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Abstract Corrosion behavior in the supercritical water (SCW) of an Al added high-Cr oxide dispersion strengthened (ODS) steel was investigated in this study. The steel was exposed to SCW at 600 °C and 25 MPa for various exposure times up to 1000 h, and the resultant oxide layers were characterized by using weight change measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopies (XPS), scanning/transmission electron microscopy (SEM/TEM), and energy dispersive spectroscopy (EDS). The weight gain after exposure for 1000 h was 6.24 mg / dm 2 , indicating the superior corrosion resistance of 16Cr-3Al ODS steel to SCW. Chemical analysis of the surface oxides showed that the content of Cr-rich oxides increased with the increase of exposure time in SCW. Cross-sectional observation of the oxide scales revealed that an outer layer composed of (Cr, Fe) 2 O 3 and an inner layer composed of Al 2 O 3 formed on the sample surface. The growth of both outer and inner oxide layers followed the parabolic law with the increase of exposure times. The corrosion mechanism was discussed in detail based on the microstructure evolution process with exposure time in SCW. Graphical abstract Unlabelled Image Highlights • The weight gain of 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 1000 h was 6.24 mg / dm 2. • The thickness of the oxide layer after exposure for 1000 h was 400 nm. • Duplex oxide layer formed on the sample's surface. • The formation of compact Al 2 O 3 inner oxide layer significantly enhances the corrosion resistance of the steel to SCW. [ABSTRACT FROM AUTHOR]

In this paper, the effects of hafnium (Hf) addition on the thermal stability of the microstructure and tensile properties of Al-containing 16Cr-ODS steels were investigated. High temperature aging tests of the ODS steels with and without Hf were conducted at 750 °C, 950 °C and 1150 °C for 100 h respectively. And the microstructure evolution after thermal aging was investigated using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The results indicated that Hf addition could improve the coarsening resistance of the oxide particles and the grains during aging treatment due to the formation of stable Y 2 Hf 2 O 7 particles. Tensile tests on the aged-steels showed that the ultimate tensile strength and yield strength of 16Cr-Hf-ODS steel were higher than those of 16Cr-ODS steel. It was considered that the excellent thermal stability and tensile properties of 16Cr-Hf-ODS steel were related with the refined microstructure. [ABSTRACT FROM AUTHOR]

The effect of zirconium (Zr) addition on the microstructure and tensile properties of aluminum (Al) added ferritic ODS steel Fe-16Cr-3Al-1.5W-0.35Y 2 O 3 (16Cr3Al ODS) was investigated. Experimental analysis indicated that the addition of Zr refined oxide particles by forming finer Y-Zr-O particles instead of Y-Al-O particles encouraged grain refinement obviously from the results of TEM. The mean size of oxide particles decreased from 9.8 ± 1.7 nm to 8.0 ± 0.9 nm and the mean inter-particle distance decreased from 14.6 ± 1.5 nm to 10.5 ± 0.7 nm when Zr was added. In addition, the 16Cr3Al ODS steel containing Zr showed preferable yield strength and ultimate tensile strength compared with the ODS steel without Zr at several test temperatures, indicating the enhancement of tensile properties by the addition of Zr due to the optimization of microstructure in ODS steel by Zr addition. [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened (ODS) steel is one of the most promising candidate structural materials for the high-temperature nuclear reactor application. In this study, two compositions of ODS steels (14Cr-ODS and 14Cr-Zr-ODS) were prepared to investigate the influence of Zr addition on the microstructures and mechanical properties of ODS steels. The microstructures, including dispersion morphology and crystal structures of oxide particles, particle-matrix interface coherency and particle-dislocation interactions, were characterized using TEM, HRTEM, and SEM, and the mechanical properties at room and high temperatures were measured using uniaxial tensile tests. Results show that Zr addition leads to the formation of finer precipitated particles, which was identified as rhombohedral Y 4 Zr 3 O 12 , with denser dispersion in the matrix. The calculation results reveal that the lattice misfit, δ , at the interface between particle and matrix increases as the particle size increases. In addition, the strength and elongation of ODS steels are improved with Zr addition due to the stronger interface bonding force between fine particles and matrix as well as the larger pinning effect of small particles to dislocation movements. [ABSTRACT FROM AUTHOR]

To investigate the effects of hafnium (Hf) addition on the microstructural and mechanical performance of an aluminum (Al) added 16Cr ODS steel, the microstructures of the 16Cr ODS steel with and without Hf were investigated by EBSD and TEM, and tensile tests were conducted at high temperature to evaluate the strength and ductility changes at different temperatures. The Hf addition is beneficial to the grain refinement of the 16Cr ODS steel. The average grain size decreases from 1.795 μm to 1.027 μm with Hf addition. In addition to the Y Al O nanoparticles, Y Hf O complex oxides with a finer size are also observed in the sample with the Hf addition, which reduces the mean size of oxides within the steel and increases the number density of the oxide particles. The yield strength and tensile strength of the 16Cr ODS steel are further improved by Hf addition due to its optimized microstructures with further refined grains. [ABSTRACT FROM AUTHOR]

To reveal the effects of Al addition on the oxidation mechanisms of 16Cr ODS steel, high temperature oxidation at 1000 °C of 16Cr and 16Cr-3Al ODS steels with the same Cr content were investigated. After oxidation, the weight gain of the 16Cr ODS steel was relatively higher than that of the 16Cr-3Al ODS steel. The 16Cr ODS steel had triple oxide layers of an outer FeCr 2 O 4 layer, an intermediate Cr 2 O 3 layer, and an inner SiO 2 layer, while the 16Cr-3Al ODS steel had double oxide layers of an outer (Cr, Al) 2 O 3 layer and an inner Al 2 O 3 layer. The oxidation mechanisms of both ODS steels were explored through thermodynamic calculations, and the significant improvement in oxidation resistance of the 16Cr-3Al ODS steel compared to 16Cr ODS steel was attributed to the lower diffusion rate of O in Al 2 O 3 than in Cr 2 O 3. The existence form of oxide particles in the oxide layers was illustrated. These findings offer new strategies for enhancement in oxidation resistance of structural materials used in high temperature environment. • The 16Cr-3Al ODS steel exhibited better oxidation resistance compared to the 16Cr ODS steel. • Double oxide layers formed on 16Cr-3Al ODS steel, while triple oxide layers formed on 16Cr ODS steel. • No segregation of Y element observed in the oxide grain boundaries of 16Cr-3Al ODS steel. [ABSTRACT FROM AUTHOR]

[Display omitted] Bonding of tungsten (W) with oxide dispersion strengthened (ODS) steel is an urgent challenge during the practical manufacturing process in many applications, including advanced nuclear reactor systems, military equipment, etc. To eliminate the generation of brittle intermetallic compounds Fe-W and relieve the residual thermal stresses, a novel composite interlayer Zr/Cu, for bonding W with ODS steel using the transient liquid phase (TLP) diffusion technology, was designed in this work. The bonding materials were prepared as a sandwiched structure of W/Zr-Cu/ODS steel in a vacuum diffusion bonding device. Interfacial structure and tensile strength of the bonded joints were investigated, and the processing parameters were optimized. After the bonding process, a reliable Cu/ODS steel interface could be built at all applied temperatures or holding times. Nevertheless, only as the bonding temperature was 1000 °C or even higher, the inserted Zr foil could be completely consumed and a stable W/Cu interface free from intermetallic compounds and micro-cracks was built. Additionally, the bonding ratio was gradually improved with increasing the bonding temperature or holding time. As a result, a high-strength W/Cu/ODS steel joint of ∼430.4 MPa could be fabricated at 1000 °C for a holding time of 60 min, and the bonding ratio was as high as 98 %, which provided a significant guide for the assembly of plasma facing components (PFCs). [ABSTRACT FROM AUTHOR]

The creep properties, microstructural evolution, and fracture mechanism of 16Cr-3Al oxide dispersion strengthened (ODS) steel were systematically investigated at 600 °C under different uniaxial tensile stresses in the range of 160–200 MPa. The minimum creep rate had a linear relationship with the mean creep rate, which followed the modified Monkman-Grant relationship with a slope of 0.57. The threshold stress was determined as 145.09 MPa based on the modified Bird-Mukherjee-Dorn (BMD) equation. Both Laves phase and Cr-rich phase were identified after creep deformation. The evolution of Cr-rich phase during creep deformation included the dissolution of W-rich σ phase and the precipitation of Si-rich σ phase. Cross-sectional cracks could be observed near the rupture surface, and these cracks mainly formed near the Si-rich σ phase and the boundaries between coarse and fine grains. It was considered that the formation of Si-rich σ phase in fine grains was the main fracture mechanism of 16Cr-3Al ODS steel during creep deformation. [ABSTRACT FROM AUTHOR]

Two oxide-dispersion-strengthened (ODS) steels with different compositions (14Cr-ODS and 14Cr-Zr-ODS) were investigated to reveal the influences of microalloying element addition on the microstructure and to clarify the refining mechanism of precipitated particles. TEM and HRTEM results indicated that precipitated particles in the Zr-containing ODS steel had finer sizes and dispersed more homogeneously within the grains. It was found that rhombohedral Y4Zr3O12 particles with complex lattice structures were formed and could pin the migration of the grain boundaries during heat treatment due to their high thermal stability. In addition, the Zr-containing ODS steel exhibited a finer and more uniform grain morphology. Tensile tests showed that microalloying element addition could significantly improve the comprehensive mechanical properties of 14Cr ODS steels at room temperature. [ABSTRACT FROM AUTHOR]

• Laves phase and σ phase exhibited different morphology at fine and coarse grains' regions. • The bimodal grain structure distribution became more obvious after thermal aging. • Yield strength decreased but still maintained high values due to the combined strengthening mechanisms. • The loss of ductility was attributed to the formation and coarsening of Laves phase at grain boundaries. Effects of thermal aging at 650 ºC up to 5000 h on the microstructure evolution and tensile properties of 16Cr-3Al ODS steel were investigated. Laves phase and σ phase were identified after thermal aging for 250 h. The Laves phase tended to form at the grain boundaries and exhibited different morphologies at fine grains' boundaries and coarse grains' boundaries, while the σ phase mainly located at fine grain zones. The grain structure and misorientation evolution during the aging process were observed by the electron back-scattered diffraction (EBSD) method. It was found that the area fraction of coarse grains increased while the geometrically necessary dislocation (GND) density decreased during thermal aging process, which induced the misorientation evolution. Grain growth mode was discussed by combination with transmission electron microscopy (TEM) analysis. In the zones with uniform grain size distribution, the growth direction of grain boundaries was from the grains with a high number density of precipitates towards those with a low number density of precipitates. Room temperature tensile tests were conducted, and the results of tensile strength, yield strength, and elongation during different stages of thermal aging indicated that 16Cr-3Al ODS steel had a relatively good thermal aging stability at 650 ºC. [ABSTRACT FROM AUTHOR]

Oxide Dispersion Strengthened (ODS) steel with a composition of Fe-14Cr-2W-0.2V-0.07Ta-1Al-0.3Y2O3 was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). All of the investigations were performed on the as-SPSed ODS sample without further heat treatment. The microstructure, particles, and tensile properties of the ODS sample were analyzed. According to the results, both submicron-sized and micron-sized grains existed in the microstructure, showing a bimodal structure. The nanoparticles were homogeneously distributed in the matrix, and the nanoparticles in the ODS sample were mainly Y-Al-O composite oxides. Two kinds of Y-Al-O nanoparticles were identified: Y3Al5O12 particles with body-centered cubic structures and Y4Al2O9 particles with monoclinic structures. The size of the nanoparticles ranged from 2 nm to 61.5 nm, and the average size of the nanoparticles was 11 ± 2.7 nm. Except for the nanoparticles, large M23C6 particles that were detrimental to the ductility of the ODS sample were also identified. These large M23C6 particles possessed a long strip or rod-like morphology, and were generally distributed along the grain boundaries. The generation of cracks generally occurred in the region where the large M23C6 particles were located during the tensile test. The as-SPSed ODS steel possessed a ductile/brittle mixed fracture mode according to the fracture surfaces analysis. [ABSTRACT FROM AUTHOR]

Hot deformation tests of 14Cr oxide dispersion strengthened (ODS) steel fabricated by mechanical alloying and hot isostatic pressing (HIP) were performed on a Gleeble-1500D simulator at temperatures ranging from 1050 to 1200 °C with the strain rate range of 0.001−1 s−1. The relationship between the rheological stress and the deformation condition was studied, and a processing map at the true strain of 0.5 was proposed. Microstructure evolution during the deformation process and the effects of deformation conditions on microstructures were also investigated, as well as the stability of nanoparticles. Results show that the 14Cr ODS steel possesses positive strain rate sensitivity. The flow stress increases with the decrease of deformation temperature and the increase of strain rate. The recrystallization process is promoted by the increase of deformation temperature and the reduction of strain rate. Nanoparticles possess excellent stability during the deformation process and are coherent with the matrix. [ABSTRACT FROM AUTHOR]

• Both uniform and nodular corrosion regions have been analyzed. • The structure of corrosion layer was investigated. • Oxides in different corrosion layers were identified. • The mechanisms of the uniform and nodular corrosion have been analyzed. The corrosion behavior of 14Cr ODS steel exposed to the supercritical water (SCW) at 600 °C under 25 MPa with the dissolved oxygen content of 300 ppb was investigated. The weight gain measurement and corrosion layers analysis were used to determine the corrosion behavior and corrosion mechanism of the 14Cr ODS steel. The results indicate that the 14Cr ODS steel exposed in this work possesses better corrosion resistance than the other ODS steel with a similar composition. Both uniform corrosion regions and nodular corrosion regions can be seen on the exposed specimens, which is due to the bimodal microstructure of the ODS steel. The corrosion products have a multi-layer structure, including the outer oxide layer, the inner oxide layer, and the transition layer. The oxide in the inner layer is FeCr 2 O 4. The oxides in the outer layer change from the initial Fe 3 O 4 to Fe 2 O 3 with the increase of exposure time. The possible corrosion mechanism is analyzed as well, combining with the microstructure characters of the 14Cr ODS steel. [ABSTRACT FROM AUTHOR]