ODS Steel Research Articles

Thermodynamic stability of oxide phases of Fe–Cr based ODS steels via quantum mechanical calculations

Using density functional theory-based calculated formation and reaction enthalpies, we have examined the relative stability of a large number of likely oxide phases of typical ODS steels based on the Fe–Cr–Al–Ti–Y–O system. Calculated formation and reaction enthalpies are in good agreement with available measured values. The relatively more negative formation enthalpies of Y2O3, Y2TiO5, Y4Al2O9, Y2Ti2O7 and YAlO3 oxides show them to be more stable corroborating with their observation in the microstructure of ODS steels. Analysis of reaction enthalpies, guided by convex-hull consideration, gives a mechanism by which reaction of Y2O3 with Al2O3 for the formation of YAlO3, Y4Al2O9, and Y3Al5O12 and reaction of Y2O3 with TiO2 for the formation of Y2TiO5 and Y2Ti2O7 are favored over other likely oxides, such as, those based on Y–Fe–O, Y–Cr–O and Fe–Cr–O.

Thermodynamic Stability of Oxide Phases of Fe-Cr Based ODS Steels via Quantum Mechanical Calculations

Abstract Using density functional theory-based calculated formation and reaction enthalpies, we have examined the relative stability of a large number of likely oxide phases of typical oxide dispersion strengthened steels based on the Fe-Cr-Al-Ti-Y-O system. Calculated formation and reaction enthalpies are in good agreement with available measured values. The relatively more negative formation enthalpies of Y2O3, Y2TiO5, Y4Al2O9, Y2Ti2O7 and YAlO3 oxides show them to be more stable corroborating with their observation in the microstructure of ODS steels. Analysis of reaction enthalpies, guided by convex-hull consideration, gives a mechanism by which reactions of Y2O3 with Al2O3 for the formation of YAlO3, Y4Al2O9, and Y3Al5O12, and reactions of Y2O3 with TiO2 for the formation of Y2TiO5 and Y2Ti2O7, are favored over other likely reactions, such as, those based on Y-Fe-O, Y-Cr-O and Fe-Cr-O systems.

Effect of Oxide Particles and Pre-Implanted Helium on Defect Evolution during Electron Irradiation

In-situ observations of ferritic/martensitic steels by electron irradiation with a 1.25MeV high voltage microscope at 573K were carried out to study damage evolution in the steels. The development of interstitial type loops and cavities in both of the two steels, F82H-IEA and F82HODS, showed smaller and more numerous defects in the ODS steel. The cavities were formed preferentially at the interface between oxide particles and matrix. The results suggest that ODS particles may function to suppress the nucleation and growth of loops and cavities arising from irradiation. The effect of pre-implanted helium was also studied. The pre-implanted helium led to a homogenous distribution of black dots and cavities in the steels, and these may act as sinks for point defects arising from irradiation, causing a suppression of the subsequent growth of loops and cavities. The hardening corresponding to the microstructural evolution was estimated by assuming parameters extracted from the ion irradiation. [doi:10.2320/matertrans.MD201318]

Helium ion irradiation induced swelling and hardening in commercial and experimental ODS steels

The radiation resistance of two promising ferritic ODS steels, PM2000 and 15CRA-3, is studied. More specifically, the effect of helium on these materials is studied by examining the swelling and hardening after He+ implantation to ∼1dpa at 4000appm/dpa.White light interferometry is used to measure the post-irradiation surface displacement whereas mirco-hardness measurements are carried out to determine the radiation hardening. These methods prove to be very efficient and fast working, thus making good statistics obtainable in a reasonable time.Low swelling is found in both alloys, although the PM2000 shows a lower resistance, with an increased swelling of ∼0.8%dpa−1 at 600°C, while the 15CRA-3 presents an almost constant swelling of around 0.4%dpa−1 throughout the complete temperature range. Radiation hardening follows a similar trend, whereby hinting towards an annealing mechanism taking place at the interfaces between the very fine distribution of Y2O3 nanoparticles and the matrix, interfaces which act as sites of enhanced point defect recombination as well as traps for the implanted helium.

Mechanism for radiation damage resistance in yttrium oxide dispersion strengthened steels

ODS steels based on yttrium oxide have been suggested as potential fusion reactor wall materials due to their observed radiation resistance properties. Presumably this radiation resistance can be related to the interaction of the particle with vacancies, self-interstitial atoms (SIAs) and other radiation damage debris. Density functional theory has been used to investigate this at the atomic scale. Four distinct interfaces, some based on HRTEM observations, between iron and yttrium oxide were investigated. It is been shown that the Y2O3–Fe interface acts as a strong trap with long-range attraction for both interstitial and vacancy defects, allowing recombination without altering the interface structure. The catalytic elimination of defects without change to the microstructure explains the improved behaviour of ODS steels with respect to radiation creep and swelling.

EFFECTS OF HEAT TREATMENTS ON MICROSTRUCTURES AND MECHANICAL PROPERTIES OF DUAL PHASE ODS STEELS FOR HIGH TEMPERATURE STRENGTH

In the present study, the effects of various heat treatments on the microstructure and mechanical properties of dual phase ODS steels were investigated to enhance the high strength at elevated temperature. Dual phase ODS steels have been designed by the control of ferrite and austenite formers, i.e., Cr, W and Ni, C in Fe-based alloys. The ODS steels were fabricated by mechanical alloying and a hot isostatic pressing process. Heat treatments, including hot rolling-tempering and normalizing-tempering with air- and furnace-cooling, were carefully carried out. It was revealed that the grain size and oxide distributions of the ODS steels can be changed by heat treatment, which significantly affected the strengths at elevated temperature. Therefore, the high temperature strength of dual phase ODS steel can be enhanced by a proper heat treatment process with a good combination of ferrite grains, nano-oxide particles, and grain boundary sliding.

Small Angle Neutron Scattering Study of Nano Sized Microstructure in Fe–Cr ODS Steels for Gen IV In-Core Applications

The nano-sized microstructures in Fe-Cr oxide dispersion strengthened steel for Gen IV in-core applications were studied using small angle neutron scattering. The oxide dispersion strengthened steel was manufactured through hot isostatic pressing with various chemical compositions and fabrication conditions. Small angle neutron scattering experiments were performed using a 40 m small angle neutron scattering instrument at HANARO. Nano sized microstructures, namely, yttrium oxides and Cr-oxides were quantitatively analyzed by small angle neutron scattering. The yttrium oxides and Cr-oxides were also observed by transmission electron microscopy. The microstructural analysis results from small angle neutron scattering were compared with those obtained by transmission electron microscopy. The effects of the chemical compositions and fabrication conditions on the microstructure were investigated in relation to the quantitative microstructural analysis results obtained by small angle neutron scattering. The volume fraction of Y-oxide increases after fabrication, and this result is considered to be due to the formation of non-stochiometric Y-Ti-oxides.

Effect of the thermal ageing on the tensile and impact properties of a 18%Cr ODS ferritic steel

The effects of the thermal ageing at 400°C, 500°C and 600°C during 5000h on the mechanical properties of a 18%Cr ODS ferritic steel are investigated. A hardening effect is observed after ageing at 400°C and 500°C, probably due to the presence of chromium rich α′ particles as suggested by the literature. The impact resistance and the ductility of the material are strongly lowered by the ageing at 600°C. This embrittlement is characterized on the fracture surfaces by the presence of cleavage facets on the whole range of testing temperatures. The intermetallic σ phase is found to be responsible for the occurrence of cleavage fracture on the material aged at 600°C, and thus for the significant embrittlement of this material. M23C6 carbides are also observed before and after thermal ageing. The lattice parameters of the σ phase and the M23C6 carbides observed in this 18%Cr ODS steel aged at 600°C during 5000h are measured.

Nano-cluster stability following neutron irradiation in MA957 oxide dispersion strengthened material

ODS steels are promising materials for Sodium cooled Fast Reactors since their fine distribution of nano-clusters confers excellent mechanical properties. However, the nano-feature stability needs to be assessed under neutron irradiation. Before irradiation, the characterizations show that nano-particles are finely distributed within the ferritic matrix and are identified to have a pyrochlore type structure. After irradiation of the MA957 alloy in the Phenix French reactor at 412°C up to 50dpa and 430°C up to 75dpa, transmission electron microscopy characterization reveals a very slight density fall but no distinguishable difference in nano-features size before and after irradiation. In addition, after both irradiations, the nano-oxides are still (Y, Ti, O) compounds with orientation relationship with the matrix. A multislice simulation of high resolution images suggests that nano-particles still have a fcc pyrochlore type structure after irradiation. A possible change of lattice parameter seems to be highlighted, possibly due to disordering by cascade effect.

Characterization of a 14Cr ODS steel by means of small punch and uniaxial testing with regard to creep and fatigue at elevated temperatures

A 14Cr ODS steel was characterized at elevated temperatures with regard to its behavior in small punch and uniaxial creep tests and in low cycle fatigue tests. A comparison of small punch and uniaxial creep tests at 650°C revealed a strong anisotropy of the material when strained parallel and perpendicular to the extrusion direction with rupture times being several orders of magnitude lower for the perpendicular direction. The stress-rupture and Larson–Miller plots show a very large scatter of the creep data. This scatter is strongly reduced when rupture time is plotted against minimum deflection rate or minimum creep rate (Monkman–Grant plot). Fatigue tests have been carried out at 650°C and 750°C. The alloy is cyclically very stable with practically no hardening/softening. Results from the tests at both temperatures can be described by a common power law. An increase in the test temperature has little influence on the fatigue ductility exponent. For a given total strain level, the fatigue life of the alloy is reduced with increasing temperature.

Development and testing of microcompression for post irradiation characterization of ODS steels

For decades, the nuclear materials community has been interested in smaller tests for obvious benefits like in limited space or reduction in activity. In this work, we are presenting micropillar tests on ODS steel with a grain size of ∼450nm and finely distributed oxides as a part of an attempt to use this method in evaluating the change in mechanical properties of nuclear materials after irradiation. Systematic works combining microcompression tests and TEM observations showed that there is no significant size effect on the measured strengths and the dislocation structures of the ODS alloy down to a pillar size of 1μm. We used both circular and square pillars and discuss the effect of pillar geometry with the evaluated strength and errors. The measured strengths at half the pillar height were found to be in good agreement with the bulk strength measured from a miniature tensile test. The cross-sectional TEM observation of the deformed pillar showed that oxide particles act as strong internal constraining sources for dislocation movement and the formation of dislocation structure.

Corrosion resistance of PM2000 ODS steel in high temperature supercritical carbon dioxide

Corrosion of PM2000 ODS steel in supercritical CO2 at 650 °C and 20 MPa for 3000 h was studied. The corrosion performance of the alloys was evaluated by weight change measurements, and the surface oxide scale was characterized by scanning electron microscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy. PM2000 ODS steel showed significantly better corrosion resistance compared to austenitic stainless steels and nickel based alloys tested in the same conditions. The high corrosion resistance is attributed to the formation of a dense and protective aluminum‐rich oxide layer on the surface. The oxide layer thickness was only about 200 nm after 3000 h exposure.

Corrosion Resistance of 9 - 15% Cr ODS Steels and its Comparison with Austenitic Stainless Steel

An oxide dispersion strengthened steels are one of the most promising high temperatures, and high performance advanced structural material being developed for future fast reactors and high-temperature Generation IV reactors. In the present work, the corrosion resistance and its correlation with the passive film compositions of 11% Cr F/M and 9-15% Cr (with Zr or Hf) ODS steels is examined and compared with AISI type 304L stainless steel in boiling 60 - 62% (~13 M) HNO3. The corrosion rate measured in 62% HNO3 for 240 h of 11% Cr F/M, 9% Cr and 15% Cr (Zr) ODS steels show high corrosion rate. On the other hand, low corrosion rate was observed in 304L stainless steel (0. 21 to 23 mm y-1). However, severe intergranular corrosion attack was revealed in type 304L SS after 240 h exposure, but none in ODS steels. Such an intergranular corrosion attack seen in type 304L stainless steel is undesirable. On the contrary, low corrosion rate (0.04 0.15 mm y-1) of 15% Cr (Hf) ODS steel in 3 M, 6 M and 9 M HNO3, comparable to that of type 304L stainless steel was observed. The improved corrosion resistance of 15% Cr (Hf) ODS steel was attributed to enrich (20 at. %) and protective Al2O3 layer formation in addition to Cr2O3 in the passive film.

Oxidation of Oxide Dispersion Strengthened Steels: II. Kinetics and Mechanism

The oxidation resistance of 18%Cr-oxide dispersion strengthened (ODS) ferritic steels with and without 5%Al has been investigated in air at 700900 °C for time period up to 540 h. The oxidation rate of ODS steels is significantly dependent on the oxidation time and temperature. Compared to Al-containing ODS steel, the finer grains of Al-free ODS steel are due to the formation of smaller coherent oxide particles which suppress the steel's grain growth. The grain refinement of ODS steels is expected to allow rapid segregation of Cr or Al to the steel surface, so that the continuous Fe-Cr spinel or alumina layer is formed quickly in comparison to the alloys without oxide particles dispersion. Therefore, the excellent oxidation resistance of ODS steels is owing to the formation of continuous, protective oxide layers which correlate with oxide nanoparticles and grain refinement.

Oxidation of Oxide Dispersion Strengthened Steels: I. Influence of Alloy Composition

Oxidation tests of 18%Cr-oxide dispersion strengthened (ODS) steels with and without 5%Al were carried out in air at 700900 °C for time period up to 540 h. No minor alloying elements affect the oxidation behavior and the Al concentration between these ODS steels is a main difference. Cr2O3 and (Fe,Cr)3O4 spinel oxides exist on the surface of 18Cr-ODS steel; however, the surface oxide of 18Cr5Al-ODS steel is comprised of only Al2O3. Oxidation resistance of the ODS steels exposed at 700 °C is much better than Incoloy800 of which the Cr content is larger and their oxidation behavior doesnt follow the aluminum content. These results suggest that oxide particles dispersion and grain refinement play a more critical role than alloy composition in the high-temperature oxidation resistance.

An in situ powder neutron diffraction study of nano-precipitate formation during processing of oxide-dispersion-strengthened ferritic steels

• In situ powder neutron diffraction to study precipitate formation in ODS steel. • First real time observation of nano-precipitate formation during processing. • Y 2 O 3 particles were fully dissolved into steel matrix during mechanical alloy. • The precipitation occurred during annealing of as-milled powder above 900 °C. The evolution of phases in a Fe–14Cr–10Y 2 O 3 (wt%) oxide-dispersion-strengthened ferritic steel during mechanical alloying (MA) and subsequent annealing was studied by high resolution powder neutron diffraction, with emphasis on the kinetics of oxide-based nano-precipitate formation. Y 2 O 3 particles were completely dissolved into the ferritic matrix during MA. The formation of nano-precipitates was then observed by in situ thermo-diffraction experiments during annealing of as-milled powder above 900 °C, supported by scanning electron microscopy. This revealed nano-precipitate coarsening with increasing annealing temperature. Powder microhardness was measured at various processing stages, and hardness changes are discussed in terms of the measured phase fractions, crystallite size and lattice strain at different temperatures and times.

Corrosion Study and Passive Film Characterization of 11% Cr F/M and 15% Cr ODS Steels

Corrosion Study and Passive Film Characterization of 11% Cr F/M and 15% Cr ODS Steels

The surface characterization and corrosion resistance of 11 % Cr ferritic/martensitic and 9–15 % Cr ODS steels for nuclear fuel reprocessing application

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 HNO3 to 9 M HNO3 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 HNO3. 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 Fe2O3, Cr2O3, and Y2O3, 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.

High temperature reaction tests between high-Cr ODS ferritic steels and U–Zr metallic fuel

Out-of-pile high-temperature reaction tests are carried out on the diffusion couples between U–Zr fuel and ODS ferritic steels, in which the concentrations of Cr and Al were systematically changed for improvement of corrosion resistance. The results show that the Al addition to ODS ferritic steel considerably improves the compatibility between U–Zr fuel and the ODS steel. The threshold temperature for reaction layer formation is roughly 50K higher in the Al-containing ODS ferritic steels than in the steels without Al addition for the testing time up to 900min. The compatibility improvement mechanism by Al addition is discussed from the viewpoint of activity change. The activity calculation results obtained using general thermodynamic data indicate the possibility that stabilization of the intact α-Zr layer by Al addition is the main mechanism and occupation of steel lattice sites by Al that does not easily diffuse into γ-U–Zr also partly contributes to suppression of the inter-diffusion.

Comparative study of hydrogen uptake and diffusion in ODS steels

In this work EUROFER 97, ODS-EUROFER, and PM2000 steels are studied in terms of their interaction with hydrogen at ambient temperature. Hydrogen uptake and effective activation energy of its diffusion and trapping are calculated from the thermal desorption spectra obtained for the studied steels in the temperature interval up to 1123K after the electrochemical hydrogen charging. It was observed that hydrogen uptake in ODS-EUROFER and PM2000 is markedly higher than in EUROFER 97 steel. It is shown that hydrogen in ODS-EUROFER has the effective diffusion activation energy, which is 0.08 and 0.06eV larger than that in EUROFER 97 and PM2000, respectively. The high-temperature components of the TDS peaks in the studied ODS steels are suggested to reflect hydrogen de-trapping from the oxide nanoparticles. The mean particle size, the size distribution and chemical composition are studied using SEM and TEM techniques.