Ferritic Oxide Dispersion Strengthened Research Articles

Development of creep resistant high yttria 18Cr ferritic ODS steel through hot powder forging route

The present work reports the creep behaviour of a high yttria Fe-18Cr-2W-0.57Ti-1Y2O3 (all compositions in wt.%) ferritic oxide dispersion strengthened (ODS) steel produced by powder forging. The creep tests were carried out in a temperature range of 873 K to1023K and a stress range of 150 MPa to 425 MPa. Creep data is analysed using the power law creep equation. A large value of stress exponents (n ∼ 10 at 873 K, ∼ 18 at 923 K and 8.5 at 973 K) and a high activation energy (Q ∼581 kJ/mol) are obtained. These high values are attributed to threshold stress (σth) generated due to attractive interaction between dislocation and oxide particles. Threshold stress for different creep conditions was determined. True stress exponent values of 4 to 5 were obtained after accounting for the threshold stress. Electron back scattered diffraction (EBSD) revealed an equiaxed strain free grain structure in the powder forged ODS steel. Transmission electron microscopy (TEM) of creep tested samples indicates that microstructure remains stable after creep with no significant coarsening of nano size oxide particles. Dislocation pinning by the oxide particles was observed in creep tested samples. The observed values of true stress exponent suggest dislocation climb over the nano oxide particles to be the creep mechanism operating in the ODS steel. Samples sections close to creep fracture surfaces were investigated by EBSD. Void formation at triple junctions followed by crack propagation normal to the loading direction was observed. All the samples exhibited mixed mode (cleavage + ductile dimple) failure. Conventional ODS steel compositions contain up to 0.35% yttria. A significant improvement in creep life as well as an increase in activation energy for creep was observed because of high (1%) yttria content used in the ODS steel studied.

Chemical and structural evolution of nano-oxides from mechanical alloying to consolidated ferritic oxide dispersion strengthened steel

Ferritic Oxides Dispersion Strengthened (ODS) steels are of great interest for nuclear fission and fusion power plants. The nano-oxides embedded into the matrix provide the main contribution to the ODS steel strength. Understanding of the precipitation mechanism of ODS steels is thus critical for optimizing the fabrication process, involving Mechanical Alloying (MA) of Fe-14Cr, Y2O3 and TiH2 powders. In this study, results from small-angle X-ray and neutron scattering, atom probe tomography and electron microscopy have been combined to investigate the nano-oxides evolution throughout the whole consolidation thermal treatment until 1100 °C. After MA clusters are observed, composed of Y, O and Ti. During heating these clusters grow and new ones nucleate, together with a sequential enrichment in Ti (from as-MA to 700 °C) and Y (between 900 and 1100 °C). A small quantity of Al is also found in the nano-oxides between 700 and 1100 °C. At 1100 °C the nano-oxides are found to be mainly Y2Ti2O7 and subsequently progressively transform to Y2TiO5 during isothermal holding. Nano-oxides display however an unchanged extremely low coarsening rate, demonstrating the outstanding stability of both Y2Ti2O7 and Y2TiO5 at 1100 °C.

Isothermal Oxidation Behavior of Ferritic Oxide Dispersion Strengthened Alloy at High Temperatures

This paper discusses the oxidation behaviors of ODS steel alloy of Fe-16Cr-4Al-1Ni-0.4ZrO2 at 700 ℃, 800 ℃, and 900 ℃. X-ray diffraction (XRD) as well as X-ray mapping in a scanning electron microscope were used to characterize the oxidation behavior of the samples. The rate of oxidation was measured based on the thickness of the oxide formed on the surface of the samples. Six types of oxides were identified in all ODS Fe-16Cr-4Al-1Ni-0.4ZrO2 alloy samples after the oxidation tests, dominated by Fe2O3, Fe3O4, Cr2FeO4, AlFeO3, Al2FeO4, and AlFe2O4. The oxidation kinetics of ODS Fe-16Cr-4Al-1Ni-0.4ZrO2 steel at 700, 800, and 900 ℃ followed logarithmic oxidation rate behavior.

Reducing Anisotropy in High Yttria Ferritic Oxide Dispersion Strengthened Steels by Powder Forging

Reducing Anisotropy in High Yttria Ferritic Oxide Dispersion Strengthened Steels by Powder Forging

Chemical and Structural Evolution of Nano-Oxides from Mechanical Alloying to Consolidated Ferritic Oxide Dispersion Strengthened Steel

Chemical and Structural Evolution of Nano-Oxides from Mechanical Alloying to Consolidated Ferritic Oxide Dispersion Strengthened Steel

Optimization of Wear Process Parameters on 17-Cr Ferritic ODS Steel

The main aim of this article deals with the wear behavior of mechanically alloyed 17-Cr oxide dispersion strengthened (ODS) Ferritic steel consolidated through Vacuum Hot Pressing (VHP) at temperature level of 1170 °C under pressure level of 60 MPa with 60 minutes as holding time and with rate of cooling of 50 ˚C /min and a vacuum level of 10-3 torr. The persuade of wear process parameters were investigated based on the load applied, sliding velocity and sliding distance at a temperature of 350°C on dry sliding track of 17-Cr Ferritic oxide dispersion strengthened steel (Fe-17Cr-0.35Y2O3 -1.5ZrO2 -4Al (%wt). Wear test was conducted in a dry atmosphere using a pinon-disc wear testing machine. Wear behavior of 17-Cr Ferritic ODS steel was analyzed by using Taguchi approach. To examine the process parameter during high temperature wear rate analysis of variance and signal to noise ratios were used. During the wear analysis sliding distance was found to be influential parameters of wear rate for 17-Cr Ferritic oxide dispersion strengthened steel succeeded by functional load and sliding velocity. The regression model was found to calculate the rate of wear for 17-Cr Ferritic oxide dispersion strengthened steel.

Effect of Sintering Parameters on Mechanical Properties of 17-Cr Ferritic ODS Steel Through Taguchi Approach

Ferritic oxide dispersion-strengthened (ODS) steels containing Fe–17Cr–0.89Mn–0.89Si–0.25Y2O3–1.5ZrO2–3Al (wt%) was prepared through mechanical alloying, and X-ray diffractogram was used to analyze crystallite size for different time intervals during milling. Milled powders were consolidated at three different temperatures 1050 °C, 1100 °C and 1170 °C at three different pressure levels of 30, 40 and 60 MPa at different cooling rates 5 °C/min, 25 °C/min, 50 °C/min. Optimization of three parameters, namely sintering pressure, sintering temperature and rate of cooling, was done in this current study through design of experiments. Analysis of variance technique inclusive of signal-to-noise ratio was employed to optimize the processing parameters through L9 orthogonal array which predicts the response variable like density, tensile strength and hardness in the present study. Density and mechanical properties of powder metallurgy component are governed by sintering parameters. From the results, it is observed that in sintering pressure increases the mechanical properties and density. Samples sintered at 1170 °C with the pressure level of 60 MPa and rate of cooling 50 °C/min exhibit higher sintered density, hardness and tensile strength than the other sintered conditions. Sintering pressure is the most prevailing factor followed by the vacuum hot pressing inducing density and mechanical properties. Analysis of variance and Taguchi method were used to find the optimum parameters for attaining the superior mechanical properties of the 17Cr ferritic ODS steel.

Corrosion behavior of ferritic ODS steel prepared by adding YH2 nanoparticles in supercritical water at 600 °C

The corrosion behavior of the ferritic oxide dispersion strengthened (ODS) steel (14Cr-3Al-2W-0.1Ti) prepared by adding YH2 nanoparticles (NPs) was investigated in supercritical water (SCW) at 600 °C for 1500 h. The mass gain of the ODS steel (215.5 mg/dm2) was lower than that of SUS430 steel (357.2 mg/dm2). A dual oxide layer generated on the surface of ODS steel after corrosion in SCW. The outer layer was composed of Fe2O3 and Fe3O4, while the inner layer composed of the spinel-type FeCr2O4 together with Al2O3. The generation of Y2Ti2O7 NPs in the ODS steel by adding YH2 NPs prohibits the formation of Y-Al-O particles and leaves more Al available to form a continuous protective oxide scale to improve the corrosion resistance. Moreover, the Y2Ti2O7 NPs act as efficient barriers to suppress the outward diffusion of metal atoms. This novel ODS steel shows potential applications in supercritical water.

High Temperature Oxidation Behavior of Fe-14Cr Ferritic Oxide Dispersion Strengthened Steels Manufactured by Mechanical Alloying Process

High Temperature Oxidation Behavior of Fe-14Cr Ferritic Oxide Dispersion Strengthened Steels Manufactured by Mechanical Alloying Process

Small Angle Neutron Scattering Study of Ferritic ODS Alloys

Small Angle Neutron Scattering (SANS) technique allows characterizing the nanomicrostructure of the ferritic Oxide Dispersion Strengthened (ODS) steels which are candidates for the structure material of fusion reactor and fast reactor. A SANS study of domestic ferritic ODS alloys are presented. The main objective is to study the evolution of the oxide dispersion nanoparticle during the different stages of the fabrication and different content.

Oxidation/dissolution of ferritic ODS steels in static lead with various oxygen content at 650 °C

The compatibility of ferritic oxide dispersion strengthened (ODS) steels with static liquid Pb at 650°C was investigated for up to 1750h. The dependence of corrosion behavior on oxygen concentration in the melt and steel structure was determined. The interaction mode changed from intensive oxidation in the oxygen-saturated melt (∼10−3mass%O), accompanied by the formation of a bi-layer magnetite/spinel scale, to formation of a thin Cr-based oxide film in oxygen-added melt (∼10−6mass%O) and to the severe steel dissolution in pure Pb (≤10−14mass%O). The effect of specific phase-structural state of ODS steels (presence of strengthening dispersive oxide particles, fine-grained structure, texture, residual porosity, etc.) on their corrosion response to Pb melts depending on the oxygen concentration is considered. The viability of the in situ formed Cr-based oxide film, as a promising way to improve the corrosion resistance of steels in high-temperature Pb melts (T>550°C) is discussed as well.

Hot hardness behaviour of ultrafine grained ferritic oxide dispersion strengthened alloys prepared by mechanical alloying and spark plasma sintering

The microstructure, hot hardness, and indentation creep of ultrafine grained (ufg) 9Cr–1Mo and 9Cr–1W based ferritic oxide dispersion strengthened (ODS) alloys prepared by mechanical alloying and spark plasma sintering are investigated. Studies on hot hardness revealed that the hardness of Mo-containing alloys is higher than that of W-containing alloys. This is attributed to the complete dissolution of Mo while W did not dissolve completely during mechanical alloying. Hardness of Ti containing alloys is higher compared to the base and Y2O3 containing alloys and this is attributed to the presence of Ti–Y–O based nanoprecipitates. Study of hot hardness behaviour of ufg ferritic ODS alloys revealed a sharp transition at around 700–800K. This is possibly due to the enhanced grain boundary mediated plasticity and pronounced reduction in Hall–Petch coefficient with increase in temperature.

Microstructural Evolution of Cr-Rich ODS Steels as a Function of Heat Treatment at 475 °C

In the current research, the effect of heat treatment on the morphology of the dispersoids and their phase composition were investigated in three Cr-rich ferritic oxide dispersion strengthened (ODS) steels: PM2000, MA956, and ODM751. The steels were aged at 475 °C for times ranging from 100 to 1,000 h. The microstructure was characterized using transmission electron microscopy. Study of the as-recrystallized samples revealed nano-scale Y–Al–O complex-oxide particles dispersed in the ferritic matrix. These dispersoids, which differ in size (10–160 nm) and geometry (polygonal and spherical), were identified as Y4Al2O9, YAlO3, and Y3Al5O12. After heat treatment, a significant change in the morphology, size, and distribution of the dispersoids was observed. Changes in the phase composition of the oxide dispersoids were also observed: YAlO3 (with perovskite structure) was identified as the most dominant phase, indicating that it is probably the most stable phase in the Cr-rich ferritic ODS steels.

Effect of mechanical alloying and consolidation process on microstructure and hardness of nanostructured Fe–Cr–Al ODS alloys

► The particle size progressively decreases with increase in milling time. ► The use of stearic acid can was found to achieve fine uniform spherical powders. ► High BPR has a significant influence on the rate of decrease of crystallite size . ► High-temperature sintering leads to relative density increase and grain growth. ► Canning-HIP technique has a significant improvement in mechanical properties. Nanostructured ferritic oxide dispersion strengthened (ODS) alloys are promising materials for both high temperature creep properties and irradiation resistance. These alloys were produced by mechanical alloying (MA) which is a complex process and a number of process variables need to be controlled to achieve the desired properties. In this study, the effect of mechanical alloying and consolidation processes on the microstructure and properties of nanostructured ferritic ODS alloy were investigated. The powder mixtures were milled in Spex mill for different milling times (1 h, 3 h, 8 h, and 15 h). The effect of process control agents (PCAs) and ball to power ratio (BPR) were studied. Two-stage sintering and canning-HIP processes were introduced as new consolidation methods to produce ODS alloys with improved mechanical properties. The results showed that crystallite size during milling decrease with milling time and higher BRP has a significant influence on the rate of decrease of the crystallite size. The use of stearic acid as PCA led to fine, uniform and spherical particles during the ball milling. Additionally, the samples sintered at the elevated temperatures over long periods resulted in a reduction in porosity; however, the microstructure became coarser, accompanied by a decrease in the hardness. The canning-HIP technique can effectively minimize the content of oxygen and carbon impurities to achieve high relative density and high hardness.

Compressive Deformation Behavior of the Ferritic Oxide Dispersion Strengthened Alloys at Elevated Temperatures

The compressive properties characterized as a function of the true stress-strain response of the ferritic oxide dispersion strengthened (ODS) alloys, fabricated by mechanical alloying, were examined at strain rates ranging from 0.01 to 0.5s-1. Based on the differential scanning calorimeter (DSC) analysis, the solidus temperature of the ferritic ODS alloys is 1446oC. Therefore, the compressive temperatures were chosen to be from 1050oC to 1300oC. Transmission electron microscopy (TEM) observation is indicative of the homogeneous distribution of the nano-yttria particles with the granularity of 100~200nm, which may exert a strong pinning effect on subgrains. The results indicate that flow stress of the ferritic ODS alloys enhances with decreasing compressive temperature and increasing strain rate. Investigation of the strain rate sensitivity exponent and apparent activation energy of the ferritic ODS alloys has been carried out in detail.

Irradiation creep and precipitation in a ferritic ODS steel under helium implantation

Ferritic oxide dispersion strengthened (ODS) steel, PM2000, has been homogeneously implanted with helium under uniaxial tensile stresses from 20 to 250 MPa to maximum doses of about 0.75 dpa (3000 ppm He) with displacement damage rates of 5.5 × 10 −6 dpa/s at temperatures of 573, 673 and 773 K. Straining of a miniaturized dog-bone specimen under helium implantation was monitored by linear variable displacement transformer (LVDT) and meanwhile by their resistance also measured by four-pole technique. Creep compliance was almost constant at 5.7 × 10 −6 dpa −1 MPa −1 for temperatures below 673 K and increased to 18 × 10 −6 dpa −1 MPa −1 at 773 K. The resistivity of PM2000 samples decreased with dose and showed a tendency to saturation. Subsequent transmission electron microscopy observations indicated the formation of ordered Fe 3− x Cr x Al precipitates during implantation. Correlations between the microstructure and resistivity are discussed.

Formation of porosity in ferritic ODS alloys on high temperature exposure

In order to investigate the formation, distribution, growth behavior and volume fraction of porosity at high temperatures in ferritic Oxide Dispersion Strengthened (ODS) superalloys MA 956, ODM 751 and PM 2000 were exposed at 1100 and 1200∘C in air and in 2% oxygen containing nitrogen atmospheres for up to 8760 h. All samples exhibited small amounts of porosity (% < 0.1) in the as-received condition. During the early stages of exposure (up to 10 h) pores were generally located close to the surface of the samples. Later they were found in the central region of the samples and the volume fraction of porosity and the mean pore size increased. After exposure for 240–1000 h, mainly depending on sample thickness, the size and volume fraction of porosity reached a maximum value and then decreased. SEM and EDS analysis results showed that some pores contained small amounts of Ti, Al and Y rich particles which were distributed over the internal surface of pores. It was noted that when the volume fraction of porosity and the size of pores began to decrease the pores had filled with the matrix material. A relation was established between the production environment of the materials and the growth behavior of the pores.

Oxide Dispersion Strengthened (ODS) Fuel Pins Fabrication for BOR-60 Irradiation Test

In order to confirm and demonstrate Oxide Dispersion Strengthened (ODS) cladding fuel pin survival to high burn-up condition, an irradiation test in BOR-60 was conducted within the framework of JNC-Russian RIAR collaborative work. Two types of 9Cr martensitic and 12Cr ferritic ODS steel claddings were manufactured in JNC, and the upper end-plugs were welded by the pressurized resistance method. In the RIAR fuel production facility, the MOX and UO2 granulated fuels as well as uranium metal getter particles were vibro-packed into the ODS steel cladding. The lower end-plug was welded by the TIG end-face method. The inspection and quality control of the fabricated ODS fuel pins were done through X-ray analysis, gamma scanning, and leak testing etc. and it was confirmed the fuel pins satisfied the BOR-60 requirements. Those fuel pins were loaded into two dismountable experimental assemblies, and irradiation was started in the BOR-60 from June 26, 2003. The cladding middle wall temperature was kept within 700±20°C and 650±20°C. Currently, the attained burnup is 5 at.%. Toward the target burnup of 15 at.%, interim examinations will be made at intervals of 5 at.%.

酸化物分散強化型 (ODS) フェライト鋼中のナノ粒子の高分解能観察

酸化物分散強化型 (ODS) フェライト鋼中のナノ粒子の高分解能観察

Mechanically alloyed, ferritic oxide dispersion strengthened alloys: structure and properties

The paper presents information concerning the production of oxide dispersion strengthened (ODS) alloys by mechanical alloying process. The macro- and microstructures of the ferritic ODS alloys, INCOLOY MA 956 and PM2000 are discussed in detail. The basic engineering properties of these alloys, namely oxidation resistance and mechanical properties, especially tensile, compression and high temperature creep, are outlined.