Oxide Dispersion Strengthened Ferritic Steels Research Articles

Effect of Nitrogen Atmosphere Annealing of Alloyed Powders on the Microstructure and Properties of ODS Ferritic Steels.

Oxide Dispersion Strengthened (ODS) ferritic steels are promising materials for the nuclear power sector. This paper presents the results of a study on the sintering process using the Spark Plasma Sintering (SPS) technique, focusing on ODS ferritic steel powders with different contents (0.3 and 0.6 vol.%) of Y2O3. The novelty lies in the analysis of the effect of pre-annealing treatment on powders previously prepared by mechanical alloying on the microstructure, mechanical, and thermal properties of the sinters. Using the SPS method, it was possible to obtain well-densified sinters with a relative density above 98%. Pre-annealing the powders resulted in an increase in the relative density of the sinters and a slight increase in their thermal conductivity. The use of low electron energies during SEM analysis allowed for a fairly good visualization of the reinforcing oxides uniformly dispersed in the matrix. Analysis of the Mössbauer spectroscopy results revealed that pre-annealing induces local atomic rearrangements within the solid solution. In addition, there was an additional spectral component, indicating the formation of a Cr-based paramagnetic phase. The ODS material with a higher Y2O3 content showed increased Vickers hardness values, as well as increased Young's modulus and nanohardness, as determined by nanoindentation tests.

Effects of Cr and Si addition on the high-temperature oxidation resistance in high-Mn alumina-forming oxide dispersion strengthened austenitic steels

Oxide dispersion-strengthened (ODS) steels are promising candidates for constructing nuclear reactors. Considering a high-temperature environment, ODS austenitic steels could provide superior performance compared to the current ODS ferritic steels. To meet the reduced activation requirement, the compositions of ODS austenitic steels have to be carefully adjust. Currently, the authors focus on the ODS austenitic steels with the austenite stabilizer of Mn, and several amounts of Al are added to improve their high-temperature oxidation resistance. This study investigates the third-element effects of Cr and Si on the high-temperature oxidation resistance of high-manganese alumina-forming ODS austenitic steels. Generally, Si-containing steel exhibits superior oxidation resistance owing to the formation of a continuous inner layer of amorphous alumina. In contrast, the Cr-containing steel does not possess a continuous alumina layer, resulting in insufficient oxidation resistance. The reported phenomenon is believed to be a reference for the development of ODS austenitic steels in the future.

Microstructural stability of secondary phases in an ODS ferritic steel after thermal aging at 873 K

An oxide dispersion strengthened (ODS) steel with nominal composition Fe–14Cr–2W–0.4Ti–0.3Y2O3 (wt%) has been manufactured by mechanical alloying of pre-alloyed powders with nanosized Y2O3, compacted by hot isostatic pressing and hot cross rolled. In order to evaluate the long-term thermal resistance of the alloy, it has been subjected to 2000 h of thermal aging at 873 K, which is a relevant temperature for nuclear reactor applications. A thermodynamic equilibrium simulation indicates the precipitation of Laves phase under the aging parameters used. This prediction is confirmed from the detailed multi-technique characterization performed. Before aging, Ti-rich oxides, Cr-W-rich precipitates (M23C6 type) and a homogeneous Y-rich nanoprecipitate dispersion are observed. After aging, some W-rich precipitates are identified as Laves phase, while M23C6 carbides, Ti-rich oxides and Y-rich nanoprecipitates remain stable. Mechanical characterization performed in a previous research showed higher hardness, a loss of total elongation and a slight shift of the ductile-to-brittle transition temperature (DBTT) towards a higher value after aging, with similar strength values before and after the long-term thermal treatment. These changes can be due to the redistribution of precipitates together with the Laves phase formation. The mechanical properties not being dramatically affected seem to be due to the observed stability of Y-rich nanoprecipitates distribution.

Development of oxide dispersion strengthened ferritic steel with enhanced strength–ductility combination through dispersion strengthening and back stress hardening

Oxide dispersion strengthened (ODS) ferritic steel is a potential structural material for fast neutron reactor and fusion reactor application. In this study, a new Fe–13Cr–2W-0.4Ti-0.6Zr/0.35Y2O3 ODS ferritic steel was designed and fabricated through hot isostatic pressing. Results show that the ODS ferritic steel consists of highly dispersive nano-oxide particles, identified as Y2(Ti0.4Zr0.6)2O7 and Y4Zr3O12, with average particle size of 5.34 nm and particle distribution density of 4.2 × 1023 m−3. In addition, heterogeneous coarse/ultrafine grains (CGs/UFGs) structure is also achieved in the matrix. The Fe–13Cr–2W-0.4Ti-0.6Zr/0.35Y2O3 ODS ferritic steel exhibits superior mechanical properties at both room temperature (σb = 1208 MPa, δ = 14%) and high temperature (σb, 700°C = 382 MPa, δ700°C = 32%). The high strength is mainly due to the synergistic effect of particle dispersion strengthening and back stress strengthening, while the good ductility can be attributed to the excellent deformation ability of the coarse grains and the back stress hardening effect induced by the heterogeneous CGs/UFGs structure.

High Temperature Mechanical Properties and Microstructure in 9Cr or 12Cr Oxide Dispersion Strengthened Steels

Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700°C or 750°C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900°C to 1350°C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, M11 and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high-temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900°C to 1100°C, but increased at 1200°C, decreased drastically at 1250°C, and increased again at 1300°C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the δ-ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300°C.

Twofold age-hardening mechanism of Al-added high-Cr ODS ferritic steels

Age-hardening of oxide dispersion strengthened (ODS) ferritic steels with various contents of Cr and Al was investigated by Vickers hardness (HV) measurement and tensile test after ageing at 475°C for up to 9000 h. The correlation between age-hardening and the evolution of nanometer-scale structure and chemistry studied by atom probe tomography (APT) reveals that the age-hardening could be interpreted in terms of twofold hardening mechanism involving α–α′ phase separation and/or the precipitation of (Al, Ti)-enriched β′ phases. The results of APT characterization indicate that the Cr concentration and volume fraction of α′ phase increase significantly while the chemical compositions and volume fraction of β′ phases don't changed considerably during ageing from 300 h to 9000 h. At the early stage of ageing, the age-hardening due to α–α′ phase separation is very limited because of the low Cr concentration of α′ phase, which results in very weak modulus strengthening. With the increasing of ageing time, the age-hardening is increased because of the increasing of Cr content in the α′ phase, resulting in considerably enhanced modulus mismatch strengthening. However, age-hardening due to β′ phase precipitation is significant once the β′ phases are formed and, thereafter, it doesn't change apparently. The metal/oxide interfaces in ODS steels provide nucleation sites for α′ and β′ phases. Based on the comprehensive data developed, the roles of Cr and Al in the age-hardening of ODS ferritic steels were elucidated.

Stability of untransformed ferrite in 10Cr ODS steel

The development of generation IV nuclear reactor requires improvements in structural and cladding materials. Oxide dispersion strengthened (ODS) steels are promising candidates because of improved creep properties. Two main families of ODS steels are known, the first one is martensitic ODS steel with a full reverse ferrite to austenite phase transformation at high temperatures. The second one is ferritic ODS steels with a ferritic matrix whatever is the temperature. Previous papers mention the existence of untransformed ferrite (UF) phase at high temperature in very particular martensitic ODS steels [S. Ukai 2009, M. Yamamoto 2010, T. Yamashiro 2016, A. Durand 2021]. UF is unexpected according to experimental observation and thermodynamic calculations of the non-strengthened materials (NR) with same chemical composition. This UF is particularly interesting because UF is a way to improve the creep resistance of martensitic grades [H. Sakasegawa 2008, A. Durand 2021] and then conciliates the benefits of both ferritic and martensitic grades. The present study is focused on the thermal stability of untransformed ferrite (UF) in a martensitic ODS steel at 10 wt% Cr. Effects of the heat treatments holding times and temperatures are studied by considering the UF fraction evolution and microstructural changes. The decrease of UF fraction is correlated with the decrease of the nano-oxides density coupled to the increase of their size. The phase transformation temperatures of ODS steels are also affected by heat treatments. Those temperatures appear as a mix of the ODS and the equivalent non-strengthened materials ones.

Experimental and Numerical Small Punch Tests of the 14Cr ODS Ferritic Steel

Abstract Nowadays, various small specimen test techniques have gained wide popularity and appreciation among researchers as they offer undoubtful benefits in terms of structural material characterisation. This paper focuses on small punch tests (SPTs) performed on small-sized disc specimens to assess the mechanical properties of 14Cr oxide dispersion strengthened (ODS) steel. A numerical model was established to support experimental data and gain deeper insight into complex strain states developing in a deformed specimen. Modern evaluation procedures were discussed for obtaining mechanical properties from the small punch force-deflection response and were compared with the literature. Applicability and universality of those relations at different test conditions were also studied. It appeared that different ball diameters used had negligible influence on yield point but strongly affected ultimate strength estimation. It was found that friction belongs to decisive factors determining strain distribution in samples, as dry conditions increase the peak strain and move its location farther from the punch pole.

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.

Powder Particle Size Effects on Microstructure and Mechanical Properties of Mechanically Alloyed ODS Ferritic Steels

Reduced activation ferritic (RAF) steels are expected to be widely used in challenging nuclear industrial applications under severe thermo-mechanical regimes and intense neutron loads. Therefore, actual research panorama is facing the strengthening strategies necessary to maximize both performance and endurance under these conditions. Oxide dispersion strengthened (ODS) RAF steels are leader candidates as structural materials in fusion energy reactors thanks to the reinforcement obtained with a fine dispersion of nanosized oxides in their matrix. In this study, the influence of the initial powder particle size and the selected processing route on the final material has been investigated. Two RAF ODS steels coming from atomized pre-alloyed powders with nominal particle powder sizes of 70 and 30 µm and composition Fe-14Cr-2W-0.4Ti-0.3Y2O3 (wt. %) were manufactured by mechanical alloying. Alloyed powders were compacted by hot isostatic pressing, hot crossed rolled, and annealed at 1273 K. Initial powder particle size differences minimize after milling. Both steels present an almost completely recrystallized material and similar grain sizes. The same type and distributions of secondary phases, Cr-W-rich, Ti-rich, and Y-Ti oxide nanoparticles, have been also characterized by transmission electron microscopy (TEM) in both alloy samples. The strengthening effect has been confirmed by tensile and Charpy impact tests. The two alloys present similar strength values with slightly better ductile brittle transition temperature (DBTT) and ductility for the steel produced with the smaller powder size.

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

Effect of Pulsed Fluxes of Deuterium Ions and Deuterium Plasma on Oxide Dispersion Strengthened Ferritic Steels

Effect of Pulsed Fluxes of Deuterium Ions and Deuterium Plasma on Oxide Dispersion Strengthened Ferritic Steels

Microstructure and mechanical properties of an ODS ferritic steel with very low Cr content

An ultra-low Cr oxide dispersion strengthened (ODS) steel with the composition of Fe-5Cr-4.5Al-0.6Zr (hereinafter abbreviated as 5CrAlZr ODS steel) were proposed and manufactured via mechanical alloying (MA) and hot isostatic pressing (HIP), followed by hot forging treatment. TEM results indicate that a large amount of fine δ-Y4Zr3O12 particles were formed within the grains and along the grain boundary, additionally, Y6ZrO11 and Al2Y4O9 nano-particles with larger size were also detected. The mean particles diameter (dp), the mean inter-particle spacing (λ) and number density (nv) of the nano-oxide particles in the 5CrAlZr ODS steel are about 10 nm, 80 nm and 1.23 × 1022 /m3, respectively. The room temperature ultimate tensile strength (UTS) and the total elongation (TE) of the fabricated 5CrAlZr ODS steel are 875 MPa and 21%, respectively, which are higher than those of traditional 5Cr-2W-0.24V steels. The UTS and TE of 5CrAlZr ODS steel at 700°C are 234 MPa and 58%, respectively. The strengthening mechanisms underlying were also discussed.

Effect of mechanical alloying on the microstructural evolution of a ferritic ODS steel with (Y–Ti–Al–Zr) addition processed by Spark Plasma Sintering (SPS)

The high-energy milling is one of the most extended techniques to produce Oxide dispersion strengthened (ODS) powder steels for nuclear applications. The consequences of the high energy mill process on the final powders can be measured by means of deformation level, size, morphology and alloying degree. In this work, an ODS ferritic steel, Fe–14Cr–5Al–3W-0.4Ti-0.25Y2O3-0.6Zr, was fabricated using two different mechanical alloying (MA) conditions (Mstd and Mact) and subsequently consolidated by Spark Plasma Sintering (SPS). Milling conditions were set to evidence the effectivity of milling by changing the revolutions per minute (rpm) and dwell milling time. Differences on the particle size distribution as well as on the stored plastic deformation were observed, determining the consolidation ability of the material and the achieved microstructure. Since recrystallization depends on the plastic deformation degree, the composition of each particle and the promoted oxide dispersion, a dual grain size distribution was attained after SPS consolidation. Mact showed the highest areas of ultrafine regions when the material is consolidated at 1100 °C. Microhardness and small punch tests were used to evaluate the material under room temperature and up to 500 °C. The produced materials have attained remarkable mechanical properties under high temperature conditions.

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

Attributes of Spark Plasma Sintering on Developing Oxide Dispersion Strengthened Ferritic Steels and Understanding Structure–Property Relation

Ferritic alloys strengthened by oxide dispersoids having a composition of Fe–16Cr–2W–0.3Ti–0.5Y2O3 and Fe–16Cr–2W–0.3Ti–0.75Y2O3 were developed through mechanical alloying followed by spark plasma sintering process. The densification behaviour of ferritic steel was understood while sintering at a temperature of 900 °C with soaking for 5 min, at the applied pressure of 50 MPa. X-ray diffraction analysis was done on milled powders and sintered bulk samples to confirm the alloying and crystallographic changes. Field emission scanning electron microscopy (FESEM) was carried out to study the morphology and FESEM-EDS to ascertain the composition of alloyed powders. Transmission electron microscopy study conceded the evolution of nanocrystalline ferritic steel during MA. Distinctive stages of mechanical alloying were observed. System with 0.5 Y2O3 displayed better densification behaviour and possessed higher mechanical properties.

Temperature dependent strengthening contributions in austenitic and ferritic ODS steels

We aim on the model-based description of the strength of ferritic and austenitic oxide dispersion strengthened (ODS) steels in the temperature range from room temperature (RT) up to 800 °C. Therefore, we present two approaches for the synthesis of austenitic alloys by mechanical alloying Y2O3, namely with (i) elemental powders at RT and (ii) with a gas-atomized master-alloy. Consolidation of both powders by field assisted sintering technique leads to a more homogenous distribution of grain size and particles in specimens from elemental powders. In the entire temperature range, the compressive strength of the austenitic ODS steels is shown to be lower compared to the one of ferritic counterparts. Above approximately 500 °C, a strong decrease in strength is observed for all ODS variants due to the onset of creep-based deformation. Multi-scale materials characterization is performed to quantitatively assess microstructural materials parameters crucial for the modeling of the temperature dependent yield strength. These data are utilized to quantitatively describe the strength contribution by Hall-Petch and Orowan strengthening as well as dislocation strengthening at RT. Lower amounts of grain boundary and dislocation strengthening are found to be crucial for the lower strength of austenitic ODS steels. Meaningful calculation of materials strength is only achieved, when both interactions of strengthening contributions and experimental uncertainties are considered. Models describing diffusion-based creep (by Coble) and dislocation-based creep (by Blum and Zeng), which are shown to provide a more appropriate description of high temperature strength, are critically assessed for temperatures at and above the strength drop. It is shown that the deformation at high temperatures is possibly dominated by the formation and annihilation of dislocations at grain boundaries.

Annealing behavior of hardening and ductility loss of a 16Cr–4Al ODS ferritic steel irradiated with high energy Ne ions

Annealing behavior of the hardening and ductility loss of an oxide-dispersion-strengthened (ODS) ferritic steel (16Cr–4Al) irradiated with high-energy heavy ions is studied. 20Ne ions with 123.4 MeV supplied by a cyclotron was used to produce a quasi-uniform atomic region of atomic displacement damage and Ne concentration from surface to a depth of 33 μm in the specimens. Two atomic displacement levels (0.16dpa, 0.70dpa) were approached at about 220 K. The irradiated specimens were subsequently thermally annealed at room temperature, 473 K and 673 K, respectively. Hardness and ductility of the specimens were investigated with nano-indentation technique and small-punch test. The data of the nano hardness were fitted by Nix-Gao model to obtain the bulk equivalent hardness values. The irradiated specimens show observable hardening, which recedes monotonously with the increase of annealing temperature. The Arrhenius plots show a good linearity in the entire temperature range investigated, with an apparent activation energy of 0.13 ± 0.01 eV for both the two damage levels. Assuming that the hardening is caused by the self-interstitial-atom (SIA) clusters initially produced by the cascade damage, an activation energy of 0.70 ± 0.05 eV is deduced for the migration and coalescence process of the SIA-clusters. The small punch test (SPT) of the high-dose specimens shows that a minor ductility loss occurred under conditions as-irradiated or subsequently thermally annealed at 473 K, while a remarkable increase of the ductility loss was observed after the thermal annealing at 673 K, which coincides with the formation of nano-scale gas bubbles in high density in the specimen. It is indicated that the formation of bubbles has minor on the irradiation hardening. A comparison of three ODS ferritic steels Ne-ion-irradiated to 0.7 dpa/260 appm(Ne) shows that the ductility loss decreases with the increase of the sink strength of the oxide dispersoids/ferritic substrate interfaces.

In situ synchrotron analysis of phase transformation at high temperatures in ODS ferritic steel

Texture and microstructure evolution in a ferritic/martensitic 9wt%Cr oxide dispersion strengthened (ODS) steels during heat treatment after cold tube forming are studied via in situ synchrotron X-ray diffraction analysis. The initial textures of cold-formed ODS steel tubes present a well-known α-fiber ( //RD). During high-temperature heat treatment, this initial sharp fiber texture, inherited from cold forming, is transformed by the phase transition to austenite into a weak fiber. During cooling, a texture memory effect is observed with a strong dependency of the fiber intensity on the cooling rate. After quenching into martensite, the fiber becomes weaker than in the initial ferrite. By contrast, after slow cooling into ferrite, the fiber develops and becomes stronger than initially. For many materials, texture memory effect can be explained by a variant selection phenomenon. Accordingly, experimental results are compared to the double Kurdjumov–Sachs (DKS) model. During heating, a limited texture heritage is exhibited with essentially a Kurdjumov–Sachs (K–S) relation between the two phases. During cooling the DKS relation correctly predicts the texture fiber components. However, the strong enhancement of texture during slow cooling remains not completely explained. A Williamson–Hall analysis of the size and strain evolutions during the heating cycle showed a remarkable stability of the crystallites size which is the same in ferrite, austenite and martensite.

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.