Oxide Dispersion Strengthened FeCrAl Research Articles

Effects of Zr and/or Ti addition on the morphology, crystal and metal/oxide interface structures of nanoparticles in FeCrAl-ODS steels

FeCrAl oxide dispersion strengthened (ODS) steel is one of the most promising candidate cladding materials for Generation IV nuclear reactors due to its superior high temperature strength and excellent resistance to creep, corrosion and irradiation. The morphologies of matrix grains and oxides, the crystal and metal/oxide interface structures of nanoparticles in two FeCrAl ODS steels, i.e., 3.3Al–0.5Zr (Fe–15Cr–2W–3.3Al–0.5Zr–0.35Y2O3) and 3.7Al–0.1Ti–0.6Zr (Fe–15Cr–2W–3.7Al–0.1Ti–0.6Zr–0.35Y2O3), were studied by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM). The average matrix grain sizes of 3.3Al–0.5Zr ODS steel and 3.7Al–0.1Ti–0.6Zr ODS steel are 1.01 and 0.97 μm, respectively. The dispersion morphology of oxides in 3.7Al–0.1Ti–0.6Zr ODS steel is much better, relative to that of 3.3Al–0.5Zr ODS steel. Crystal & interface structures of 88 and 171 oxides in 3.3Al–0.5Zr ODS steel and 3.7Al–0.1Ti–0.6Zr ODS steel were characterized, respectively. The proportions of Y–Zr–O, Y–Al–O, Y–Cr–O, ZrO2 and Y2O3 in 3.3Al–0.5Zr ODS steel are 77.2%, 9.1%, 2.3%, 2.3% and 2.3%, respectively and, however, the proportions of Y–Zr–O, Y–Ti–O, Y–Al–O and ZrO2 in 3.7Al–0.1Ti–0.6Zr ODS steel are 86.5%, 8.3%, 3.5% and 1.7%, respectively. The much better dispersion morphology of oxides in 3.7Al–0.1Ti–0.6Zr ODS steel, relative to that of 3.3Al–0.5Zr ODS steel, is due to the much higher proportion of Y–Zr–O, the considerably lower proportion of Y–Al–O and the fairly high number fraction of Y–Ti–O. The mechanisms of the formation and polymorphic transition of various kinds of oxides and, moreover, the strengthening mechanisms in both ODS steels were discussed.

Effects of Zr Content on the Microstructure of FeCrAl ODS Steels

FeCrAl oxide dispersion strengthened (ODS) steels are an important kind of cladding material for accident-tolerant fuels. Their radiation resistance and mechanical properties are closely related to the grain size and dispersed second phases. In order to tailor the microstructure and provide an experimental basis for the composition design of FeCrAl ODS steels, in this paper FeCrAl ODS steels with different Zr contents were prepared by mechanical alloying and the subsequent hot isostatic pressing (MA-HIP) process. The effects of Zr content on the grain size distribution and the precipitation of dispersed second phases in FeCrAl ODS steels were investigated by electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results showed that the grain size decreased first and then increased as the Zr content increased, and that the average grain size achieved the minimum value of 2.092 μm when the Zr content was 0.6 wt.%. The Zr content had a negligible effect on the grain orientation of FeCrAl ODS steels, but the dispersed second phase changed from the Al2Y4O9 phase with monoclinic structure to the Y4Zr3O12 phase with hexagonal structure as the Zr content increased.

A comparison study of change in hardness and microstructures of a Zr-added FeCrAl ODS steel irradiated with heavy ions

FeCrAl oxide dispersion strengthened (ODS) steel is regarded as one of the most promising candidate materials for nuclear fission reactors such as supercritical pressurized water reactor (SCPWR), sodium-cooled fast reactor (SFR), and lead bismuth-cooled fast reactor (LFR) and so on, since it has excellent irradiation resistance and good corrosion resistance of super-critical water and Pb–Bi coolants. In the present work, irradiation response of a Zr-added FeCrAl ODS steel (15Cr–4Al-0.1Ti-0.6Zr) together with an Al-free ODS (16Cr-0.1Ti) steel and a non-ODS steel T91 was studied. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) were used to characterize the microstructures including grains, dislocations, and oxides in the pristine specimens and irradiation defects in the irradiated specimens. Nano-indentation tests were used to evaluate the change in hardness after irradiation. The oxides in the 15Cr–4Al-0.1Ti-0.6Zr specimen are slightly larger than those in the 16Cr-0.1Ti specimen, but with a similar number density. After irradiated with 9.45 MeV 83Bi35+ to 26 dpa at ambient temperature, hardening was observed in all the specimens. Irradiation hardening of the 15Cr–4Al-0.1Ti-0.6Zr specimen is slightly higher than the 16Cr-0.1Ti specimen but significantly lower than the non-ODS steel T91. The irradiation defects in the ODS steel specimens are apparently smaller than those in T91 with a similar number density. By calculating sink strength of the initial microstructures such as grains, dislocations and oxides, the evolution of irradiation defects was discussed. A linear relationship between the irradiation hardening and the sink strength was established. A similar linear relationship between the calculated irradiation hardening using the TEM data (based on dispersion barrier hardening model (DBH) and Friedel-Kroupa-Hirsch (FKH) model, respectively) and sink strength was also found. The calculated irradiation hardening is relatively lower than the experimental value, indicating the possible contribution of Cr segregation to hardening which needs further study.

Formation Mechanism of Nanoparticles in Fe–Cr–Al ODS Alloy Fabricated by Direct Oxidation Method

This study presents the fabrication of 14Cr Fe–Cr–Al oxide dispersion strengthened (ODS) alloy by a direct oxidation process. In order to explain how oxide nanoparticles are formed in the consolidation process, the powders after oxidation are subjected to vacuum thermal treatment at high temperatures. Differential scanning calorimeter, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy techniques are used to detect the generation, evolution of oxides on both the surface and interior of the powder, as well as the type of oxide nanoparticles in the fabricated ODS alloy. It is found that an iron oxide layer is formed on the surface of the powder during low temperature oxidation. And the iron oxide layer would be decomposed after thermal treatment at high temperature. In the consolidation process, the oxygen required by the reaction of alumina and yttrium oxide to produce nanoscale Y–Al–O particles mainly derives from the decomposition of iron oxide layer at elevated temperature and the inward diffusion of oxygen. Using the direct oxidation process, YAlO3 nanoparticles are dispersed in the grains and at the grain boundaries of Fe–Cr–Al ODS alloy.

High temperature properties of AlN coatings deposited by chemical vapor deposition for solar central receivers

There is an increasing interest for tower concentrated solar power (CSP) systems which can work at temperatures higher than 1073 K to optimize the efficiency. One of the challenges is to design the receiver that will be heated at high temperatures in air. On the contrary to coatings in gas turbine engine, the coating/substrate system must have a high thermal conductivity to ensure a good heat transfer to the fluid. Aluminum nitride (AlN) coating, deposited by chemical vapor deposition at 1373 K at a growth rate of 10–50 μm h−1, is selected for its high thermal conductivity, low thermal expansion coefficient, high temperature stability and its ability to develop stable alumina scales above 1273 K. Cast and ODS (Oxide Dispersion Strengthened) FeCrAl alloys, also alumina-formers, are chosen as model substrates to reduce the influencing parameters in real-life receivers and to study the potential of these coatings. Accelerated cyclic oxidation tests and emissivity measurements allow the evaluation of AlN coatings as materials for high temperature CSP receivers. The multilayered systems show low degradation after hundreds of thermal cycles at 1073 K in air and can support higher temperatures (1373 K) for 100 to 500 h depending on the coating thickness. Nevertheless the fast cyclic oxidations in solar furnace generated cracks through the coatings. The measurement of the optical properties also revealed a decrease of the absorptivity after oxidation.

Fe<sup>+</sup> Implantation Induced Damage in Oxide Dispersion Strengthened Steels Investigated by Doppler Broadening Spectroscopy

Open volume defects created by 1 MeV Fe+ implantation up to a damage of 15 displacements per atom into Fe14wt%Cr and into oxide dispersion strengthened (ODS) Fe14wt%Cr were investigated by positron annihilation spectroscopy, especially single Doppler broadening (DBS) and coincidence Doppler broadening spectroscopies (cDBS). The influence of W and Ti alloying elements on the evolution of defects during ion implantation were probed in addition. Whereas no effect of the W and Ti alloying constituents on the line-shape parameters S and W could be detected both before and after ion implantation, fine dispersed Y2O3 particles showed important changes in the structural properties compared to pure FeCr steel, which could be detected by DBS and cDBS. The distribution of the electron momenta in implanted ODS Fe14Cr, obtained with cDBS, shows that oxide particles form obstacles for expanded vacancy mobility and that the vacancies are localized around the oxide particles.

Microstructure and mechanical properties of ODS Fe14Cr model alloy processed by equal channel angular pressing

A nanograin sized model oxide dispersion strengthened (ODS) ferritic steel with nominal composition Fe–14Cr–0·3Y2O3 (wt-%) was produced by mechanical alloying and consolidated by hot isostatic pressing. The alloy was submitted to severe plastic deformation by equal channel angular pressing (ECAP). Microstructural and mechanical characterisation was performed before and after ECAP. It was found that ECAP decreases and homogenises grain size without altering the nanoparticle dispersion, in addition to enhancing ductility and shifting the strength drop at high temperatures.

Inhibited aluminization of an ODS FeCr alloy

Aluminide coatings are of interest for fusion energy applications both for compatibility with liquid Pb–Li and to form an alumina layer that acts as a tritium permeation barrier. Oxide dispersion strengthened (ODS) ferritic steels are a structural material candidate for commercial reactor concepts expected to operate above 600°C. Aluminizing was conducted in a laboratory scale chemical vapor deposition reactor using accepted conditions for coating Fe- and Ni-base alloys. However, the measured mass gains on the current batch of ODS Fe–14Cr were extremely low compared to other conventional and ODS alloys. After aluminizing at two different Al activities at 900°C and at 1100°C, characterization showed that the ODS Fe–14Cr specimens formed a dense, primarily AlN layer that prevented Al uptake. This alloy batch contained a higher (>5000ppma) N content than the other alloys coated and this is the most likely reason for the inhibited aluminization. Other factors such as the high O content, small (~140nm) grain size and Y–Ti oxide nano-clusters in ODS Fe–14Cr also could have contributed to the observed behavior. Examples of typical aluminide coatings formed on conventional and ODS Fe- and Ni-base alloys are shown for comparison.

High-Temperature Oxidation Behavior of ODS-Fe3Al

The high-temperature oxidation behavior of an oxide dispersion-strengthened (ODS) Fe3Al alloy has been studied during isothermal and cyclic exposures in oxygen and air over the temperature range 1000 to 1300°C. Compared to commercially available ODS–FeCrAl alloys, it exhibited very similar short-term rates of oxidation at 1000 and 1100°C, but at higher temperatures the oxidation rate increased because of increased scale spallation. Over the entire temperature range, the oxide scale formed was α-Al2O3, with the morphological features typical of reactive-element doping and was similar to those formed on the ODS–FeCrAl alloys. Although initially this scale appeared to be extremely adherent to the Fe3Al substrate, an undulating metal–oxide interface formed with increasing time and temperature, which led to cracking of the scale in the vicinity of surface undulations accompanied by a loss of small fragments of the full-scale thickness. In some instances, the surface undulations appeared to have resulted from gross outward local extrusion of the alloy substrate. Similar features developd on the FeCrAl alloys, but they were typically much smaller after a given oxidation exposure. The ODS–Fe3Al alloy has a significantly larger coefficient of thermal expansion (CTE) than typical FeCrAl alloys (approximately 1.5 times at 900°C) and this appears to be the major reason for the greater tendency for scale spallation. The stress generated by the CTE mismatch was apparently sufficient to lead to buckling and limited loss of scale at temperatures up to 1100°C, with an increasing amount of substrate deformation at 1200°C and above. This deformation led to increased scale spallation by producing an out-of-plane stress distribution, resulting in cracking or shearing of the oxide.

Study of the Reactive Element Effect in ODS Iron-Base Alumina Formers

Iron aluminide (Fe{sub 3}Al) and FeCrAl compositions were dispersed with 15 different oxides in order to study the effect of oxygen- active dopants on high-temperature growth and adhesion of {alpha}- Al{sub 2}O{sub 3} scales. In these model-type, oxide dispersion strengthened (ODS) systems, the chemical effects of various cation dopants were compared to the baseline effect of an Al{sub 2}O{sub 3} oxide dispersion. By conducting isothermal and cyclic oxidation tests and by characterizing the oxidation product, effects on scale adhesion, growth rate and microstructure were evaluated. The dopants were categorized based on effectiveness in modifying the alumina scale. An Al{sub 2}O{sub 3} dispersion yielded some improvement in oxidation behavior apparently by strengthening the relatively weak substrate. However, the type of improvements in adhesion and change in growth mechanism associated with addition of reactive elements such as Y were not achieved. In general, due to the weaker substrate and the inherently faster interfacial void formation, the dispersions were less effective in ODS Fe{sub 3}Al than in ODS FeCrAl.