ODS Steel Research Articles

Helium in swift heavy ion irradiated ODS alloys

Combination of room temperature He ion implantation and swift heavy ion (SHI) irradiation in the multiple ion track overlapping regime has been used to study helium porosity in EP450 ODS steel containing crystalline or fully amorphous Y-Ti nano-oxides. It was found that helium bubbles formed during post-irradiation annealing have much better adhesion to amorphized precipitates in comparison with crystalline ones. However, this does not affect the overall porosity of the alloy which is defined by helium bubbles formed on structural defects inside the ferrite grain body and on grain boundaries.

Recrystallization behavior of a two-way cold rolled 12Cr ODS steel

Recrystallization behavior of a two-way cold rolled 12Cr ODS steel during annealing in the range of 1273–1473 K was systemically investigated by means of scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and hardness testing. Results show that original grains with <110> axis were rotated to <111> axis parallel to normal direction after ∼60% reduction in thickness. Recovery stage was obvious at 1273 K annealing taking about 100 min, but at higher temperatures it was shortly followed by recrystallization. ∼84% recrystallization was observed after annealing at 1473 K for 60 min, indicating that the new cold rolling process decreases the recrystallization temperature in comparison with the as-fabricated specimen completing recrystallization at 1673 K. In addition, the microtexture evolution was characterized as recrystallization proceeded. The intensity of {111}<110> and {111}<112> components decreased, while intensive {110}<001> component was obtained. Dependence of the stored energy on the orientation of deformed grains is presumed for the reduction of {111}<110> and {111}<112> components, and oriented growth is probably responsible for the enhancement of {110}<001> texture after recrystallization.

Ferritic and martensitic ODS steel resistance upset welding of fuel claddings: Weldability assessment and metallurgical effects

Development and characterization of clad-to-plug resistance upset welds is presented for two oxide dispersion strengthened steels candidates for sodium-cooled fast reactors fuel cladding: 9Cr ferrito-martensitic steel and 14Cr ferritic steel. A comparative approach is adopted regarding these two materials implying weldability studies and weld mechanical strength tests. A special attention is paid on welding metallurgical effects on the specific microstructure of these materials. Among others, some major grain property modifications by dynamic recrystallization and a slight modification of precipitate sizes and volume fraction are found and discussed.

Effect of Cold Rolling on Recrystallization Behavior of Al-Free and Al-Added 15Cr-ODS Ferritic Steels

The effect of cold rolling on the recrystallization behavior of Al-free and Al-added 15Cr-oxide dispersion strengthened (ODS) ferritic steels was investigated. The recrystallization of both steels are enhanced by cold rolling. The Al-free ODS steel with finer oxide particles is harder to recrystallize than Al-added ODS steel with coarser oxide particles. The effect of Al addition on the recrystallization behavior is evident. It is estimated that the recrystallization temperature of Al-free and Al-added 15Cr-ODS ferritic steel is 900 °C and 1250 °C with the annealing period of 1 h. In Al-free ODS steel, a small hardening was observed in the temperature range between 850 °C and 1200 °C, while no such phenomenon was observed in Al-added ODS steel, which is indicative of retardation of recovery by finely dispersed oxide particles. Oxide particle growth is mostly dependent on annealing temperature, while recrystallization and grain growth are controlled by not only the temperature, but the cold rolling ratio, which alters the multiple factors such as dislocation density, initial grain shape and oxide particle dispersion morphology. The cold rolling direction also influences the grain morphology and grain orientation in Al-added ODS steel, and the second rolling in a perpendicular direction to the first cold rolling direction induces the rotation of the grains from &lt;110&gt; to &lt;112&gt;. The recrystallization temperature is not significantly changed by the cold rolling direction. Recrystallization after cold rolling appears to increase the {111} grain orientation on the cold rolled specimen surface.

Precipitation of various oxides in ODS ferritic steels

The reactive elements (Ti, Al and Zr) have been widely used for the improvement of the strength or the oxide resistance of ODS steels. In the present work, we studied the effect of the reactive elements on the precipitation of oxide particles, especially focusing upon the appropriate amount of the oxide composing elements, by addition of the large amounts of Y2O3 and reactive elements. The mechanically alloyed powders are annealed in a wide temperature range from 773 to 1423 K and then analysed by various analysis methods (small-angle X-ray scattering and X-ray diffraction by synchrotron radiation X-ray, scanning transmission electron microscope combined with energy dispersion X-ray spectroscopy, high-resolution transmission microscopy and 3D-atom probe). Ti, Al and Zr enhanced the growth of oxide particles, opposite to the many studies of Ti- and Zr-added ODS steels, as a result of the addition of Y2O3 and these elements ten times larger than those for usual use. The nucleation and growth of oxide particles are discussed by simulating the critical radius of nucleation and the growth based on the LSW theory. Y4Zr3O12 is stable and easy to precipitate even at lower annealing temperature. All reactive elements enhance the growth of oxide particles because of their big molar volume as compared to that of Y2O3.

Effect of the heating rate on the microstructure of a ferritic ODS steel with four oxide formers (Y-Ti-Al-Zr) consolidated by spark plasma sintering (SPS)

The proposed ODS ferritic steel alloyed with (Y-Ti-Zr-Al) was produced by mechanical alloying (MA) and spark plasma sintering (SPS) to obtain a complex nanostructure. To densify the material, a sintering cycle by SPS was performed at 1100 °C using fast heating rates (from 100 to 600 °C/min). During the attrition of MA powders, the uneven distribution of deformation level and of alloying elements has produced an inhomogeneous recrystallization during the consolidation step. Influence of processing condition was studied by modifying the heating rate of SPS to promote a heterogeneous material with bimodal grain size distribution. The final microstructures were characterized by X-ray diffraction and electron microscopy (SEM and TEM). The mechanical behaviour at R.T. was characterized by means of the Vickers microhardness and micro tensile tests. The good balance obtained between ductility (∼22–26%) and yield stress (800–910 MPa) at room temperature is provided by the bimodal grain size distribution. To predict the experimental values depending on the processing conditions, a yield strength model is presented. This model covers the contribution of different strengthening mechanism from solid solution, grain size, dislocation density and oxides precipitation. The model indicates the dislocation density as the major strengthening contribution. In addition, small punch (SP) tests were performed to analyse the response of the material at high temperatures where remarkable properties have been achieved.

Investigation of the mechanical properties of ODS steels at high temperatures using nanoindentation technique

In this study, mechanical properties of two ODS steels, characterized by the same chemical composition yet manufactured by using two different production methods, namely High Isostatic Pressure (HIP) and Hot Extrusion (HE), have been investigated at high temperature by nanoindentation technique. The hardness of the material was evaluated from room temperature up to 600 °C, while initial creep behavior was estimated at room temperature and at 600 °C. In order to better understand the mechanical response of the materials, especially impact of the surface effects, mechanical maps (hardness and reduced young modulus) have been created.

Ultra-high temperature creep rupture and transient burst strength of ODS steel claddings

9Cr-ODS steel claddings consisting of tempered martensitic matrix, showed prominent creep rupture strength at 1000 °C, which surpassed that of heat-resistant austenitic steels although creep rupture strength of tempered martensitic steels is generally lower than that of austenitic steels at high temperatures. The measured creep rupture strength of 9Cr-ODS steel claddings at 1000 °C was higher than that from extrapolated creep rupture trend curves formulated using data at temperatures from 650 to 850 °C. This superior strength seemed to be owing to transformation of the matrix from the α-phase to the γ-phase. The transient burst strengths for 9Cr-ODS steel were much higher than those for 11Cr-ferritic/martensitic steel (PNC-FMS). Cumulative damage fraction analyses suggested that the life fraction rule can be used for the rupture life prediction of 9Cr-ODS steel and PNC-FMS claddings in the transient and accidental events with a certain accuracy.

Investigation of the Fracture Mechanics Properties of Small Tubes from Oxide Dispersion Strengthened Steels

The paper presents one part from the RATEN ICN contribution to the European FP7 MatISSE Project objectives, which is focused on the fracture mechanics properties of small tubes made from ODS steels (Oxide Dispersion Strengthened steels). The ODS tubes are foreseen as cladding tubes for gen IV reactors, and therefore the mechanical properties are very important for working in the most aggressive environment (irradiation and high temperatures) during the gen IV reactor operation. The fracture toughness, KIC, could be obtained for tubes with small diameters by means of the PLT-type mechanical test (acronym for Pin-Loading Test). This kind of test is a non-standard (ASTM) mechanical test, and during last decade it is still worldwide under development. The specific specimens for PLT-tests has been prepared from ODS tubes, which were provided to the project by the CEA France, a partner in the FP 7 MatISSE, in two different compositions: Fe -9cr ODS and Fe -14 Cr ODS. The paper highlights the PLT experimental test methodology, starting with obtaining of the geometric function, description of the experimental set-up and results processing.

The Study of the Mechanical Properties of Small Tubes Made from Oxide Dispersion Strengthened Steel by Using the Ring Tension Tests

The paper aims to investigate the mechanical properties of tubes having a small diameter and made from new steels called ODS steels Oxide Dispersion Strengthened steels). These new materials are candidate materials for fuel claddings in the generation IV reactors. This work contributes to original results to the experimental program carried out by RATEN ICN Pitesti in the framework of the FP7 European MatISSE Project (Materials� Innovations for a Safe and Sustainable nuclear �n Europe). The experimental method used for the investigating of the mechanical properties of ODS steel tubes is a non-standard method, known as Ring Tension Test (acronym - RTT). It is recommended for the mechanical testing of tubes with thinner walls and reduced inner diameter. The main advantage of RTT is that the tested sample maintains the initial geometry of the component from which has been made. For the experimental RTT tests, the ODS tubes were provided by MatISSE Project participant, CEA France. Tubes are in two different compositions: Fe-9Cr ODS and Fe-14Cr ODS. For a better understanding of the ODS mechanical properties, some further investigation on the scanning electron microscopy is carried out.

Nano-scale Inclusions in ODS 12Cr–0.2V–0.3Ti Steel and Its Stability under Fe Ion Irradiation

Advanced mechanical properties of ODS steels are mainly due to the high number density of homogeneously distributed oxide inclusions. It is well known that some alloying elements like Ti, V, Zr, … play important role in oxide/nanocluster formation and influence on number density and size of these inclusions. In this work, we studied an ODS steel contained both vanadium and titanium. The ODS 12Cr-1.1W-0.2V-0.3Ti steel was characterized with TEM and APT. Different types of oxides inclusions were revealed in the steel: large (>70 nm) Ti2O3 oxides, small (~ 2–15 nm) Y-Ti-O oxides, and nanoclusters (2–5 nm). It was shown that the number density of these nanoclusters was considerably high than the one of the oxides and nanoclusters are enriched in Y, O, Cr, Ti, and V. Moreover, in spite of the fact that V and Ti are present in the material in approximately close value, clusters are predominantly enriched in Ti. APT samples of this material were irradiated with Fe ions up to ~ 8 dpa at room temperature. APT analysis of the irradiated material revealed decreasing of nanocluster number density with increasing their sizes. Changing in nanocluster composition was detected after the irradiation, but the decrease of Ti concentration in clusters was considerably less than the one for V. However, the Y/Ti, Y/(Ti + V), and (Ti + V + Y)/O composition ratios of nanoclusters remained stable under Fe ion irradiation to 8 dpa at room temperature.

Oxide-Dispersion Strengthened Radiation-Resistant Steels

The better characteristics of the long-term strength of the EP-450-ODS (oxide dispersion strengthened) steel are determined compared to the well-known oxygen-free reactor steels. Using the Mossbauer analysis of the deformed powder mixture 56Fe3O4 + 57Fe, the possibility of the deformation-induced dissolution of iron oxides in the steel matrix, which makes it possible to obtain high quality ODS steels in the process of subsequent high-temperature annealing (sintering), is shown. The enhanced strength characteristics are obtained in recrystallized samples of ODS iron (YS0.2 ~ 570 MPa, UTS ~ 632 MPa) in the absence of any other alloying elements except for oxygen of air.

Joint inhomogeneity in dissimilar friction stir welded martensitic and nanostructured ferritic steels

ABSTRACTDissimilar welding between oxide dispersion strengthened ferritic (ODS) steel and reduced activation martensitic steel would be required for constructing the advanced blanket of progressive fusion reactors. In this study, we achieved dissimilar joints by friction stir welding, and aimed to characterise and ameliorate joint inhomogeneity. Main results reveal that the joint inhomogeneity is generated from discrepant microstructural evolutions within the martensitic and ODS ferritic steels. The ODS steel achieves evolution by the dynamic recrystallisation, while the martensitic steel undergoes phase transformation that drastically hardens the stir zone. By a proper post-weld heat treatment, the joint inhomogeneity can be effectively ameliorated due to carbide reprecipitation and stress relief in the joint.

Ab initio modelling of the Y, O, and Ti solute interaction in fcc-Fe matrix

Abstract Strengthening of the ODS steels by Y2O3 precipitates permits to increase their operation temperature and radiation resistance, which is important in construction materials for future fusion and advanced fission reactors. Both size and spatial distribution of oxide particles significantly affect mechanical properties and radiation resistance of ODS steels. Addition of the Ti species (present also as a natural impurity atoms in iron lattice) in the particles of Y2O3 powder before their mechanical alloying leads to the formation of YTiO3, Y2TiO5, and Y2Ti2O7 nanoparticles in ODS steels. Modelling of these nanoparticle formation needs detailed knowledge of the energetic interactions between ions. We have performed detailed first principles calculations of the interactions between two Y and O atoms, two Ti and O atoms as well as between Y, Ti, and O atoms which will be used in further kinetic Monte Carlo modelling of the nanoparticle formation.

Characterization of ODS steel friction stir welds and their abnormal grain growth behaviour

We have characterized three friction stir butt welds of MA956 ODS steel produced using different traverse speeds of the welding tool, by a combination of micro-hardness testing and optical and electron microscopy. The welds were also given a high temperature heat treatment at 1380 °C for one hour to induce abnormal grain growth. The mean grain size at all measured locations increased with a lower welding speed, due to the increased thermal energy into the weld. The grain size changed gradually across the stir zone of the weld, with larger grains present towards the top of the weld and on the advancing side. This was accompanied by lower hardness values at those locations. Shear banding, in the thermo-mechanically affected zone, and a deformed region of the base material, was clearly observed for all welds at the weld border. The post-weld heat treatment was able to induce abnormal grain growth in all the welds, creating a very coarse microstructure with grain sizes in the order of hundreds of microns or millimetres. The coarse-grained structure seemed to develop from the top of the stir zone, close to the surface fine-grained layer, and progressed downwards until it generally covered the entirety of the welds' stir zone. Abnormal grain growth did not occur in the border region of the welds, most likely due to the observed local particle pile-up in that region.

Effects of Ti addition on microstructure and mechanical property of spark-plasma-sintered transformable 9Cr-ODS steels

By adding Ti consistent, dual-phase 9Cr ODS steel with martensite and transformed ferrite is fabricated by spark plasma sintering, and M23C6 carbides are identified in transformed ferrite. Although Ti is strong carbide forming element, Ti concentration has a negligible effect on the distribution of M23C6 within ferrite, as well as martensite size. With Ti addition increasing, the amount of ferrite is increased, and more fine Y-Ti-O complex oxides are produced. Annealing treatment will not change the ferrite distribution, but the density and size of oxide nanoparticles in steels will be affected. It is found that Ti addition can effectively improve the tensile strength of 9Cr ODS steels, which is related to the Y-Ti-O oxides with small size. The large-sized ferrite grains will induce ductile-brittle mixed fracture, deteriorating the ductility of transformable 9Cr ODS steels.

Why Do Secondary Cracks Preferentially Form in Hot-Rolled ODS Steels in Comparison with Hot-Extruded ODS Steels?

Secondary cracks are known to absorb energy, retard primary crack propagation and initiate at lower loads than primary cracks. They are observed more often in hot-rolled than in hot-extruded ODS steels. In this work, the microstructural factors responsible for this observation are investigated. Better understanding of these factors can lead to tailoring of improved ODS steels. Fracture toughness testing of two batches of 13Cr ODS steel, one hot-rolled and the other hot-extruded, was carried out. The fracture behaviour of secondary cracks was investigated using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Crystallographic texture and grain morphology play a predominant role in preventing secondary cracks in hot-extruded ODS steels. At lower temperatures, secondary cracks occur predominantly via transgranular cleavage. The fracture mode changes to ductile and intergranular at higher temperatures.

Effect of Zr and Al addition on nanocluster formation in oxide dispersion strengthened steel - An ab initio study

Conventional oxide dispersion strengthened steels are characterized by thermally stable, high density of Y–Ti–O nanoclusters which are responsible for their high creep strength. Ti plays a major role in obtaining a high density of ultrafine particles of optimum size range 2–10 nm. In Al containing ODS steels developed for corrosion resistance, Y–Al–O clusters formed are of size range 20–100 nm and Ti fails in making dispersions finer in presence of Al. Usage of similar alloying elements like Zr in place of Ti is widely considered. In this study, binding energies of different stages of Y–Zr–O–vacancy and Y–Al–O–vacancy complexes in bcc Iron matrix are studied by first principle calculations. It is shown that in all the stages of formation, Y–Zr–O–vacancy clusters have higher binding energy than Y–Al–O–vacancy clusters. Hence in a ferritic steel containing both Zr and Al, Y–Zr–O clusters are more stable and favored to nucleate over Y–Al–O clusters. The bonding nature in each stage is analysed using charge density difference plots to help establish the plausible reason for the higher stability of Y–Zr–O clusters.

Strengthening mechanisms of different oxide particles in 9Cr ODS steel at high temperatures

The influence of different oxide particles on deformation mechanisms of 9Cr ODS steels at temperatures 400–600 °C is tested. Two ODS variants with Al2O3 and Y2O3 dispersed particles respectively and one variant without particles have been prepared by powder metallurgy. The deformation mechanisms are investigated by the tensile and stress-relaxation test at temperatures 400 °C and 600 °C. The analysis is based on a comparison of athermal and thermal stress components of the flow stress, apparent activation volume and a strain rate hardening. Microstructural parameters are evaluated upon SEM and TEM observations. The contributions of different strengthening mechanisms are assessed using the Taylor, Hall-Petch, dispersed barrier and Orowan models. Alumina particles provide the highest strengthening which can be explained by the stress decrease around the particles induced by a high difference between Young's moduli of alumina and ferritic matrix. This stress field can trap the dislocation in the vicinity of particles. Due to this stress decrease, alumina particles provide strengthening even at 600 °C in contrast to yttria particles that lose their effect due to their overcoming by a non-conservative motion of dislocations.

Effects of pre-deformation on microstructural evolution of 12Cr ODS steel under 1473–1673 K annealing

The effects of various thickness reductions in cold rolling on the microstructural evolution of the 12Cr ODS steel after the subsequent 1473–1673 K annealing were studied by means of scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Results revealed that a larger thickness reduction generated higher stored energy and more homogeneous γ-fiber with {111} parallel to the normal direction, which evidently facilitates the recrystallization process. Moreover, heterogeneous distribution of voids was developed, and the size and area fraction were reduced with higher deformation at 1473 K and 1573 K. Partial recrystallization was observed with a reduction of 60% while abnormal grain growth took place with larger reductions, leading to the dominant {110} 〈 001 〉 component. Additionally, the average diameter and number density of nanoscale oxide particles were not significantly dependent on the deformation strain, whereas the grain size increased with a larger thickness reduction.