Iron Chromium Aluminium Research Articles

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.

Distinctive Oxide Films Develop on the Surface of FeCrAl as the Environment Changes for Nuclear Fuel Cladding

The corrosion-resistant properties of IronChromium–Aluminum (FeCrAl) alloys have been known for nearly a century. Since the 1950s, they have been explored for application in the generation of nuclear power. In the last decade, the focus has been on the use of FeCrAl as cladding for uranium dioxide fuel in light water reactors (LWRs). The corrosion resistance of this alloy depends on the oxide that it can develop on the surface. In LWRs in the vicinity of 300 °C, the external surface oxide of the FeCrAl cladding could be rich in Fe under oxidizing conditions but rich in Cr under reducing conditions. If there is an accident and the cladding is exposed to superheated steam, the cladding will protect itself by developing an alpha aluminum film on the surface.

Improved and Innovative Accident-Tolerant Nuclear Fuel Materials Considered for Retrofitting Light Water Reactors—A Review

Since 2011, there has been an international effort to evaluate the behavior of newer fuel rod materials for the retrofitting of existing light water reactors (LWR). These materials include concepts for the cladding of the fuel and for the fuel itself. The materials can be broadly categorized into evolutionary or improved existing materials and revolutionary or innovative materials. The purpose of the newer materials or accident-tolerant fuels (ATF) is to make the LWRs more resistant to loss-of-coolant accidents and thus increase their operation safety. The benefits and detriments of the three main concepts for the cladding are discussed. These include (i) coatings for existing zirconium alloys; (ii) monolithic iron–chromium–aluminum alloys; and (iii) composites based on silicon carbide. The use of ATF materials may help extend the life of currently operating LWRs, while also being a link to material development for future commercial reactors.

Experiments and modeling of the macroscale mechanical responses of neutron-irradiated FeCrAl alloys

Iron chromium aluminum (FeCrAl) alloys have been regarded as candidate materials for accident-tolerant fuel cladding. In this study, the stress-strain curves of various neutron-irradiated and un-irradiated FeCrAl alloys at the test temperatures of 300 K and 653 K are obtained through the uniaxial tensile tests, with the fast neutron fluences of 2 × 1019 n/cm2 and 8 × 1019 n/cm2 (E ≥ 1 MeV). Simultaneously, improved shear strain rate model and damage evolution model are proposed in the crystal plasticity theoretical frame, with the irradiation effects involved. The developed models are numerically implemented, with the simulation results of the macroscale stress-strain curves agreeing well with the experimental data. The irradiation hardening and embrittlement phenomena are captured, together with the characteristics of strain hardening and softening. The influences of alloy compositions, temperatures, and fast neutron fluences on yield strength, ultimate tensile strength, yield ratio, and uniform elongation are discussed. Macro mechanical properties such as yield strength and yield ratio are found to be near-linearly correlated with crystal plasticity model parameters. A model for multi-objective optimization of the mechanical properties is established, with which the optimum design strategies of alloy composition for the materials in the in-reactor service environments are proposed. This study could provide a theoretical basis for the research and development of advanced FeCrAl alloys.

Effect of Zr addition on coarse Laves phase stimulated nucleation of recrystallization in FeCrAl alloy

This study investigates the coarse Laves phase stimulated nucleation of recrystallization behavior in iron–chromium–aluminum (Fe–Cr–Al) alloys. The results reveal that the samples with coarse Laves phases in the matrix have finer grains because of the occurrence of particle stimulated nucleation (PSN). Due to the lower diffusion coefficient and lower solubilities of Zr in the bcc matrix, Zr addition increases the proportion of the coarse Laves phase, which promotes the PSN, resulting in a fine grain size in the Fe–Cr–Al alloys. The ultimate tensile strength of the Zr-modified Fe–Cr–Al sample with coarse Laves phases in the matrix reaches ∼600 MPa, and the elongation is up to ∼28%. Additionally, the high elongation of this sample could greatly improve its formability. The superior combination of strength and ductility is attributed to grain refinement and discontinuous deformation. This study could be expected to provide a better understanding of the coarse Laves phase, such as accelerated recrystallization behavior and beneficial effects on the mechanical properties, and provides potential freedom for further design and microstructure control of Fe–Cr–Al alloys.

Influence of Zr on grain orientation, texture, and mechanical properties in hot- and warm-rolled FeCrAl alloys

Iron–chromium–aluminum (Fe–Cr–Al) alloys have great potential application as an accident-tolerant fuel cladding material in light-water reactors. It has excellent processing ability, resistance to steam oxidation at high temperatures (HTs), and irradiation swelling-resistance. It is necessary to improve the room- and high-temperature mechanical properties and reveal the strengthening mechanism. In this study, we analyzed the effect of Zr on the microstructure and room- and high-temperature tensile properties of hot- and warm-rolled FeCrAl alloys. The results show that the Laves phase could be effectively stabilized by Zr addition, and the proportion of low-angle grain boundaries increased. Moreover, the yield strength at room and high temperatures of the samples after Zr addition were improved, owing to the precipitation and sub-grain boundary strengthening by Zr. In addition, Zr changed the grain orientation, but had no relevance to the number of easier activated slip systems. The movable dislocation cannot slip during high-temperature tensile tests because of the pinning effect of the dynamic precipitated Laves phase, which could be the reason for the decrease in elongation at high temperatures. • Zr addition stabilized the Laves phase effectively in Fe–Cr–Al alloys. • Zr addition increased the proportion of LAGBs in hot and warm rolled Fe–Cr–Al alloys. • Zr addition did not influence the number soft orientation grains in Fe–Cr–Al alloys. • The elongation of Fe–Cr–Al alloys decreased at the high temperature tensile tests.

Effect of Ferric Ion Irradiation on the Incipient Plasticity of an Additively Manufactured Iron–Chromium-Aluminum Alloy During Spherical Nanoindentation

Effect of Ferric Ion Irradiation on the Incipient Plasticity of an Additively Manufactured Iron–Chromium-Aluminum Alloy During Spherical Nanoindentation

Burst characteristics of advanced accident-tolerant FeCrAl cladding under temperature transient testing

Assessment of burst characteristics of accident-tolerant fuel (ATF) claddings is essential to evaluate the safety margins of nuclear reactors, and better understanding can enable accelerated licensing for reactor concepts that include the new materials. Therefore, this study investigated the burst behavior of an ATF candidate of C26M, which is an iron–chromium–aluminum alloy (FeCrAl) under transient testing like the simulated loss-of-coolant accident (LOCA) conditions, except the water-quenching phase, in light-water reactors (LWRs).The effect of LOCA specimen length was assessed in terms of post-burst tube parameters. No critical length effect was determined on the burst pressure, burst location, and burst size measurements. Burst temperature showed larger variation likely due to its measurement approach used in this study. The post-test diametral strain was identified as the critical parameter to reduce the specimen length as compared to tube burst length or width. Postmortem optical metrology and digital image correlation were employed to determine the local strain-state during the simulated LOCA, which showed the loading path was close to equibiaxial conditions in regions away from the burst location, rather than internally pressurized conditions (or “plane-strain” tension).Pst-test microstructural characterizations of the FeCrAl revealed that ductile damage was present at the edge of the outer radial surface while the rest of the material was ruptured via cleavage at LOCA burst temperatures. Furthermore, internal grain boundary cracks were observed at locations away from the tube burst region. This behavior was considered to occur during the cooling down period of the simulated LOCA test. Overall, this study aimed to provide essential data for fuel performance code development by considering micromechanics of the deformation and failure.

A damage-effect-involved phenomenological crystal plasticity model and computational methods for mechanical responses of FeCrAl alloys

In this study, a new damage-effect-involved phenomenological crystal plasticity model is proposed to capture macroscale straining hardening and strain softening behaviors of iron chromium aluminum (FeCrAl) alloys. The constitutive model is described with co-rotational tensors. Damage factor evolves with the effective plastic work calculated by the resolved shear stress and shear strain rate on all slip systems. A stress update algorithm is specially established, and the corresponding self-defined subroutines are incorporated into the finite element analysis via VUMAT in ABAQUS/Explicit. Fast convergence can be achieved with the maximum value of damage factor set very close to 1. The simulated macroscale stress-strain curves can agree well with the experimental ones for various FeCrAl alloys. The yield strengths and ultimate tensile strengths of these alloys can be well captured. The dependence of damage model parameters on the processing conditions, alloy compositions are discussed and analyzed. This work provides a basis for the multiscale simulation research on the mechanical performances of polycrystal alloys.

Mechanical failure of fresh nuclear grade iron–chromium–aluminum (FeCrAl) cladding under simulated hot zero power reactivity initiated accident conditions

The reactivity initiated accident (RIA) is a postulated accident in light water reactors initiated by a rapid increase in reactivity which causes an increase in fission rate and fuel temperature. One potential mode of fuel system failure during RIA is pellet cladding mechanical interaction (PCMI) due to the rapid thermal expansion of the fuel pellet. Simulated PCMI experiments were performed by rapidly pressurizing iron-chromium-aluminum (FeCrAl) cladding tube samples using a hydraulic modified burst test system, causing the specimens to burst under a biaxial loading path. Deformation and rupture of the specimens were tracked with a telecentric lens and high-speed camera system. Outer surface strains were calculated using digital image correlation (DIC) on speckle patterns painted on the specimens’ outer surfaces. Experiments were conducted at approximately 275 °C, representative of hot zero power RIA conditions. The failure hoop strain was DIC-calculated between approximately 1.8–3.4%, corresponding to quasi-static energy depositions of approximately 110–260 calories per gram UO2 assuming initial pellet-cladding contact. These conditions are very similar to the proposed energy deposition limit of 150 calories per gram UO2 for unirradiated zirconium-based cladding in US Nuclear Regulatory Commission (NRC) Draft Regulatory Guide 1327. A very small strain rate dependence was observed in the data, with the magnitude of the failure hoop strain decreasing slightly with increasing strain rate. This observed dependence may be relevant because the FeCrAl cladding strain rate will be approximately 20% higher than in zirconium-based cladding due to the harder neutron spectrum and resultant shorter neutron generation time.

Mesoscale simulation research on the homogenized elasto-plastic behavior of FeCrAl alloys

Iron chromium aluminum (FeCrAl) alloys have been considered as a promising candidate in nuclear power applications. In order to accelerate the advanced fabrication of these alloys, it is necessary to perform numerical simulations to predict the macroscale mechanical properties for FeCrAl alloys. In this study, modified crystal plasticity models and stress update algorithm are proposed to describe the deformation behavior of single crystals. Computational homogenization methods for polycrystalline aggregates are provided to obtain the macroscopic properties from the mesoscale stress and strain fields in the representative volume element (RVE). The involved models and algorithms are implemented in ABAQUS with a self-written VUMAT subroutine, and validated with experimental data. The homogenized elasto-plastic curves for FeCrAl alloys are numerically obtained, which match well with the experimental results for different alloy compositions, grain sizes and processing conditions. The relations of the shear-rate model parameters with the compositions, microstructures and processing conditions of FeCrAl alloys are discussed. The research results indicate that the phenomenological shear-rate model and the modified crystal plasticity models can be adopted in Materials Genome Initiative (MGI) to achieve the multi-scale simulation of polycrystalline mechanical performances. The simulation results could also provide a reference for the optimization of FeCrAl alloys.

Iron–Chromium–Aluminum (FeCrAl) Cladding Oxidation Kinetics and Auxiliary Feedwater Sensitivity Analysis—Short-Term Station Blackout Simulation of Surry Nuclear Power Plant

Accident tolerant fuels (ATF) and steam generator (SG) auxiliary feedwater (AFW) extended operation are two important methods to increase the coping time for nuclear power plant safety response. In light of recent efforts to investigate such methods, we investigate both FeCrAl cladding oxidation kinetics and SG AFW sensitivity analyses, for the Surry nuclear power plant Short-Term Station Blackout simulation using the MELCOR YR 1.8.6 systems code. The first part describes the effects of FeCrAl cladding oxidation kinetics. Zircaloy cladding and two different oxidation models of FeCrAl cladding are compared. The initial hydrogen generation time (>0.5 kg) is used as the evaluation criterion for fuel degradation in a severe accident. Results showed that the more recent oxidation correlation by ORNL predicts much less hydrogen generation than Zircaloy cladding. The second part investigates the effects of three different methods of AFW injection into the SG secondary side. We considered three different methods of water injection; i.e., constant water injection into the secondary side (case 1); water injection based on secondary side water level in boiler region (case 2); water injection based on secondary side water level in the downcomer region (case 3). The case of constant water injection is the most straightforward, but it would have the tendency to overfill the SG with excess water. Water injection with downcomer level control is more reasonable but requires DC power to monitor level and to control AFW injection rate.

Performance of iron–chromium–aluminum alloy surface coatings on Zircaloy 2 under high-temperature steam and normal BWR operating conditions

Iron-chromium-aluminum (FeCrAl) coatings deposited on Zircaloy 2 (Zy2) and yttria-stabilized zirconia (YSZ) by magnetron sputtering have been tested with respect to oxidation weight gain in high-temperature steam. In addition, autoclave testing of FeCrAl-coated Zy2 coupons under pressure-temperature-dissolved oxygen coolant conditions representative of a boiling water reactor (BWR) environment has been performed. Four different FeCrAl compositions have been tested in 700 °C steam; compositions that promote alumina formation inhibited oxidation of the underlying Zy2. Parabolic growth kinetics of alumina on FeCrAl-coated Zy2 is quantified via elemental depth profiling. Autoclave testing under normal BWR operating conditions (288 °C, 9.5 MPa with normal water chemistry) up to 20 days demonstrates observable weight gain over uncoated Zy2 simultaneously exposed to the same environment. However, no FeCrAl film degradation was observed. The 900 °C eutectic in binary Fe–Zr is addressed with the FeCrAl-YSZ system.

Decrease in electrical resistance of surface oxide of iron–chromium–aluminium alloy by La0.6Sr0.4Co0.2Fe0.8O3 coating and heat treatment for the application of metal-supported solid oxide fuel cells

We have investigated the property of a Fe–Cr–Al-type stainless steel as a porous alloy substrate for metal-supported solid oxide fuel cells (SOFCs) especially on the cathode side. We found that the microstructure and electrical resistance of the surface oxide layer of the alloy changes depending on the heat-treatment conditions. A relatively low electrical resistance was obtained when the porous alloy substrate was coated with La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and heat treated at 700–800 °C in air. The morphology of the surface oxide layer observed by high-resolution transmission electron microscopy was a columnar structure of γ-Al2O3 polycrystal and Sr3Al2O6 growing outward in the same direction. In contrast, the surface oxide layer of the alloy showed a high electrical resistance when the uncoated porous alloy substrate was heat treated. The morphology of the surface oxide layer in that case was a columnar structure consisting of only γ-Al2O3 growing outward in various directions.

The effect of co-firing sewage sludge with used wood on the corrosion of an FeCrAl alloy and a nickel-based alloy in the furnace region

The effect of digested sewage sludge as a fuel additive to reduce corrosion of furnace walls has been studied. The nickel base alloy Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT™ were exposed for 14.25h at the furnace wall in a power boiler burning 100% used (also known as waste or recycled) wood. The test was then repeated with the addition of sewage sludge to the waste wood. The samples were chemically analysed and thermodynamically modelled and the corrosion mechanisms were investigated. The results showed that the co-firing of sewage sludge with recycled wood leads to a reduction in the corrosion. Attack by a potassium–lead combination appeared to be the main corrosion mechanism in Alloy 625 during waste wood combustion, while attack by alkali chloride was found to be dominant in APMT alloy.

High-resolution chemical analysis by STEM-EELS of nanosized oxide particles in a mechanically-alloyed FeCrAl intermetallic

The chemical composition of nanosized oxides has been analysed in a mechanically-alloyed (MA) iron–chromium–aluminium intermetallic containing yttria additions using an aberration-corrected, high-resolution scanning transmission electron microscope (STEM). The oxide particles are seen to contain yttrium and oxygen only, but very little of the matrix metallic elements, while the matrix in the immediate vicinity shows a very low iron content. Possible reasons for the change of matrix composition outside the particle-matrix interface are discussed.

Microstructural stability of Fe–Cr–Al alloys at 450–550 °C

Iron–Chromium–Aluminium (Fe–Cr–Al) alloys have been widely investigated as candidate materials for various nuclear applications. Albeit the excellent corrosion resistance, conventional Fe–Cr–Al alloys suffer from α–α′ phase separation and embrittlement when subjected to temperatures up to 500°C, due to their high Cr-content. Low-Cr Fe–Cr–Al alloys are anticipated to be embrittlement resistant and provide adequate oxidation properties, yet long-term aging experiments and simulations are lacking in literature. In this study, Fe–10Cr–(4–8)Al alloys and a Fe–21Cr–5Al were thermally aged in the temperature interval of 450–550°C for times up to 10,000h, and the microstructures were evaluated mainly using atom probe tomography. In addition, a Kinetic Monte Carlo (KMC) model of the Fe–Cr–Al system was developed. No phase separation was observed in the Fe–10Cr–(4–8)Al alloys, and the developed KMC model yielded results in good agreement with the experimental data.

Corrosion of furnace wall materials in waste-wood fired power plant

Corrosion tests were performed with four different materials exposed at the furnace wall in a power boiler burning recycled wood, with the aim of evaluating coatings to reduce the corrosion. The nickel base Alloy 625 and the iron–chromium–aluminium alloy Kanthal APMT had the lowest corrosion rates followed by the stainless steel 310S. The low alloy steel 16Mo3, from which the walls are constructed, had the highest rate. Different corrosion mechanisms were found to occur according to the alloy type. Thermodynamic modelling showed that chlorine gas exists at extremely low levels under the prevailing conditions and the hydrated form is thermodynamically favoured.

Methane/air premixed combustion in a two-layer porous burner with different foam materials

Selecting appropriate materials is the most important consideration in designing a porous medium burner. The choice determines the performance and service life of the burner. The premixed combustion of methane/air in a two-layer burner packed with 3mm-diameter alumina (Al2O3) beads at the upstream section and cellular foam at the downstream section was studied. Cellular foam was selected as the alternative material for alumina, zirconia (ZrO2), iron–chromium–aluminum (FeCrAl), or silicon carbide (SiC). The flame stability, flame temperature profile, flame temperature, pressure drop, and pollutant emission of these four materials were investigated. The effect of pore density on SiC foam was also discussed. Results indicated that flame stability limits expanded with increased foam conductivities but shrank with increased pore density. Carbon monoxide (CO) emission was not sensitive to the materials (S>30cm/s). The CO emission of 30pores per inch (PPI) foams was notably lower than that of foams with other pore densities because of the higher flame temperature. Hydrocarbon (HC) concentration was almost constant for ZrO2, Al2O3, and FeCrAl foams. However, the HC concentration of SiC foams decreased with increased flame speed. HC emission increased in the order of 20, 25, 30, and 10PPI foams. Nitrogen oxide (NOx) emission was relatively low (below 3ppm) because of low flame temperature.

On laser welding of thin steel sheets

This paper presents a process–structure–property relationship study of laser welds as a continuous consolidation method for joining thin monophased steel foils, thereby providing a more effective, less costly method to construct automotive catalytic converters. A body centred cubic (bcc) iron–chromium–aluminium alloy doped with Mischmetal was utilised in this study. Both pulsed and continuous wave modes were used to establish the limit welding diagrams for lap joint configuration. Actual laser welding parameters were selected using several testing conditions. The laser welds behaved substantially different from the base material under creep and high temperature oxidation. The difference was mainly attributed to the changes in grain morphology, precipitation of aluminium nitrides and carbides, and relocalisation of the reactive elements during liquid metal flow upon keyhole formation, solidification and cooling.