Fe-Cr Alloys Research Articles

Elucidating precipitation in FeCrAl alloys through explainable AI: A case study

A primary challenge of using FeCrAl in high temperature industrial settings is the formation of α′-precipitates that causes brittleness in the alloy, resulting in failure through fracture. The precipitation causes hardness change during thermal aging which is sensitive to both alloy composition and experimental condition (i.e., temperature and time of heat treatment). A Gaussian Process Regression (GPR) model is built on the hardness data collected at GE Research. Subsequently, for the first time, SHapley Additive exPlanations (SHAP) built upon the GPR is used as an Explainable Artificial Intelligence (XAI) tool to understand the effect of feature values in driving the hardness change. SHAP analysis has confirmed that the primary chemical driver for α′ age hardening in the FeCrAl system is Cr as expected. However, the analysis also indicated that Al does not have a clear trend of only suppressing the formation of α′ which contradicts current literature. This lack of a trend on the effect of Al on age hardening may be due to Al ability to both suppress thermodynamically and enhance kinetically the formation of α′. Similarly, SHAP analysis points towards Mo having no clear trend towards either enhancing or suppressing α′. This study indicates that more in depth studies focusing on both the chemistry and different aging temperatures (to study kinetics) should be performed to better understand the aging of this system.

Microstructure and phase investigation of FeCrAl-Y2O3 ODS steels with different Ti and V contents

FeCrAl-based steels are considered promising materials for high-temperature nuclear applications. Over the past years, various compositions have been studied to assess their mechanical properties, structural integrity, and radiation damage resistance. However, the microstructure and phase composition of FeCrAl-ODS steels with the addition of different alloying elements are less commonly studied than pure FeCrAl alloys. The paper presents a novel research path for developing FeCrAl matrix ODS steels with Y2O3, Ti, and V additions. The materials synthesis consisted of mechanical alloying of pure metallic components with yttrium oxide in a planetary ball mill under an argon atmosphere. Titanium was added in the amount of 1.0 wt.% to both samples, while 0.5 wt.% of vanadium was added to one sample to verify its impact on the structural stability and hardness. The spark plasma sintering (SPS) technique was used to consolidate the powders. Afterward, the microstructure, chemical composition, phase composition, and hardness were assessed using SEM-EDS, EBSD, TEM-EDS, XRD, XRF, Nanoindentation, and Vickers microhardness. The experimental data reveal rather homogeneous powders after mechanical alloying and dense bulk samples after SPS. The microstructure observations show oxide particles and carbides on the grain boundaries and inside grains of the bcc matrix, which suggests elevated radiation damage resistance. The presence of nanoscale oxide particles (15–50 nm) in the matrix could significantly reduce the impact of aging embrittlement at high temperatures by affecting the chromium diffusion pathways in the ODS steel matrix. The addition of vanadium leads to an improvement of hardness to 4.83±0.43 GPa compared to 3.78±0.34 GPa for the sample without vanadium. Presented experimental results are promising in terms of research and development of FeCr and Al-based ODS materials tailored to operate under harsh conditions in generation IV fission reactors and fusion reactors.

MD simulation of vacancy and interstitial diffusion in FeCr alloy

The diffusion mobility of iron and chromium atoms in Fe-9%Cr and Fe-20%Cr alloys for temperature up to 1000 K has been studied by molecular dynamics simulation. Both vacancy and interstitial migration of atoms has been considered. Corresponding diffusion coefficients of iron and chromium atoms, as well as the diffusion coefficients of a vacancy and self-interstitial configuration, have been evaluated. The values obtained for the vacancy diffusion satisfactory agree with experimental results taken from the literature.

Designing ultrastrong and thermally stable FeCrAl alloys with the fine-grained structure

Designing microstructurally stable FeCrAl alloys with excellent strength-ductility synergy is highly desirable for their engineering applications. However, due to the preference nucleation of precipitates at grain boundaries (GBs), the improved precipitation strengthening of these alloys is usually accompanied by intergranular embrittlement. Here, we propose a novel thermomechanical processing route coupled with the Si alloying strategy via precipitation of coherent deformable Laves precipitates inside equiaxed fine-grains to achieve FeCrAl alloys with ultrahigh yield strength over ∼992 MPa, excellent uniform elongation of ∼7.6% at room temperature, and superior thermal stability at temperature ∼1200 °C. The Si alloying not only decreases the stacking fault energy of Laves precipitates favorable for their stacking-fault-mediated deformation but also hinders grain coarsening at 1200 °C due to the Si-enrichment favorable for GB pinning effects. Our results prove the possibility of achieving the collaborative enhancement of mutually exclusive properties in alloys, such as strength-ductility-thermal stability via nanoprecipitation engineering, and offer a promising route to prepare dispersion-strengthened materials.

Flash electropolishing of BCC Fe and Fe-based alloys

The preparation of transmission electron microscopy (TEM) samples is a critical step in the characterization of materials, and the focused ion beam (FIB) technique is a commonly used method. However, a significant limitation of this technique is the FIB-induced damages on the foil surfaces, which can obscure the real features of interest, particularly in radiation effects studies. To overcome this limitation, this study presents a detailed description of the flash electropolishing technique, which can be used to remove the FIB damage from samples. The flash electropolishing technique has been successfully applied to a range of materials, including Fe, Fe-based model alloys, commercial Fe-Cr alloys, and advanced Fe-Cr alloys, both in their as-received and ion-irradiated states. Furthermore, the parameters used for Fe-Cr model alloys can be adjusted for commercial and advanced alloys with minimal modifications. The study also examined the effects of electropolishing variables, such as perchloric acid concentration, electropolishing temperature, Cr concentration, and voltage. Qualitatively, a general trend and scoping test strategy is explored in our experiments. Overall, the flash electropolishing technique offers a promising solution to the challenges posed by FIB-induced damages in the preparation of TEM samples.

Effect of Fe on the oxidation, sulfidation and carburization behaviors of alloys in CO2 containing SO2

The effect of Fe on the oxidation, sulfidation and carburization behaviors of alloys in CO2 containing SO2 at 1000 ℃ was investigated using multiple evaluation methods, including first-principles, molecular dynamics simulation, high-temperature corrosion experiment and thermodynamic analysis. The adsorption capacity of both gas molecules on FeCr alloy surface was stronger than that on NiCr alloy surface. The molar content of O, C and S atoms in Fe-25Ni-22Cr alloy was higher than that in Ni-25Fe-22Cr alloy. The increase of Fe content promoted the reactions between alloy elements and O, C, S atoms, which caused serious corrosion of Fe-Ni-Cr alloys.

Microstructure Characterization and Failure Mechanism of Laser Welded Nuclear-grade FeCrAl Alloy

FeCrAl alloy is considered a new type of promising accident-tolerant fuel cladding material. Laser welding technology, which has been widely applied in the connection of various metal materials, uses a high energy density laser heat source with the advantages of efficiency and precision. However, due to the inherent characteristic of high speed, the laser weld is generally exposed to rapid solidification, resulting in the inevitable formation of defects such as coarse grain size and porosity in the weld. The presence of pores is a troublesome question because they will reduce the bearing area of the joint and accelerate the initiation of cracks, resulting in a significant reduction of the tensile strength. Based on these considerations, in this study, the effect of pores on laser-welded nuclear-grade FeCrAl alloy is investigated by quasi-in-situ tensile testing, and the fracture mechanism of the joint is revealed. The maximum ultimate tensile strength measured by the experiment is 550 MPa, and the elongation of welds is 9.6%. In addition, there are obvious stress concentrations and cracking phenomena generated in the vicinity of a pore. The joint is eventually fractured in the fusion line near the fusion zone. A large number of pores can be found at the fracture failure position from the fracture morphology observation, indicating that the pores are presumably to be the intrinsic reason for the cracking of the welded joints.

Application of a deep learning semantic segmentation model to helium bubbles and voids in nuclear materials

Imaging nanoscale radiation-induced defects using the transmission electron microscope (TEM) is a key factor in the successful implementation of materials for nuclear energy structural applications. Analyzing each defect in a TEM micrograph is currently a manual task. To identify the defects in a single image can take anywhere from 15 min to an hour and a project can require the analysis of anywhere from tens to ≥ 100 images. Here, we use artificial intelligence (AI) models to automate this task. For simplification, we evaluated images with only a single type of defect; helium bubbles. We performed semantic segmentation of these helium bubble defects in electron microscopy images of irradiated FeCrAl alloys using a deep learning DefectSegNet model. This model, which was previously used to classify crystal defects, is inspired by the classic DenseNet and U-Net image segmentation models. It claims high spatial resolution, but has poor performance at object boundaries. Our paper improves the DefectSegNet model’s application by adding two new features. First, the DefectSegNet model is applied not only to perform calculation pixel-wise but also object (or feature) wise. Because object-wise metrics are directly relevant to our final goal of detecting bubbles, whereas pixel-wise classification is only an intermediate step, it is an important part of our study. Second, a distance map loss (DML) function has been added to increase its performance at object boundaries. It is crucial to accurately represent defects boundaries, especially bubbles, in order to track the bubble-induced swelling caused by irradiation. The boundary-focused DML function is also compared to other loss functions like Cross-entropy, Weighted Binary Cross Entropy (WBCE), Dice and Intersection over Union (IOU). Finally, by incorporating new features, we found a marked improvement on segmentation quality and better shape preservation at the boundaries and areas of the bubbles.

Effect of molybdenum addition on oxidation behavior and secondary protection mechanism of FeCrAl coatings

FeCrAl alloy coatings are widely used for their excellent oxidation resistance at high temperatures, but they can suffer from rapid material degradation due to breakaway oxidation of the coating. Composition manipulation is an essential way to prompt the oxidation resistance of the coatings. In this study, we prepared Fe-13Cr-7Al (at.%) alloy coatings with varying Mo contents (Mo = 0, 1.5, 3, 6 at.%) by magnetron sputtering and investigated their oxidation behaviors at 550 °C. The surface and cross-sectional morphologies of the coatings before and after oxidation were examined using electron microscopy. The results showed that the crystalline grain and particle size of the coatings decreased significantly with the increase of Mo content, leading to a notable increase of grain boundaries and further enhancing element diffusion during the oxidation tests. As a result, a protective and stable oxide scale rapidly grew on the surface of FeCrAlMo coatings during oxidation. Moreover, FeCrAlMo coatings demonstrated improved secondary protection mechanisms with increasing Mo contents. This was attributed to the formation of an internal oxidation layer of a spinel barrier (M3O4, M = Fe, Cr, Al, Mo) beneath the outer oxidation layer. The spinel barrier impeded inward diffusion of oxygen ions and outward diffusion of metal cations, resulting in enhanced oxidation resistance of the spinel-type oxide. The thermodynamic calculations were used to evaluate the oxidation mechanisms of the coatings. These results provide a deeper understanding of the effect of Mo addition on the oxidation behavior and underlying protection mechanisms of FeCrAl coatings, and can be beneficial for designing and developing materials for use in oxidation environments.

Prediction and theoretical investigation of dehydrogenation enthalpy of V–Ti–Cr–Fe alloy using machine learning and density functional theory

The dehydrogenation enthalpy, a crucial thermodynamic parameter, determines the difficulty of hydrogen desorption in V–Ti–Cr–Fe alloy, but it is unaffordable to predict and modify the dehydrogenation enthalpy and investigate the synergistic effects of structural and chemical parameters relying on conventional experimental methods. In this paper, an extreme gradient boosting (Xgboost) model with feature selection is developed, which can obtain a relative error under 15% in prediction on most of the data. Moreover, the genetic algorithm is utilized for optimizing alloy composition having suitable dehydrogenation enthalpy. The accuracy of prediction and optimization is demonstrated by the subsequent experimental validation, where the error between calculation and experiment is within 4.5%. Density function theory (DFT) is used to analyze the key features provided by machine learning, with results suggesting that the volumes, valence electrons, and bulk module factors have a significant impact. This research supplies an efficient way of using machine learning to predict and optimize the dehydrogenation enthalpy. In addition, DFT calculation also validates the feature importance outputted by machine learning and reveals the principle of feature effects at the microscopic aspect.

Effects of Cr and Mo contents on the flow-accelerated corrosion behavior of low alloy steels in the secondary side of pressurized water reactors

Carbon steels have been extensively used as structural materials in the secondary side of pressurized water reactors, which are susceptible to flow-accelerated corrosion (FAC). To overcome this issue, low-alloy steels containing 2.25Cr-1Mo (P22) have been introduced, as Cr and Mo are effective alloying elements in mitigating FAC. However, the behavior of these alloying elements in the secondary water chemistry is not well-understood, and therefore, there is a need to explore alternative materials that can address the issue of FAC. In this study, three model alloys were manufactured based on Ducreux's model on FAC rate to examine the effects of Cr and Mo on the corrosion behavior of low-alloy steels compared to that of commercial P22. Microstructure of P22 was ferritic/pearlite structure while the model alloys was ferritic/tempered bainite structure due to the existence of Mn and Si. The difference in microstructure led to the difference in hardness values, and elimination of Mo from the model alloys resulted in reduction in strain. The FAC rate was mainly influenced by the Cr content in the oxide layer rather than Mo. The passive film layer hinders the exposure of the inner layer to H2O in the solution, resulting in Cr3+ dissolution into the Fe oxide layer. The continuous passivation leads to the formation of two compact layers, one amorphous and one crystalline Cr, creating Fe oxide substitutes. It is also revealed that the solubility of Cr species is much lower than that of Fe species resulting in the enrichment of Cr in the outer layer, and thus the higher Cr content reinforces the passivity of the steels. The Fe-Cr alloy exhibited promising corrosion resistance, suggesting that it could be a potential substitute for Fe-Cr-Mo alloy.

Graded microstructure and mechanical properties of spark plasma sintered Fe-Cr alloys

Developing graded microstructure and mechanical properties is critical for accelerating the design and optimization of structural materials for a wide range of applications. In this study, Fe-Cr alloys were fabricated by spark plasma sintering (SPS) technique. The microstructure, microhardness, and tensile properties of the as-fabricated and thermally annealed variants were investigated. Graded grain structures were created in the as-fabricated cylinders along both axial and radial directions. Grain size was gradually reduced from the sample periphery to the center. Microhardness measurements reveal a gradual decrease of hardness towards the periphery of the samples. The as-fabricated Fe-Cr alloys show a desired combination of tensile strength and elongation, primarily due to the formation of a high density of oxides, voids, dislocations, and grain boundaries. After in-situ thermal annealing at 600 °C under 60 MPa for 2 h using SPS, the Fe-Cr alloys underwent minimal grain growth, and the graded grain structures were retained. The hardness was more uniformly distributed in the annealed variants, and the tensile strength was reduced with an increase in the total elongation, which is attributed to the dissolution of nano-sized oxide particles and the relief of the residual stress. This study demonstrates that SPS coupled with subsequent heat treatment can tailor the graded microstructure and control the mechanical properties of Fe-Cr alloys, showing potential applications in other alloy systems.

Effects of grain boundaries and temperature on spinodal decomposition in a binary Fe-Cr alloy: A phase-field simulation

Due to its excellent oxidation resistance, corrosion resistance and mechanical properties, Fe-Cr based stainless steel is a promising candidate for advanced nuclear power systems. At the temperature of 300–600 °C, spinodal decomposition usually occurs in Fe-Cr alloys. However, irradiation can greatly accelerate this process. Fe-Cr based stainless steel decompose into a Cr-enriched α′ phase and a Fe-enriched α phase during spinodal decomposition, and the Cr-enriched α′ phase plays an important role in the hardening/embrittlement of Fe-Cr based stainless steels. In the present work, the evolution of vacancy and the Cr-enriched α′ phase in Fe-Cr alloy is investigated using phase-field method. The simulation results show that the vacancy concentration has a great influence during the spinodal decomposition. The Cr-enriched α′ phase occurs initially at the positions with high vacancy concentration. During spinodal decomposition, the vacancy forms a defect concentration loop around the Cr-enriched α′ phase. In addition, with the increase of temperature, the concentration of Cr atoms in the center of the Cr-enriched α′ phase decreases, and the coarsening rate increases, while the number of the Cr-enriched α′ phase formed in the alloy decreases. At low temperature, α′ phase is mainly coarsened by interlinking; while at high temperature, α′ phase is mainly coarsened by Oswald maturation. Moreover, the Cr-enriched α′ phase occurs at the grain boundary (GB) and the Fe-enriched α phase occurs at both sides of the GB during the spinodal decomposition. At the same time, in polycrystalline system, GB movement during grain growth is inhibited during the spinodal decomposition.

Effect of Yttrium Addition on Nonmetallic Inclusions in FeCrAl Alloys

To investigate the effect of yttrium (Y) addition on nonmetallic inclusions in FeCrAl alloys, a series of laboratory experiments and thermodynamic calculations are carried out as a function of reaction time and Y addition amount. Without Y addition, the main inclusion in FeCrAl alloys is AlN, and only a small amount of Al2O3 inclusions is evident. The higher content of Cr in FeCrAl alloy promotes the formation of Al2O3. Y addition can effectively modify Al2O3 inclusions to YAlO3 or YS inclusions. With the addition of 0.0046 wt% Y, the evolution route of inclusions in FeCrAl alloys with reaction time is Al2O3 → YAlO3‐Al2O3 → YAlO3. The variation in Al2O3 inclusions with increasing Y content is Al2O3 →YAlO3 → YS. The experimental results are consistent well with the thermodynamic calculation results.

Excellent adhesion of protective α-Al2O3 layer formed on ODS FeCrAl alloys

The growth of α-Al2O3 layer on ODS FeCrAl alloys NF12 (Fe-12Cr-6Al-0.5Ti-0.4Y-0.4Zr-0.25Ex.O) and SP10 (Fe-15Cr-7Al-0.5Ti-0.4Y-0.4Zr-0.24Ex.O) was investigated by means of the oxidation tests under air atmosphere at 1273 K up to 10 h. The inward oxygen diffusion was promoted through the oxides of reactive elements (i.e., Ti, Y and Zr) formed along the grain boundaries of α-Al2O3 layer, and the inward growth of the layer was accelerated. The shearing stress required for the layer exfoliation was quantitatively evaluated by micro-scratch tests. The adhesion of α-Al2O3 layer formed on the ODS alloys was reinforced by so-called anchoring and pegging effects.

Mixing enthalpy near grain boundaries in Fe-Cr alloy: The results of atomistic simulation

Energy characteristics of symmetric tilt (twin) grain boundaries (GBs) Σ5(210) and Σ5(310) with the tilt axis [001] and boundaries Σ3(111) and Σ3(112) with the tilt axis [11¯0] in a bcc random binary FeCr alloy have been studied using atomistic simulation. The simulation has been carried out based on DFT calculations and two-band semi-empirical potential. We have proposed the formula for calculating the mixing enthalpy near GBs. The relationship between the mixing enthalpy and the energy of the boundary in the alloy has been revealed. Both simulation methods have shown a decrease in the mixing enthalpy near a GB compared to the ideal crystal. Moreover, DFT has predicted a stronger decrease in the mixing enthalpy compared to estimates based on semi-empirical interaction. Based on the obtained dependences of the mixing enthalpy on the alloy composition, phase diagrams for the considered GB regions have been constructed. It has been shown that the GB region is characterized by a higher equilibrium concentration of chromium, and the precipitates of the α'-phase have a lower concentration of chromium compared to the bulk. The smallest difference in equilibrium compositions has been observed for the Σ5(310) and Σ3(112) boundaries, and the largest one for the Σ3(111) boundary, which has the highest energy among considered GBs.

High-temperature corrosion of weld overlay coating/bulk FeCrAl exposed in O2 + H2O + KCl(s) at 600 °C – A microstructural investigation

This work investigates the impact of high-temperature corrosion behavior of the newly developed FeCrAl alloy Kanthal® EF101 bulk material and weld overlay coating in the presence of KCl(g)/KCl(s) at 600 °C. The oxide scale formed within the secondary corrosion regime after exposure and the impact of alloy microstructure on corrosion behavior was investigated using scanning transmission electron microscopy. The findings indicated the key microstructural differences is the alloy grain size which influences the formation of a protective scale. In addition, It is indicated that coating exhibited inferior performance than the bulk material, primarily attributed to the microstructural differences.

Microstructure and texture evolutions in FeCrAl cladding tube during pilger processing

The microstructure of FeCrAl cladding tubes depends on the fabricating process history. In this study, the microstructural characteristics of wrought FeCrAl alloys during industrial pilger processing into thin-walled tubes were investigated. The hot extruded tube showed ∼100 μm equiaxed grains with weak α∗-fiber in {h11}<1/h12> texture, while pilger rolling process change the microstructure to fragmented and elongated grains along the rolling direction. The pilgered textures could be predicted with the VPSC model. The inter-pass annealing at 800–850 °C for 1 h results in recovery and recrystallization of the ferric matrix and restoration of ductility. The final finished tube shows fine recrystallized grains (∼11 μm) with dominant γ-fiber in three dimensions. Pilger rolling enhanced α-fiber while annealing reduced α-fiber and enhanced γ-fiber. Microstructural evolution in the Laves precipitates followed the sequence of faceted needle-like → spherical → faceted ellipsoidal. Thermomechanical processing resulted in cladding tubes with an area fraction of ∼5% and a number density of 5 × 10−11 m−2 in Laves precipitates, which is half that of the first-pilgered tube. Laves precipitates pin the grain boundaries to control the microstructure and prevent grain coarsening.

The elemental weakening & aggregation effect on the hydrogen adsorption on FeCrAl (110) surface

FeCrAl alloys are promising candidates for Accident Tolerant Fuel cladding in Light Water Reactors. Understanding the adsorption mechanism of hydrogen is important for understanding the hydrogen embrittlement of FeCrAl alloys. In our previous work, we find that the adsorption site can significantly influence the H adsorption energy (Eads.). However, the widely adopted Special Quasi-random Structure (SQS) method for constructing FeCrAl structure hinders comprehensive investigations on all the possible H adsorption sites. In this work, a manually created surface structure is used to systematically study all the possible H adsorption sites. It is found that when Cr/Al atoms are dispersed on the surface, the HCr/Al bonding can be weakened. The weaker HCr/Al interactions are attributed to the charge deficit of Cr/Al caused by their relatively higher electronegativity than Fe. In contrast, when Cr/Al atoms aggregate on the surface, the HCr/Al bonding can be reinforced. The reinforcement comes from the increased charge density of Cr/Al due to aggregation.

Molecular dynamics simulation on the dissolution and diffusion characteristics of FeCrAl alloy in liquid LBE

Lead-bismuth eutectic (LBE) corrosion-resistant materials are crucial for the development of future fourth-generation nuclear reactors, and the dissolution and diffusion of material components are important processes in LBE corrosion-resistant materials. FeCrAl alloy is an important structural material for fourth-generation reactors, and it is essential to study its dissolution and diffusion processes in liquid LBE. Molecular dynamics simulations (MD) were used to investigate the mutual diffusion process between the FeCrAl alloy and liquid LBE. The dissolution and diffusion abilities of Fe, Cr, and Al atoms in liquid LBE were evaluated using mean-square displacement (MSD) and diffusion activation energy (DAE). Fe atoms exhibited the highest degree of diffusion, while Al atoms showed the lowest degree of diffusion, resulting in a distinct difference in the elemental distribution at the solid–liquid interface (SLI). There was a significant difference in the depth of penetration of Pb and Bi atoms into the FeCrAl alloy matrix, with Bi atoms penetrating deeper. Furthermore, there is a thermal vibration process during the invasion of Pb and Bi atoms into the FeCrAl alloy matrix, which promotes the invasion process of Pb and Bi atoms. In addition, an increase in temperature exacerbates the diffusion of Fe, Cr, and Al atoms in liquid LBE and the process of Pb and Bi atom invasion into the FeCrAl alloy matrix. Our research may contribute to the design of future nuclear-grade FeCrAl alloy and the development of next-generation reactors.