Fe-Cr Alloys Research Articles

Plasticity in irradiated FeCrAl nanopillars investigated using discrete dislocation dynamics

In this paper, we investigate plasticity in irradiated FeCrAl nanopillars using discrete dislocation dynamics simulations (DDD), with comparisons to transmission electron microscopic (TEM) in situ tensile tests of ion- and neutron-irradiated commercial C35M FeCrAl alloy. The effects of irradiation-induced defects, such as a/2<111> and a<100> type loops and composition fluctuations representative of phase separation in irradiated FeCrAl alloys, are investigated separately as well as superposed together in simulations. We explore the effects of defects on the stress-strain behavior, specifically yield strength and hardening response, of FeCrAl nanopillars. Our simulations confirm the widely accepted fact that irradiated alloys exhibit a stress-strain response with higher yield strength as compared to unirradiated alloys. However, our DDD calculations reveal an atypical superposition of the hardening contributions due to composition inhomogeneity and irradiation loops wherein composition inhomogeneity annihilates the hardening due to irradiation loops at small scales. As a result, we observe that the yield strength in irradiated alloys, after taking into consideration the effects of both composition inhomogeneity and irradiation loops, is smaller than the yield strength of the alloys with only irradiation loops and is approximately same for the alloy with composition inhomogeneity alone. This is referred to as “destructive interference” between the hardening contributions due to composition fluctuations and irradiation loops in the paper. We also identify this destructive interference in the superposition in our parallel TEM in situ tensile tests on unirradiated, ion-irradiated, and neutron-irradiated C35M FeCrAl alloy. This destructive interference in the hardening contributions contrasts with the dispersed barrier hardening (DBH) models widely utilized by the experimental community to model the hardening contributions due to different irradiation induced defects. The effects of the loading orientations on the yield strength and hardening are investigated and the mechanisms for the hardening in irradiated FeCrAl alloys are also reported.

The elemental effects on the H2 dissociative adsorption on FeCrAl (110) surface

As a promising candidate for accident-tolerant fuel (ATF) cladding materials, FeCrAl alloys are susceptible to hydrogen embrittlement (HE) under high-temperature radioactive water environments. Understanding the HE mechanism of FeCrAl is crucial, and studying H2 dissociative adsorption can provide valuable insights since it is the first step in the HE processes. In this paper, we investigated the H2 dissociative adsorption on Fe (110) and FeCrAl (110) surfaces using first-principles calculations based on Density Functional Theory (DFT). Our results indicate that the decomposition of H2 molecules on FeCrAl (110) surfaces is significantly affected by surface elemental effects compared to Fe (110) surfaces. Specifically, Al atoms weaken both the H2 decomposition and the H binding strength of Al-containing sites. Cr atoms decrease the decomposition tendency of H2 in certain configurations, but Cr aggregation enhances the binding strength of H atoms on Cr-containing sites. The mechanism underlying the Al/Cr weakening effects on H2 decomposition is due to the charge deficiency through alloying. Our findings have generalized implications for studying the interactions between iron-based alloys and hydrogen in various applications such as hydrogen storage, transportation, and prevention.

Evolution of intermetallics between solid Fe-Cr/Fe-Ni alloys and molten aluminium

Investigations of the evolution of intermetallic compounds (IMCs) at the interface between iron-based alloys and molten aluminium have important technical and scientific significance in many key areas. Most previous studies of IMCs formed at molten aluminium/solid steel interfaces have been focused on relatively long reaction times, but less research has been performed on the evolution of IMCs at the aluminizing interfaces of Fe-Cr and Fe-Ni alloys within a few seconds. In this study, hot-dip aluminizing experiments under natural convection conditions were employed to study the evolution of two interfacial IMCs (Fe-15Cr alloy and Fe-35Ni alloy with molten aluminium) at 700–900°C within 30 s. The IMCs for Fe-15Cr consisted of an Fe4Al13 layer next to the aluminium and an Fe2Al5 layer adjacent to the steel. In contrast, only one layer of Fe4Al13 was detected at the interface between the Fe-35Ni alloy and molten aluminium. The growth of Fe2Al5 was described by a parabolic rate law. An activation energy of 7.53 kJ/mol was determined for the temperature range of 700–900°C. The evolution of Fe4Al13 for the two alloys was similar: their thicknesses decreased anomalously with increasing dipping temperature, and their kinetics were controlled by both diffusion and dissolution. In this study, based on reaction-diffusion theory, growth and dissolution equations of the IMCs were established, and the diffusion coefficients of Al atoms in Fe2Al5 and Fe4Al13 were obtained for the first time. The similarities and differences in the IMCs for the two alloys were explained mainly by the different diffusion coefficients.

Wear prediction of BTA drill based on finite element method for drilling laminated Inconel 625 deposited metal and FeCr alloy material

This study investigated the wear prediction of BTA drills for laminated Inconel 625 deposited metal and FeCr alloy materials using 3D finite element modelling. High interface temperature owing to increased cutting speed enhances the abrasive wear mechanism of Inconel 625, while FeCr alloy exhibits a predominantly adhesive wear mechanism. The interface temperature and normal pressure required as inputs for tool-wear prediction were obtained using response surface methodology (RSM), which was developed by combining the central composite design (CCD) with finite element method simulation. This approach significantly improved the computational efficiency and provides accurate predictions with the calibrated friction model. A modified Usui wear model was established considering the sensitivity of temperature to wear rate for Inconel 625. A significant improvement in prediction accuracy was observed compared with the original Usui equation. The proposed tool-wear prediction model was validated through experimental tests. The maximum predicted flank wear width deviations for the outer and intermediate edges of BTA drill were 2% and 7%, respectively. The results demonstrate the effectiveness of the proposed BTA drill wear-prediction approach for laminated materials with different wear characteristics, providing theoretical guidance for cutting-parameter optimisation and tool-replacement strategies.

Insights into oxidation and nitridation of alumina-forming FeCrAl alloy after supercritical water exposure: Effect of pre-corrosion on high-temperature damage behavior

This work investigated the corrosion characteristics of FeCrAl alloy in supercritical water (SCW), and focused on the effect of SCW pre-corrosion on the alloy oxidation and nitridation at 900 °C. FeCrAl alloy formed outer Fe-rich oxides (mainly Fe2O3) and inner Fe-Cr-Al oxides during SCW exposure. SCW pre-corrosion made FeCrAl alloy exhibit different morphologies and phase compositions in the same medium. SCW pre-corrosion promoted the deposition of Ni and Mo elements from the autoclave, filling the gaps in flaky oxides by forming Ni- and/or Mo-rich products in air and N2. Moreover, SCW pre-corrosion avoided the appearance of accelerated nitridation zones in N2. However, pre-corrosion caused the preferential diffusion of N2 and air. Outward-diffused Cr ions during SCW exposure reacted with O impurities in N2 to form Cr2O3 that easily dissociated N2, leading to the development of nitridation towards the alloy interior.

Interplay between diffusion and magnon-drag thermopower in pure iron and dilute iron alloy nanowire networks

Results of measurements on the thermoelectric power of 45 nm diameter interconnected nanowire networks consisting of pure Fe, dilute FeCu and FeCr alloys and Fe/Cu multilayers are presented. The thermopower values of Fe nanowires are very close to those found in bulk materials, at all temperatures studied between 70 and 320 K. For pure Fe, the diffusion thermopower at room temperature, estimated to be around − 15 upmuV/K from our data, is largely supplanted by the estimated positive magnon-drag contribution, close to 30 upmuV/K. In dilute FeCu and FeCr alloys, the magnon-drag thermopower is found to decrease with increasing impurity concentration to about 10 upmuV/K at 10% impurity content. While the diffusion thermopower is almost unchanged in FeCu nanowire networks compared to pure Fe, it is strongly reduced in FeCr nanowires due to pronounced changes in the density of states of the majority spin electrons. Measurements performed on Fe(7 nm)/Cu(10 nm) multilayer nanowires indicate a dominant contribution of charge carrier diffusion to the thermopower, as previously found in other magnetic multilayers, and a cancellation of the magnon-drag effect. The magneto-resistance and magneto-Seebeck effects measured on Fe/Cu multilayer nanowires allow the estimation of the spin-dependent Seebeck coefficient in Fe, which is about − 7.6 upmuV/K at ambient temperature.

The Role of Oxygen Reduction Reaction in Determining Pit Stability of FeCr Alloys

Pitting corrosion is one of the most dangerous forms of corrosion, leading to sudden catastrophic failures in engineering system. Once initiated, pit propagation rates are largely dependent on the magnitude of the supporting cathodic current available from the oxygen reduction reaction (ORR) occurring on the external passive film. Here the ORR kinetics on conventional and high Cr content FeCr alloys made by arc-melter were investigated by experimental and computational methods. DFT calculations suggested that the overpotentials for the ORR on FeCr oxides are in the order Cr2O3 > Fe3O4 > Fe2O3 > FeCr2O4, which was experimentally confirmed on the FeCr alloys by rotating disc electrode experiments in both O2 saturated 0.1 M NaOH and 3.5 wt% NaCl. Koutecký-Levich analysis demonstrated that ORR follows a 4-electron pathway on all surfaces investigated. Moreover, the investigation of the Pourbaix diagrams of FeCr revealed that at potentials where pitting corrosion initiates the main constituents of the passive films should be a mixture of Fe2O3 and Cr2O3, rather than FeCr2O4, which is proved to be a good ORR catalyst. The results support the postulation that one role Cr additions play in the prevention of pitting corrosion is to suppress the ORR reaction.

Steady-state and power transient critical heat flux experiments of FeCrAl and Zircaloy claddings under saturated pool boiling

FeCrAl and Zircaloy cladding tubes are vertically placed in the saturated pool boiling chamber to perform steady-state and power-transient critical heat flux (CHF) experiments. Three FeCrAl-variants, including FeCrAl-C26M, FeCrAl-B126Y, and FeCrAl-B136Y, are tested to compare their pool boiling thermal performances with traditional Zircaloy claddings including Zirlo, Zr705, and Zircaloy-4. In the steady-state pool boiling experiments, it was confirmed that FeCrAl alloys have higher pool boiling CHF values than Zircaloys. Among these tested materials, FeCrAl-C26M has the highest pool boiling CHF, but the FeCrAl-C26M nucleate boiling heat transfer coefficient is less than that of FeCrAl-B136Y. It reconfirms that CHF is enhanced on the post-CHF oxidized samples. However, as the cycling number of post-CHF oxidization increases, the CHF enhancement becomes less appreciable due to the limited physics contributions of the oxide layer. It is noteworthy that the finalized CHF enhancement can vary a lot between different cladding materials. For example, in three Zircaloy variants, their saturated pool boiling CHF results converge to 980 kW/m2 on the post-CHF oxidized samples. In three FeCrAl variants, their pool boiling CHF results are finalized to 1400 kW/m2. This CHF enhancement difference could be possibly attributed to the chemical constituents of oxide layers presented on cladding surfaces. The Fuchs-reactivity initiated accident transients are simulated by direct current programmable power supply to characterize thermal responses of different claddings to various pulse-width power transients. It is found that the heat transfer coefficients show monotonically increasing parametric trend with respect to faster power-transient rates, i.e., shorter pulse widths. Different from the steady-state pool boiling, the transition boiling regime is phenomenologically reflected in the power-transient pool boiling curve. This potentially implies that power-transient CHF featured by surface temperature overshoot may indicate transient thermal-safety margins not so accurately as the power-transient maximum heat flux. The difference gap of power-transient maximum heat flux between various cladding materials can be gradually closed by the progressive increasing of power transient rates.

Kinetics of Transformation, Border of Metastable Miscibility Gap in Fe–Cr Alloy and Limit of Cr Solubility in Iron at 858 K

The study was aimed at determination of the position of the Fe-rich border of the metastable miscibility gap (MMG) and of the solubility limit of Cr in iron at 858 K. Towards this end, a Fe73.7Cr26.3 alloy was isothermally annealed at 858 K in vacuum up to 8144 hours and Mössbauer spectra were recorded at room temperature after every step of the annealing. Three spectral parameters viz. the average hyperfine field, 〈B〉, the average isomer shift, 〈IS〉, and the probability of the atomic configuration with no Cr atoms in the two-shell vicinity of the probe Fe atoms, P(0,0), gave evidence that the transformation process took place in two stages. All three parameters could have been well described in terms of the Johnson–Mehl–Avrami–Kolmogorov equation, yielding kinetics parameters. The first stage, associated with the phase decomposition, proceeded much faster than the second stage, associated with the alpha-to-sigma phase transformation. The most reliable estimation of the position of the MMG and that of the value of the Cr solubility limit was obtained from the annealing time dependence of 〈B〉, namely 24.5 at. pct Cr for the former and 20.3 at. pct Cr for the latter. A comparison of these figures with the recent phase diagrams pertinent to Fe–Cr system was done.

Regulating the dissolution of brittle nano-precipitated phase in FeCrAl alloy by pulse current to repair aging properties

Usually, heat treatment is required to elevate the entire material to a higher energy state, thereby eliminating harmful precipitates and repairing the mechanical properties of aged materials. However, the energy utilization rate of this high-temperature treatment method is low, and the regulation of precipitates dissolution is overly dependent on temperature. In this study, an efficient and energy-saving pulse current method was used to control the brittle Cr-rich α′ phase dissolution in aged Fe-25Cr-5Al grade alloy (Fe-24.32Cr-5.67Al-0.043C in wt.%). Compared with conventional heat treatment, pulse current treatment significantly reduces the dissolution temperature and activation energy of the precipitate, and as the current density increases, the non-thermal effect promoting brittle phase dissolution becomes more pronounced. This study is of great significance for the microstructure control and performance design of materials using pulsed current.

Effects of yttrium on the oxidation behavior of Fe13Cr6AlY alloys under 1200 °C steam

FeCrAl alloys are a candidate material for accident-tolerant fuel cladding in nuclear power plants owing to their high-temperature oxidation resistance, which can be further enhanced by the addition of Y. The main objective of this study was to investigate the effects of Y on the oxidation behavior of Fe13Cr6Al alloys in a 1200 °C steam environment with varying Y concentrations. Initially, Y was present as intermetallic particles in the matrix, which were identified as YFe4Al8. Once the oxidation process began, Y-rich intermetallics served as Y reservoirs and caused two conflicting effects: Y segregation at the oxide grain boundaries and Y-rich oxide formation. The Y segregation caused the inward diffusion of oxygen to be dominant, which changed the oxide growth mechanism and led to enhanced oxidation resistance and oxide adherence. On the other hand, Y formed Y2O3 at the grain boundaries near the oxide/metal interface, which eventually became Y3Al5O12. The Y3Al5O12 pegs allowed rapid oxygen penetration into the metal substrate and caused internal oxidation. Therefore, there is an optimal Y content as a result of these conflicting effects of Y addition. The reticular structure was also carefully analyzed to determine any relationship with the Y-rich oxide pegs. The reticular structure observed on the oxidized surface was identified as Y3Al5O12, which was formed at the grain boundaries of the metal or buried in the Al oxide. The size of the reticular structure coincided with the grain size of the metal near the surface. Data AvailabilityThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.

Integrated effect of aging and heavy ion radiation on FeNiCrAl duplex alloy for accident-tolerant fuel cladding

Alumina-forming duplex FeNiCrAl alloy with superior mechanical properties and high-temperature oxidation resistance has been regarded as an accident-tolerant fuel cladding material. This work investigated the effect of aging and the radiation resistance of FeNiCrAl duplex alloy after up to 2000 hours thermal aging and 3.5 MeV Fe13+ ion radiation. The ferrite phase containing B2 phase shows much fewer radiation defects and lower radiation hardening rate than conventional FeCrAl alloy. The coherent B2 phase precipitate is critical in hindering dislocation and inhibiting the formation of radiation induced defects which is proved via experiment and molecular dynamics simulation. Besides, the Al depletion of B2 phase and nano-sized precipitation are characterized by using Time-of-Flight Secondary Ion Mass Spectrometry and Atom Probe Tomography. These findings effectively illuminate microstructure evolution of FeNiCrAl duplex alloy after long-term aging and radiation and lead to new insight for designing advanced accident-tolerant fuel material.

Electrolytic coating of iron-chromium alloy of sulphate-chloride electrolyte in machine parts recovery process modelling

Introduction. The technological process of restoring a part should provide more than 80% of the resource and no more than 50% of the cost of a new part. The wear resistance of the applied coating determines the resource of the restored part operating under waterjet and boundary friction. The microhardness of an iron-based electrolytic coating is one of the key indicators of wear resistance. The study of the results of statistical processing and the factors most affecting the increase in the microhardness of the iron-based coating will make it possible to recommend and optimize the deposition conditions for obtaining the most wear-resistant coatings.Materials and methods. The studies on equipment that let to obtain the necessary data with the required accuracy were carried out. The mathematical processing using modern statistical data processing tools that excluded possible errors, let to obtain the dependence of factors with the necessary accuracy was carried out.Results. During the study of the developed sulfate-chloride electrolyte for the deposition of an iron–chromium coating, it became necessary to determine the effect of deposition conditions – ‘factors’ (temperature, acidity of the electrolyte, cathode current density) on the microhardness of the coating – ‘response’. It was found that the combination of the factors ‘temperature’ and ‘cathode current density’ in the coded values of 1.5. 2.0 and -2 are the most significant. The deposition conditions in order to obtain a coating with maximum microhardness are optimized.Discussion and conclusions. As a result of the conducted studies for the effect of the deposition conditions of the Fe-Cr alloy from the sulfate-chloride electrolyte on the microhardness of the coating, it was found that in order to obtain high-quality coatings of the iron-chromium alloy with high microhardness, it is necessary to strictly comply with the requirements of the deposition conditions and acidity to a greater extent. As the current density and temperature of the electrolyte, compared with the acidity of the electrolyte, iron baths are installed and regulated by equipment.

Effect of cryogenic laser surface modification on the microstructure evolution and corrosion resistance of accident-tolerant FeCrAl alloys

Accident-tolerant FeCrAl alloys require a thinner cladding wall than zirconium cladding to overcome the extra neutron loss, thus requiring enhanced corrosion resistance. Surface treatment was used in this study to address this issue via cryogenic laser surface modification. A finite element model based on heat-transfer theory was raised to reveal the temperature field change during cryogenic laser treatment. The relationship between the laser processing parameters and surface morphology was studied with various characterizations to better understand the process mechanism and verify the correctness of the model. Hydrothermal corrosion test was proceeded in an autoclave at 290 °C and 7.5 MPa for 150 h to examine the enhancing corrosion resistance. STEM observations were used to study the microstructure evolution by cryogenic laser processing and corrosion. Electrochemical tests were performed to analyze the relationship between processing parameters and corrosion resistance before and after corrosion. The oxide film thickness and corrosion rate of the cryogenic laser-modified sample were 66 % and 80 % smaller than the as-received sample. The enhanced corrosion resistance was attributed to the microcrystalline layer and the thin duplex passive film that formed during the cryogenic laser process.

Dynamic Precipitation of Laves Phase in FeCrAl Alloy with Zr Addition

FeCrAl alloy is one of potential candidates for accident‐tolerant‐fuel (ATF)‐cladding materials due to its excellent oxidation and corrosion resistance at accident temperature, combining good mechanical properties at service temperature. Alloying strategy is an important way for improving comprehensive properties of FeCrAl alloy through the precipitation of fine Laves phase. Zr alloying can stabilize the Laves phase due to its lower diffusion coefficient and solubility in body‐centered‐cubic ferrite matrix. Herein, it is found that Zr addition changes the dynamic precipitation features of Laves phase in FeCrAl alloy during high‐temperature deformation, from only one type of Fe2M (M = Nb, Mo, Ta) Laves phase to Fe2Zr combining Fe2M‐type Laves phase. The Fe2Zr‐type Laves phase precipitates dynamically first, and the interface precipitates between which with ferrite matrix creates more nucleation sites for subsequent precipitation of Fe2M Laves phase. The results can be possibly applied for alloy design and microstructure tailoring in series of FeCrAl alloys used for ATF cladding in the near future.

Chromium and iron recovery from hazardous extracted vanadium tailings via direct reduction magnetic separation

The large accumulation of extracted vanadium tailings not only causes a waste of resources but also leads to environmental pollution. In this work, a technique was proposed that involves direct reduction followed by magnetic separation for recovering Fe and Cr while removing Na from the extracted vanadium tailings. Thermodynamic analysis and process parameters such as the use of additives and the reduction temperature were experimentally determined. Thermodynamic results showed that a reduction temperature higher than 1250 °C is required for the simultaneous removal of sodium, iron, and chromium. The experimental results indicated that when the Ca (OH)2 addition was added at a rate of 30 %, and the reduction temperature was maintained at 1350 °C for 2 h, Fe-Cr alloys were formed by the reduction of iron and chromium and could be separated by magnetic separation. The addition of Ca (OH)2 greatly facilitates the reduction of the Fe-containing phase in the extracted vanadium tailings, resulting in the formation of Fe-Cr alloy with reduced Cr. Sodium metal was volatilized, with removal rates of 65.9 %, 93.3 % and 93.32 % for Cr, Fe and Na, respectively. The recovery efficiencies for Fe and Cr were 85.9 % and 65.4 % respectively. The magnetic concentrate contained 83.84 % total Fe and 7.25 % Cr, and its main phase was Fe-Cr alloy. In contrast, the magnetic tailing consisted mainly of CaTiO3. The simultaneous method has a significant impact on removing valuable metals and reducing the environmental risk of the extracted vanadium tailings.

Positron annihilation investigation of hardening behavior induced by Fe2+ irradiation in low-Cr FeCrAl alloys

Compared with 13Cr–FeCrAl alloy, micro-alloyed low-Cr FeCrAl alloy is expected to provide higher potential for application as accident-tolerant fuel (ATF) cladding material. In this study, low-Cr FeCrAl alloys with 7–10 wt.% Cr contents were irradiated to different doses of 1 and 5 dpa at 400 °C with 3 MeV Fe2+ ion. The 13Cr–FeCrAl alloy C35MN developed by Oak Ridge national laboratory was also studied for comparison. Samples were examined by Doppler Broadening of Annihilation Radiation (DBAR) and nanoindentation before and after irradiation. Nanoindentation hardness tests showed that irradiation hardening occurred in FeCrAl alloy samples with different Cr contents after irradiation. The hardening response of the 13Cr–FeCrAl alloy increased with the increase of irradiation dose, while the hardness of low-Cr micro-alloyed FeCrAl alloy changed little or decreased after higher dose irradiation. DBAR shows that the defect type of all samples before irradiation is the same and there are a large number of vacancy-type defects in all unirradiated samples. High temperature irradiation reduces the density of the internal vacancy defect of FeCrAl alloy, which is probably caused by the annealing-induced recovery effect on atomic displacement. The defect density of 13Cr-alloy increases with the increase of irradiation dose, while that of 10Cr–FeCrAl alloy decreases with the increase of irradiation dose. This is related to the much finer grain/precipitate size and the higher number density of precipitates, and thus the higher fraction of boundaries in the 10Cr–FeCrAl alloy. The results also showed that the vacancy recovery rate of the 10Cr–FeCrAl alloy is higher.

A Study of the Structural and Energy Properties of (210) and (130) Boundaries in Iron and an Fe–Cr Alloy

The structure and energy properties of symmetric tilt boundaries Σ5 (130)[001] and Σ5 (210)[001] in iron and low-concentration Fe–Cr alloys are investigated from first principles and by the molecular statistics method. It is shown that the boundary strongly changes the interplane distances. The sequence of multilayerrelaxation comprises damped oscillations, gradually decreasing into the grains. The energy for the replacement of iron with chromium atoms near the boundaries is lower than in pure iron. Our calculations indicate the tendency to accumulate Cr atoms and vacancies near the grain boundaries.

A Study of the Structural and Energy Properties of (210) and (130) Boundaries in Iron and an Fe–Cr Alloy

A Study of the Structural and Energy Properties of (210) and (130) Boundaries in Iron and an Fe–Cr Alloy

Sound Generation by Metro Trains during Braking and Its Suppression Method

The causes of acoustic noise occurring during braking of electric vehicles, in particular, in the metro, were analyzed. The spectrum of such noise was measured, and it is shown that high-frequency discrete tones are excited by braking resistors, applied for electrodynamic braking of electric vehicles. Three proposed mechanisms of electromechanical interaction in a Fecral plate, which is a braking resistor element, were considered: the Ampère force and linear and nonlinear magnetostriction. It was shown that predominant odd harmonics indicate the presence of a significant piezomagnetic effect and the inverse effect of linear magnetostriction in the Fecral alloy. A phenomenological approach was used to calculate the piezomagnetic tensor elements for Fecral. Calculations by the finite element method showed that asymmetric cuts introduced into the plate may significantly decrease intensity of acoustic vibrations excited by the braking resistor.