Fe-Cr Alloys Research Articles

Influences of Fluorine on Corrosion Behavior of FeCrAl Alloy in Lithiated/borated Water at 360 °C/18.5 MPa

Abstract As an accident-tolerant fuel cladding material, Iron-Chromium-Aluminium (FeCrAl) alloy shows a big potential in replacing zirconium alloy as the cladding tube for pressurized water reactors. The presence of fluorine impurity in the primary coolant of pressurized water reactors will accelerate the corrosion of fuel cladding tubes. However, studies on the influences of fluorine impurity on the service performance of FeCrAl alloy are relatively limited. In this work, corrosion behavior of FeCrAl alloy in fluorine-containing lithiated/borated water at 360 °C and 18.5 MPa was characterized by Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS), X-ray diffractometer (XRD), and Raman spectroscopy. The results indicate that FeCrAl alloy exhibited relatively good resistance to fluorine attack. Fluorine ions changed the diffusion behavior of alloying element Cr, so facilitating the formation and dissolution of FeCr2O4. Deposition of Ni and B was observed on the alloy surface. The maximum content of B reached 24.20 at.% after fluorine-containing lithiated/borated water exposure while that was 12.50 at.% in the absence of fluorine ions. Despite this, no specific compounds of B were detected in the corrosion products, but Ni existed in the form of NiFe2O4.

Atomistic insight into the interfacial reaction and evolution between FeCr alloys and supercritical CO2 with impurities

Atomistic insight into the interfacial reaction and evolution between FeCr alloys and supercritical CO2 with impurities

On the origin of superior stability of coherent nanoparticles under ion irradiation

Recently, a new anti-irradiation mechanism relying on reversible disorder-ordering transition of coherent nanoparticles was discovered, which significantly improves the microstructural stability and void swelling resistance of metallic materials. However, the factors that govern the outstanding stability and superb radiation tolerance are still not clear. Here, two kinds of FeCrAl alloys were designed, each strengthened by a different type of coherent phase: L21-ordered Fe2AlV and B2-ordered Ni (Al, Fe). It was observed that the two alloys exhibited disparate responses to high-dose ion irradiations at elevated temperatures. The L21-Fe2AlV precipitates were found to be completely dissolved after 50 dpa of ion irradiation at 500–600 °C, whereas the B2-NiAl precipitates remained stability even after 200 dpa irradiation. This research challenges the conventional wisdom that the stability of nanoparticles is governed by the balance between radiation-enhanced coarsening and radiation dissolution. Instead, it demonstrates the re-nucleation and subsequent interface-controlled solute reshuffling processes govern the stability of coherent chemically-ordered nanoparticles under radiation at high temperatures. We demonstrate that the low interfacial energy, which is related to both the simply chemically-ordered lattice structure and low lattice misfit interface, is crucial for enabling such short-range elemental reshuffling process to repeatedly form nanoprecipitates that cannot be suppressed by radiation.

A data-driven strategy for phase field nucleation modeling

We propose a data-driven strategy for parameter selection in phase field nucleation models using machine learning and apply it to oxide nucleation in Fe-Cr alloys. A grand potential-based phase field model, incorporating Langevin noise, is employed to simulate oxide nucleation and benchmarked against the Johnson-Mehl-Avrami-Kolmogorov model. Three independent parameters in the phase field simulations (Langevin noise strength, numerical grid discretization and critical nucleation radius) are identified as essential for accurately modeling the nucleation behavior. These parameters serve as input features for machine learning classification and regression models. The classification model categorizes nucleation behavior into three nucleation density regimes, preventing invalid nucleation attempts in simulations, while the regression model estimates the appropriate Langevin noise strength, significantly reducing the need for time-consuming trial-and-error simulations. This data-driven approach improves the efficiency of parameter selection in phase field models and provides a generalizable method for simulating nucleation-driven microstructural evolution processes in various materials.

Interfacial microstructure and corrosion behavior of ODS FeCrAl alloy in oxygen-saturated lead-bismuth eutectic at 450 °C

The interfacial microstructure and corrosion behavior of oxide dispersion strengthening (ODS) FeCrAl alloy with 20 wt% Cr (e.g. 20Cr ODS FeCrAl alloy) exposed to static oxygen-saturated lead-bismuth eutectic (LBE) at 450 °C for 20 h, 40 h, 60 h, 80 h and 100 h as exposure time respectively have been investigated. The results show that multilayer corrosion films of 20Cr ODS FeCrAl alloy can exist at the corrosion interface. Due to formation of protective scales, the primary corrosion mechanism of 20Cr ODS FeCrAl alloy in LBE is oxidation rather than dissolution and penetration of liquid LBE. Specially, the spinel-shaped nano-sized dense external layer without any microcracks is obviously observed. Moreover, the dense and compact scale with micron-meters, i.e. Al-rich Fe-Cr spinel, generates at interface as the middle layer to effectively defend inward diffusion of oxygen and penetration of LBE. Meanwhile, a reinforced and continuous internal layer mixed with Cr2O3 scale and the ferrite inner oxidation zone (IOZ) existing in the form of the interlocking interface between the layers and the substrate can further promote the adhesion between corrosion layers and the substrate of 20Cr ODS FeCrAl.

Corrosivity of KOH(g) towards superheater materials in a simulated air reactor environment for chemical looping combustion of biomass

Chemical looping combustion (CLC) of biomass has the potential to improve the electrical efficiency in power plants that utilize carbon capture and storage (CCS). This is attributed to the mild corrosive environment anticipated in the air reactor (AR). However, previous studies have measured alkali emissions in the AR, likely in the form of KOH(g), suggesting that alkali may transfer from the fuel reactor (FR) to the AR. To investigate the corrosive effect of KOH(g) release in the AR, a novel experimental set-up was developed to simulate two scenarios for the AR 1) Clean scenario: no release of KOH(g) in the AR (5 % O2 + 3 % H2O + N2 bal.) and 2) Alkali slip scenario: continuous release of KOH(g) in the AR (5 % O2 + 3 % H2O + N2 bal. + 16 ppm KOH(g)). The exposure was carried out at 700 °C and six alloys, relevant as superheater material, ranging from stainless steels to nickel-base (Ni-base) alloys as well as a FeCrAl alloy were investigated. The samples were exposed for a total of 168 h and the morphology of the corrosion products was investigated using SEM-EDX and XRD. The presented results suggest that KOH(g) significantly accelerates the corrosion of the stainless steels and Ni-base alloys investigated, by rapidly destroying the protective Cr-rich oxide scale, resulting in the formation of a multilayer oxide scale with inferior protective properties. On the contrary, the FeCrAl alloy retained a protective Al-rich oxide scale irrespective of the presence of KOH(g). The findings in this study highlight that the release of KOH(g) in the AR during combustion of biomass in CLC could introduce corrosion challenges for installed superheaters that can be significantly mitigated by utilizing FeCrAl alloys.

Experimental and simulation studies to optimize the mechanical alloying process of ODS-FeCrAl alloy powder

Oxide dispersion strengthened (ODS) FeCrAl alloy exhibits superior mechanical qualities at high temperatures and resistance to neutron irradiation, making it a highly promising material for accident-tolerant fuel cladding. The preparation of ODS-FeCrAl alloy powder by mechanical alloying can mix Y2O3 and FeCrAl alloy powder uniformly and promote the uniform dispersion of nanoscale oxide particles in the alloy matrix. The preparation of ODS-FeCrAl alloy powder by the mechanical alloying method is greatly influenced by the rotational speed. Therefore, this study uses a combination of experiments and numerical simulation to investigate the effects of different rotational speeds on the microscopic morphology, particle size and force of alloy powder during the ball milling process. The mechanically alloyed powder was subjected to XRD inspection to determine the optimum ball milling time at each rotational speed by means of the maximum lattice distortion, and then the powder micromorphology and particle size were analyzed. Then the forces on the powder and grinding ball during the ball milling process were analyzed by numerical simulation, and it was found that the collision force and normal force played a dominant role in the ball milling process. The combined experimental and simulation results determined that the ODS-FeCrAl alloy powder obtained by ball milling at 300 rpm for 35 h had the largest lattice distortion, fine grain size and uniform particle size, which achieved the best mechanical alloying effect.

High-strength WC-Co/Low carbon steel connected by Ni–Cr–Fe alloy through brazing for roller sleeves

Enhancing the bond strength between WC-Co and steel is crucial for broadening its applications. In this study, WC-Co was bonded to low-carbon steel (LCS) using a Ni–Fe–Cr alloy at 1180 °C in an argon atmosphere furnace. The WC-Co/LCS (WL) interface was characterized by XRD, SEM, and EPMA, revealing no cracks or pores and confirming robust metallurgical bonding. The bonded material's shear strength reached approximately 321–328 MPa, significantly exceeding that achieved with Ag–Cu–Zn–Cd, Cu–Zn, Cu–Ni–Al, Ag–Cu–Zn + Ni/Mn, and Ag–Cu–In–Ti filler materials in the welding process. EPMA analysis showed that Fe, Cr, and Ni from the Ni–Fe–Cr alloy diffused into the WC-Co over distances of approximately 802–815 μm, 803–817 μm, and 632–641 μm, respectively. Initially, WC decomposed at sharp corners to form W₂C, which subsequently reacted with Fe and Cr to form M₆C and M₇C₃. This methodology was also applied to producing roller sleeves for vertical mills, increasing their lifespan by 1.9 times.

Effects of Ti and Y on resistance to corrosion in Fe–Cr-X Alloys

The effect of Ti and Y on the corrosion behavior of the FeCr alloy was investigated in a 3.5% NaCl solution. Ti was found to inhibit the formation of the σ phase, significantly reducing the presence of pitting in the material. In addition, Ti acted indirectly on the oxide layer, increasing the concentrations of Cr and Fe, consequently generating a probable enrichment of Cr2O3 and FeOOH, which enabled the formation of a more stable passive film resistant to attack by chloride ions. On the other hand, Y caused the precipitation of a phase identified as Fe17Y2, which was accompanied by an elongated σ phase, both located at the grain boundaries, besides the formation of Cr23C6 and Y2O3. Its addition resulted in an improvement in the corrosion resistance of the FeCr alloy by reducing the precipitation of the σ phase. However, its effect was much less important than that of Ti, since the results showed that the FeCrY alloy was still highly susceptible to localized forms of corrosion.

Plastic deformation mechanism of γ phase Fe–Cr alloy revealed by molecular dynamics simulations

Abstract Due to their outstanding mechanical properties, anti-corrosion properties, and anti-irradiation swelling properties, Fe–Cr alloys have been fully improved and developed for nuclear energy applications as structural materials. To ensure the performance stability of γ-phase Fe–Cr alloys, the present study adopted molecular dynamics (MD) simulations to explore the plastic deformation mechanism of these alloys. The slip model was constructed, and the generalised stacking fault energy (GSFE) and Peierls–Nabarro (P–N) equations were solved, revealing that {110}<111> is the preferentially activated slip system. The twinning model was constructed and the generalised plane fault energy was solved, demonstrating that twinning is preferred over slipping in the {112}<111> system. The above findings are also verified through MD simulations in which Fe–Cr specimens are stretched along the [100] direction. In addition, in the 15 at.%–25 at.% Cr range, an increase in the Cr content has a negative effect on slip but a positive effect on twin formation.

Chemical and structural durability of α-Al2O3 and γ-LiAlO2 layers formed on ODS FeCrAl alloys in liquid lithium lead stirred flow

The protection performance of an α-Al2O3 layer formed on oxide dispersion-strengthened (ODS) FeCrAl alloys in liquid LiPb flow was investigated by means of the corrosion tests under stirred-flow condition at 873 K for 1000 h. The result of STEM/EELS analysis indicated the Li adsorption and diffusion into α-Al2O3 layer. The ODS FeCrAl alloys in-situ formed a durable γ-LiAlO2 layer on their bare surface in liquid LiPb. The results of micro scratch tests on the protective layers indicated that they revealed strong resistance against the exfoliation in a shearing direction after the immersion to liquid LiPb.

Hydrogen permeation in iron-chromium-aluminum (FeCrAl) alloys and the effects of microstructure and surface oxide

Iron–chromium–aluminum (FeCrAl) class alloys are candidates for use as cladding for accident-tolerant fuels and moderators. In this context, hydrogen isotope permeation in FeCrAl alloys is an important material property. In the present work, the apparent permeability, effective diffusivity, and apparent solubility of hydrogen in the FeCrAl alloys C26M and Kanthal D (KD) were measured with gas-driven hydrogen permeation. Permeation measurements were conducted at temperatures of 400 to 700 °C and at gas-driven pressures from 1 to 100 kPa. In particular, the effect of grain size on hydrogen transport was studied with KD samples with three different microstructures: nanocrystalline (NC), ultra-fine grained (UFG), and coarse-grained (CG). The UFG and NC specimens had higher apparent activation energies (73.4 kJ mol-1 and 65.2 kJ mol-l, respectively) for hydrogen permeability than the CG sample (46.9 kJ mol-1). An aluminum oxide layer formed on the primary- and secondary-side surfaces of all samples subjected to permeation experiments which demonstrated the propensity of FeCrAl alloys to form these innate oxide permeation barriers.

Influence of hydrogen on the elastic properties and dislocation behavior in Fe–Cr and Fe–Ni alloys by DFT calculations

The effect of interstitial hydrogen on the elastic properties of bcc Fe, bcc Fe–Cr, and bcc Fe–Ni was investigated using density functional theory calculations. Our results indicate that the elastic moduli decrease linearly with increasing hydrogen concentration. The consequences of hydrogen for the mechanical properties of bcc Fe, bcc Fe–Cr, and bcc Fe–Ni were analyzed, considering various factors such as the ideal shear stress, Peierls stress, number of dislocation pile-ups, and critical crack growth lengths. At the same hydrogen concentration, compared to the bcc Fe and bcc Fe–Ni systems, fewer dislocation pile-ups and shorter critical crack growth lengths can facilitate the nucleation and propagation of cracks in the bcc Fe–Cr system. Finally, we propose a mechanism to explain the influence of Cr and Ni on hydrogen embrittlement.

Post irradiation examinations of FeCrAl cladding in PWR conditions

FeCrAl alloys have been one of the prominent Accident Tolerant Fuel (ATF) cladding material candidates, primarily due to their excellent oxidation resistance in high-temperature steam conditions when compared to Zr-based alloys. Prototypic irradiation of fueled FeCrAl rods is a fundamental step in confirming the integral performance behavior during in-pile conditions. Early generation C26M cladding, fabricated through wrought metallurgy techniques, was fueled with UO2 fuel pellets and irradiated in a pressurized water loop in the Advanced Test Reactor (ATR) at the Idaho National Lab (INL) to a burnup (BU) of ∼25 GWd/tHM. After irradiation, the rodlets were nondestructively and destructively examined. Nondestructive examinations included visual exams, profilometry, and gamma scanning. These examinations highlight the unique deposits, BU profile of the rodlets as well as the migration path of fission gases within the rodlet, and typical diametrical morphology of fuel rodlets. During handling, brittle failure of one end cap on one pin occurred. Destructive examinations included microscopy and mechanical testing. Radial cross sections of the cladding were analyzed metallographically through light optical microscopy (LOM), scanning electron microscopy (SEM) highlighting a unique corrosion morphology and micro-cracking (∼20–30 μm past the main oxide layer) at the metal-oxide interface. Ring compression testing (RCT) was used to elucidate the mechanical property change of the FeCrAl cladding after neutron irradiation. In contrast to the non-irradiated material, which remained ductile at all test temperatures between 25 °C and 250 °C, the irradiated cladding fractured in a brittle manner at 50 °C and below. The results of the tests show that some challenges remain in the development of FeCrAl cladding for LWR cladding applications including improvement of in-reactor waterside corrosion performance and the retention of ductility after neutron irradiation.

The effect of differing Y concentrations on the evolution of the structure and mechanical properties of FeCrAl alloy

The influence of different Y concentrations on the evolution of the structural and mechanical properties of FeCrAl alloys is investigated in this work. The ingot casting process was used to produce alloys with Y mass fractions of 0%, 0.03%, and 0.25%, respectively. They were then annealed for 15 min at 850°C, 950°C, and 1050°C before being quenched by water to room temperature. Microstructure and mechanical properties were analyzed using OM, EPMA, EBSD, and tensile tests. The addition of 0.03% Y to FeCrAl alloy and annealing at 850°C resulted in the greatest overall performance of the alloy. This had a tensile strength of 635.62 MPa, yield strength of 465.48 MPa, and elongation of 27.04%. With an increase in Y concentration or annealing temperature, the tensile, yield, and elongation properties decrease. This phenomenon can be attributed to the incorporation of Y, which creates a fine Y-rich second phase in the alloy. These phases act as heterogeneous nuclei, pinning the grains during solidification, effectively reducing grain growth and refining the grains. Grain refinement and an increase in the number of grain boundaries hinder dislocation movement, thereby enhancing the alloy's tensile and yield strength. Simultaneously, this prevents the disappearance of α-fiber textures and the creation of γ-fiber textures during recrystallization and reduces the strength of the γ-fiber textures in the alloy microstructure. However, as the annealing temperature increases, microstructure grain growth and the number of grain boundaries decrease. Moreover, higher Y concentrations result in the formation of large, striated Fe-Y phases, which impair the forming properties of the alloy and consequently reduce its tensile properties. Therefore, future research should focus on effectively avoiding the formation of coarse second phases in the microstructure of FeCrAl alloy to enhance its overall performance.

Exploring the impact of microstructure refinement on high temperature steam oxidation behavior of FeCrAl alloy

Microstructure refinement is an effective way to improve the mechanical properties and radiation resistance of FeCrAl alloy, which may also increase the susceptibility to high temperature steam oxidation (HTSO) due to the increased surface chemical activity. To investigate the microstructural effects on the HTSO behavior, a FeCrAl alloy was subjected to controlled annealing strategies to produce three types of microstructures: “fine sub-grains + fine precipitates”, “coarse grains + fine precipitates” and “coarse grains + coarse precipitates”. The three alloys were exposed to high temperature steam during a heating and holding procedure. Large oxide nodules and rough multilayer-structured oxide films were formed during heating, which subsequently evolved into a single α-Al2O3 layer during the isothermal oxidation. Microstructure refinement can promote the oxidation and slightly increase the weight gain. However, the variation of isothermal oxidation parabolic rate constants brought by the microstructure refinement is 1-2 orders of magnitude lower than that brought by alloying. Microstructure refinement can improve other service performance of FeCrAl alloy without significantly deteriorating the HTSO resistance, indicating its advantages for engineering applications.

Corrosion behavior of additively manufactured FeCrAl in out-of-pile light water reactor environments

Iron-Chromium-Aluminum (FeCrAl) alloys are candidate materials for the cladding of light water reactor (LWR) fuels. The FeCrAl alloys in general range in Cr composition from 12% (C26M) to 21% (APMT). In this work, the general corrosion behavior of Additively Manufactured (AM) C26M coupons was compared to the behavior of traditional Powder Metallurgy (PM) coupons. Immersion testing were conducted for 12 months at 288 °C and 330 °C in pure water containing either oxygen or hydrogen. Results show that the mass change of AM specimens in hydrogenated water was like the mass change of PM specimens. In oxygenated water, the mass change of AM coupons was higher and less reproducible than for the PM coupons. Porosity in the AM specimens makes their behavior less predictable in high-temperature water.

Solidification precipitation behaviour and growth kinetics of AlN inclusions in FeCrAl alloys

AlN inclusions are the major inclusions in FeCrAl alloys and precipitate during solidification. The precipitation behaviour and growth kinetics of AlN inclusions in FeCrAl alloys were theoretically calculated based on the Clyne–Kurz model and a diffusion-controlled growth model. The results indicated that AlN inclusions precipitated at a solid fraction of 0.45. Following precipitation, the Al concentration exhibited a slight decrease compared to scenarios where AlN inclusion precipitation was not considered, although it still showed an increasing trend. Conversely, the N concentration exhibited a significant turnaround, gradually dropping below its initial value. In addition, varying the cooling rate to 0.17, 0.83, 1.67, and 16.67 K/s, the final sizes of AlN inclusions were 46.5, 21.1, 14.8, and 4.7 μm, respectively. Fixing other factors, the solid fractions for AlN precipitation at initial Al concentrations of 4.19, 4.69, and 5.19 wt% were 0.535, 0.45, and 0.365, respectively, and the final sizes of AlN inclusions were 16.2, 14.8, and 13.6 μm, respectively. Similarly fixing the other factors and varying the initial N concentrations to 0.0045, 0.0053, and 0.0060 wt%, the solid fractions of AlN were 0.515, 0.45, and 0.40, respectively, and the final sizes of AlN inclusions were 14.6, 14.8, and 15.0 μm, respectively. This indicates that increasing the cooling rate, increasing the initial Al concentration, and decreasing the initial N concentration could effectively reduce the precipitation size of AlN inclusions. This work provides guidance for the investigation of the precipitation and growth kinetics of AlN inclusions in FeCrAl alloys.

High‐Temperature Mechanical Behavior of Single‐Crystal FeCrAl Alloy Under In Situ Micropillar Compression

FeCrAl cladding is one of the candidate materials for the near‐term accident‐tolerant fuel technologies under development. Research on high‐temperature mechanical behaviors of single‐crystal FeCrAl alloy is rather limited. Previous studies have reported the mechanical property of low‐index orientation in single‐crystal FeCrAl alloy at room temperature. However, the critical resolved shear stress to activate slip systems can be orientation and temperature dependent. Here, single‐crystal grains in a coarse‐grained FeCrAl alloy with different crystallographic orientations are selected to preferentially activate {110}<111> slip systems or {112}<111> slip systems. Micropillars are fabricated in the selected single‐crystal grains and tested at elevated temperatures in situ in a scanning electron microscope. The critical resolved shear stresses of {110}<111> slip systems and {112}<111> slip systems are determined at various temperatures. The critical resolved shear stress shows a temperature dependence and orientation independence. This study provides important insight for understanding the deformation mechanisms of FeCrAl alloys at elevated temperatures.

Characterization of Fe-Cr alloys irradiated by neutrons at intermediate temperature

A series of Fe-Cr alloys, with 3 at.% - 18 at.% Cr, was irradiated in the Advanced Test Reactor (ATR) at the Idaho National Labs (INL), USA, up to a dose of ∼6.7 dpa at a temperature of ∼456 °C. Transmission electron microscopy (TEM) samples were extracted using a focused ion beam (FIB) instrument, and the resulting microstructural defects, such as voids, dislocation loops, network dislocations, Cr rich precipitates, etc., were characterized using a TEM. It was found that the size and number density of these defects varied widely over the different alloys with varying Cr content. As expected, there were no Cr rich precipitates in samples with Cr up to 9 %, and they started appearing only in samples with 12 at.% Cr and above. The particle size decreased from about 15 nm at 12 % Cr to 8 nm at 18 % Cr, while the number density increased from ∼7e20 /m3 to 6e22 /m3 for the same Cr contents. Grain boundary segregation of Cr, along with a precipitate free zone, was observed in the cases where a boundary was present in the sample. Large voids (>1–2 nm) were almost invisible in the Fe-3at.%Cr sample, while the average void size remained almost constant between 15 and 18 nm for samples with 6–15 at.% Cr and increased slightly at 18 at.% Cr to ∼22 nm. Fe-3 %Cr showed a high density of small voids(<2 nm), estimated to be about 1e23/m3. The number density of large voids increased from ∼0 at 3 % Cr to a peak of ∼6.4e20 /m3 at 12 % Cr, then decreased to about 1.1e20 /m3 at 18 % Cr. The dislocation loops, which appeared in linear arrays in the Fe-9 %Cr sample, were analysed in detail using both invisibility criteria and image simulations using the Oxford University TEMACI software, and it was found that they are most likely to be ½ 〈111〉 type loops on {100} planes. These loops seem shaped like sections of helices in some places and are most likely formed by loops near screw dislocations climbing into a helix and then collapsing into a loop array. Similar dislocation analysis was performed in the other samples as well wherever feasible, and it was found that there is a mixture of ½ 〈111〉 and [100] dislocation loops.