanket design of Chinese fusion engineering test reactor (CFETR) may utilize pressurized water as coolant. The fusion structural materials will endure high temperature, high pressure and result in corrosion in the pressurized water. It is necessary to verify corrosion resistance of the materials. In the present study, three kinds of advanced fusion structural materials with excellent mechanical properties and neutron irradiation resistance, i.e., 9Cr-ODS steel, 9CrODS-Al steel, and 12CrODS-Al steel were immersed in pressurized water at 325 °C under the pressure of 15.5 MPa up to 859 h. Room-temperature tensile properties were investigated for the materials before and after corrosion. The oxidation layers of the steels were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ODS steels have good corrosion resistance in the pressurized water, especially the 9CrODS-Al and 12CrODS-Al steels. The outer oxide layer is identified as Fe2O3 for all the steels. The inner layer is mainly FeCr2O4 for the 9CrODS steel, and Fe(Al,Cr)2O4 for the ODS steels with Al element which better protect the steels from further corrosion. The relevant properties data can provide reference for the blanket materials choice of CFETR and the future fusion reactors.

Prediction of creep rupture life of ODS steels based on machine learning

Creep rupture life is a key material parameter for service life and mechanical properties of ODS steels. Therefore, accurately predicting the creep rupture life of ODS steels holds significant importance. This study aims to build quantitative models to predict the creep rupture life of ODS steels by ML techniques. Before the construction of models, the SFS and RF methods were used for input selection. Seven features were selected as inputs and used to predict creep rupture life. The ML algorithms including LR, SVR, MLP, KRR, RFR, and XGBoost are employed, among which XGBoost performs best in predicting the creep rupture life of ODS steels with high accuracy (RMSE of 0.78 h and 0.88 h, MAE of 0.62 h and 0.71 h, and R2 of 0.96 and 0.91 for training and testing sets, respectively). Feature importance coefficients of the XGBoost model were used to show the magnitude of the effect of descriptors on creep fracture life. For the chemical composition, the parameters that have a great influence on the creep rupture life are Ti, Cr, W and Y2O3 with the values of importance coefficients of 0.154, 0.04, 0.039 and 0.034, respectively. For the heat treatment process, the parameters that have a great influence on the creep rupture life are ST and FT with the values of importance coefficients of 0.127 and 0.025, respectively. For the test conditions, the parameters that have a great influence on the creep rupture life are CR and CS with the values of importance coefficients of 0.342 and 0.041, respectively. The findings of the feature importance of the XGBoost model are similar to those of the RFR model. This study developed an effective ML model to predict the creep rupture life of ODS steels, providing a strategy to predict the creep rupture life of ODS steels and significant insights as additional guidelines for the research and development of novel type ODS alloys with high performance.

Towards enhancing ODS composites in laser powder bed fusion: Investigating the incorporation of laser-generated zirconia nanoparticles in a model iron–chromium alloy

Oxide dispersion-strengthened (ODS) steel is a sought-after composite material known for its high demand in high-temperature and corrosive environments. Achieving the desired ODS steel properties requires specific conditions for the size and nanoparticles (NP) distribution in the printed part. Laser ablation in liquid (LAL) enables precise NP size adjustment. At the same time, the dynamic melt pool solidification in the Laser Powder Bed Fusion (PBF-LB/M) process complements this by creating favorable conditions for successful ODS processing. In this study, ZrO2 NP with a small and narrow particle size distribution (d50 = 3.8 nm; d90 = 10 nm) is produced by LAL. Dielectrophoretic deposition achieves the homogeneous, deformation-free coating of the binary Fe20Cr (wt.-%) matrix powder with NP. PBF-LB/M printed parts out of the oxide-additivated powder exhibit a crack-free structure and a density of up to 98%. Expectedly, the metal matrix grain sizes and room temperature microhardness (~ 220 HV) are not affected by NP addition. NP tracing by 2D simulation indicates a homogeneous NP distribution and less than 10% NP to be agglomerated in the solidified part. Thus, a promising perspective for a complete laser-based process chain for generating and processing ODS alloys is outlined.Graphical abstractTable of Contents

Cyclic plastic response of rotary swaged Fe-14Cr-10Al-4Y2O3 ODS alloy at 1000–1200 °C

This study is dedicated to the investigation of the cyclic stress-strain response of a newly developed ferritic steel reinforced with a high-volume fraction of oxide nanoparticles. The mechanically alloyed powder was consolidated by hot rotary swaging into cylindrical rods with diameters of 15 mm and 12 mm. Subsequently, the rods were annealed at 1200 °C for 4 h to achieve a fully recrystallized coarse-grained microstructure optimal for high-temperature applications. The cyclic tests were carried out in controlled deformation mode in a symmetrical push-pull cycle with triangular waveform at 1000 °C and 1200 °C in air. The Fe-14Cr-10Al-4Y2O3 ODS steel exhibits extraordinary cyclic strength of up to 140 MPa at temperature of 1200 °C. The results show that rotary swaging to a smaller diameter does not affect the resulting stress-strain response. The coarsening of the Y2O3 nanoparticles occurs only very slowly even at 1200 °C. In areas with significant plastic deformation, a slight rotation of the coarse grains from the ideal [111] direction and partial dynamic recrystallization was observed.

Irradiation hardening of FeCrAl ODS alloys with Zr/Hf addition after high-energy self-ions irradiation

FeCrAl ODS steel is the promising candidate cladding material for the accident tolerant fuel (ATF) of the light water reactors (LWR), due to the formation of Al2O3 in alloys which could suppress the oxidation reaction with hot steam. In the present work, change in microstructures and the corresponding irradiation hardening of two kinds of FeCrAl ODS alloys (15.5Cr-4Al), with 0.6 wt% Zr (denoted as P1) or 0.8 wt% Hf addition (denoted as P2), were investigated. Electron backscattered diffraction (EBSD) and transmitted electron microscopy (TEM) were used to characterize grains and dispersion morphology of oxides, respectively. Compared to P1, P2 has finer and denser oxides. 56Fe17+ ions with kinetic energy of 352.8 MeV were used to produce a quasi-uniformly distributed damage region from the near-surface to a depth of 24 μm in the specimens by using an energy degrader. A damage level of 0.8 dpa (displacement per atom) was attained in the specimens of both alloys simultaneously at 200℃. The Vickers hardness test shows that both specimens exhibited an apparent irradiation hardening. The extent of hardening of P2 (6 %) is significantly lower than P1 (12 %). TEM results show that irradiation-induced dislocation loops occur in both specimens but with a relatively smaller average size in P2. The results demonstrate that compared to the 0.6 wt% Zr addition, the 0.8 wt% Hf addition improves more significantly the hardening resistance, which was discussed in terms of the calculated sink strength originating from the oxides/matrix interface.