Fe-Cr Ferritic Alloys Research Articles

The role of Cr concentration and temperature on cavity swelling with co-injected helium in dual-ion irradiated Fe and Fe-Cr alloys

The level of chromium plays an essential role in irradiation tolerance of Fe-Cr ferritic alloys. However, conflicting results have been reported regarding the dependence of cavity swelling under irradiation on Cr level and temperature. Here, we have performed a comprehensive set of simultaneous dual-ion (Ni + He) irradiations to high dose (∼30 displacements per atom, dpa) at 400–550 °C on a series of ultra-high purity Fe and Fe-Cr binary alloys (3–14 wt.%Cr). Helium co-implantation rates of 0.1 and 10 appm He/dpa were selected to examine He synergistic effects relevant for fission and fusion reactor conditions, respectively. Cavities were observed in all irradiated samples by transmission electron microscopy. The results show that higher He implantation rate causes a shift in the swelling peak to higher temperatures in both Fe and Fe-Cr alloys. When assuming smaller cavities as biased sinks, the non-monotonic nature of the cavity swelling behavior is related to the ratio of biased to unbiased point defect sink strengths. Cr-enriched precipitates were observed in Fe-14Cr irradiated at 400 °C by atom probe tomography. Our analysis suggests the formation of Cr-enriched precipitates could suppress cavity swelling for Fe-Cr alloys with Cr content above 10 wt%.

Helium causing disappearance of a/2<111> dislocation loops in binary Fe-Cr ferritic alloys

Single and dual-beam self-ion irradiations were performed at 500°C on ultra-high purity Fe14%Cr alloy to ∼0.33 displacements-per-atom (dpa), and 0 or 3030 atomic-parts-per-million (appm) helium/dpa, respectively. Using transmission electron microscopy, we reveal that helium can drastically modify the dislocation loop Burgers vector in Fe-Cr alloys. Helium co-implantation caused complete disappearance of a/2<111> type dislocation loops, and the microstructure consisted of only a<100> loops. Conversely, a/2<111> type loops were predominant without He co-implantation. The total loop density remained largely unaffected. The results strikingly contrast literature asserting that helium stabilizes a/2<111> type loops in bcc Fe alloys, based on low temperature irradiations. Collectively analyzing the results with literature suggest that the small positive interaction between helium and self-interstitial atoms (SIA) in Fe predicted by atomistic simulations maybe insufficient to holistically explain the dislocation loop microstructure development in presence of helium. Helium-SIA positive binding inadvertently implies elevated a/2<111> loop fraction and higher loop densities that the present results contradict. Helium induced high cavity density causing a preferential loss of highly glissile <111> clusters, leaving the matrix saturated with <100> type clusters is proposed as a potential mechanism. Further, the in-situ irradiations combined with Burgers vector analysis strengthened the evidence of Cr-induced dislocation loop mobility reduction that appears to stabilize the a/2<111> type loops and causes higher loop densities in Fe-Cr alloys.

Long-Term Oxidation Behavior of SOFC Interconnects Alloy ZMG232G10 for 40,000 Hours

ZMGTM232G10 is Fe-Cr ferritic alloy, which has better oxidation resistance for SOFC interconnects. Oxidation behavior of ZMG232G10 was investigated in air at 750 and 850°C for 40,000 hours. Bulk specimens of ZMG232G10 showed stable oxidation behavior according to parabolic law. However, thinner specimens showed anomalous oxidation behavior for 40,000 hours at 850°C. Furthermore, ferrite to austenite phase transformation was observed in a part of specimens which was oxidized at 850°C. This unstable oxidation behavior resulted from Cr consumption in alloy matrix by oxidation.

Temperature-Dependent Cavity Swelling in Dual-Ion Irradiated Fe and Fe-Cr Ferritic Alloys

Fe-Cr ferritic-martensitic (FM) steels are promising structural material candidates for fusion and advanced fission reactors due to their attractive mechanical properties and volumetric swelling resistance. However, significant discrepancies exist regarding the effect of solutes and irradiation temperature on cavity swelling under ion versus neutron irradiation conditions. In this study, simultaneous dual ion irradiations (8 MeV Ni3+ ions and energy-degraded 3.5 MeV He2+ ions) were used to quantify the cavity swelling behavior in ultra-high purity Fe and Fe-Cr alloys (3-14 wt.% Cr), Fe-10 wt.% Cr-780 wt.ppm C, and Eurofer97 FM steel. The irradiations were conducted over a wide temperature range (400-550 °C) with a mid-range dose of ~30 displacements per atom (dpa) and 0.1 appm/dpa He implantation rate. Using state-of-the-art transmission electron microscopy (TEM), we reveal that pure Fe has a ~50 °C lower peak swelling temperature than Fe-Cr alloys, which is attributed to higher vacancy mobility in pure Fe. Chromium solute appears to strongly suppress cavity swelling in Fe-Cr alloys for temperatures below ~470 °C, but seems to have little effect or slightly enhances swelling above ~470 °C. Cavities were observed in all the irradiated samples between 400-550 °C. This indicates that the narrow temperature range of observable cavities reported in prior ion irradiated Fe-Cr ferritic alloy studies is likely an artefact associated with the use of low ion energies (<5 MeV), which leads to pronounced near-surface and implanted ion effects that suppress cavity swelling even at midrange depths (particularly at high temperatures).

In situ nanoindentation during electrochemical hydrogen charging: a comparison between front-side and a novel back-side charging approach

The effects of hydrogen in metals are a pressing issue causing severe economic losses due to material deterioration by hydrogen embrittlement. A crucial understanding of the interactions of hydrogen with different microstructure features can be reached by nanoindentation due to the small volumes probed. Even more, in situ testing while charging the sample with hydrogen prevents the formation of concentration gradients due to hydrogen desorption. Two custom electrochemical cells for in situ testing were built in-house to charge the sample with hydrogen during nanoindentation: “front-side” charging with the sample and the indenter tip immersed into the electrolyte, and “back-side” charging where the analyzed region is never in contact with the solution. During front-side charging, surface degradation often occurs which also negatively influences analyses after hydrogen charging. The back-side charging approach proposed in this work is a promising technique for studying in situ the effects of hydrogen in alloys under mechanical loads, while completely excluding the influence of the electrolyte on the nanoindented surface. Hydrogen diffusion from the charged back-side toward the testing surface is here demonstrated by Kelvin probe measurements in ferritic FeCr alloys, used as a case study due to the high mobility of hydrogen in the bcc lattice. During nanoindentation, a reduction on the shear stress necessary for dislocations nucleation due to hydrogen was observed using both setups; however, the quantitative data differs and a contradictory behavior was found in hardness measurements. Finally, some guidelines for the use of both approaches and a summary of their advantages and disadvantages are presented.Graphical abstract

Temperature-dependent cavity swelling in dual-ion irradiated Fe and Fe-Cr ferritic alloys

Fe-Cr ferritic-martensitic (FM) steels are promising structural material candidates for fusion and advanced fission reactors due to their attractive mechanical properties and volumetric swelling resistance. However, significant discrepancies exist regarding the effect of solutes and irradiation temperature on cavity swelling under ion versus neutron irradiation conditions. In this study, simultaneous dual ion irradiations (8 MeV Ni3+ ions and energy-degraded 3.5 MeV He2+ ions) were used to quantify the cavity swelling behavior in ultra-high purity Fe and Fe-Cr alloys (3-14 wt.% Cr), Fe-10 wt.% Cr-780 wt.ppm C, and Eurofer97 FM steel. The irradiations were conducted over a wide temperature range (400-550°C) with a mid-range dose of ~30 displacements per atom (dpa) and 0.1 appm/dpa He implantation rate. Using state-of-the-art transmission electron microscopy (TEM), we reveal that pure Fe has a ~50°C lower peak swelling temperature difference than Fe-Cr alloys, which is attributed to higher vacancy mobility in pure Fe. Chromium solute appears to strongly suppress cavity swelling in Fe-Cr alloys for temperatures below ~470°C, but seems to have little effect or slightly enhances swelling above ~470°C. Cavities were observed in all the irradiated samples between 400-550°C. This indicates that the narrow temperature range of observable cavities reported in prior ion irradiated Fe-Cr ferritic alloy studies is likely an artifact associated with the use of low ion energies (<5 MeV), which leads to pronounced near-surface and implanted ion effects that suppress cavity swelling even at midrange depths (particularly at high temperatures).

Quantitative evaluation of global and local chromium contents with the EPR test on ferritic and martensitic stainless steels

The cause of localised corrosion phenomena in stainless steels is often related to the element chromium, which is essential for the formation and resistance of the passive layer. A lower than nominally stated content of chromium occurs when the element is highly concentrated and locally bound in chromium-rich phases or precipitates, mostly carbides, and thus locally or globally absent for passivation. The method of electrochemical potentiodynamic reactivation (EPR) is uniquely suited to demonstrate the effect of chromium depletion on the passivation. Here, the steel surface is initially active in sulfuric acid and passivation is achieved by external dynamic polarization. After passivation, the polarization direction is reversed and in chromium-depleted areas a reactivation takes place, which is detected by an increase in current. The detection limit for a reduced chromium content depends largely on the strength of the sulfuric acid used in the EPR test and the polarization speed. The relationship between the sulfuric acid concentration and the electrochemical parameters from the EPR test is shown here using the example of ferritic Fe-Cr alloys with graded Cr contents in the range of approx. 6 to 18 %. From the passivation and reactivation current densities of the EPR data, models for the detection of global and local chromium contents (detection limits) are derived. The developed models are applied to the martensitic stainless steel X20Cr13 in different heat treatment conditions and the influence on the distribution of chromium in the microstructure, which is responsible for the passivation and thus the corrosion resistance, is shown.

High Temperature Cathodic Protection of Fe-Cr Alloy

Power plant industry extensively using ferritic Fe-Cr alloys as boiler materials due to its good resistant to creep, low in thermal shock and thermal expansion. However, the development of new generation of Ultra Supercritical (USC) power plant requires boiler materials to be operate at higher temperature and pressure, thus make it susceptible to high temperature oxidation. The USC power plant successfully reaching the target of working temperature at temperature more than 873K (600°C). By increasing the operating temperature, the efficiency of power plant also increases at the expense of acceleration of high temperature oxidation. This project explored the application of impressed current cathidic protection (ICCP) at 1073 K. At high temperature, gas may dissociates into charged ions that make it possible to conduct electrons. Typical boiler material of T91 was used in high temperature oxidation at 1073K, under Ar-20%O2 mixed gas. The sample is connected to potentiostat with Pt wire to measure the value of corrosion potential (Ecorr), and Icorr. Blank experiment is conducted to determine the presence of reaction by the presence of current conductivity at high temperature in oxygen condition. The weight gain is following parabolic’s rate law, which shows that diffusion is the rate determing step. Blank test shows that as potential induced, current flows from one Pt terminal to another Pt terminal through intermediate mixed gas. Corrosion potential (Ecorr) of the sample show the value is greater than value at room temperature. It is concluded that ICCP is feasible to be conducted for controlling corrosion at high temperature.

Temperature-Dependent Cavity Swelling in Dual-Ion Irradiated Fe and Fe-Cr Ferritic Alloys

Temperature-Dependent Cavity Swelling in Dual-Ion Irradiated Fe and Fe-Cr Ferritic Alloys

Spinodal decomposition during isothermal gas-solid equilibration – Its effects and implications

The possibility for a system to sample non-classical chemical pathways while attempting to attain thermodynamic equilibrium under conditions of restricted kinetics has been elucidated in this work through the analogy of gas phase nitriding of iron based alloys. Detailed thermodynamic assessment has for the first time shown evidence for the occurrence of in-situ isothermal spinodal decomposition during controlled nitridation of ferritic Fe-Cr alloys. Recognition of this phenomenon has led to the explanation of certain reported anomalous experimental observations: dependence of nitridation kinetics of ferritic Fe-Cr alloys on N diffusion and the paradox of Cr-nitrides precipitation sequence not following the predictions of the phase diagram in nitrided Fe-Cr alloys. Finally, the nitridation kinetics of Fe-Al alloys being controlled by Al diffusion has been attributed to the absence of a possibility for spinodal decomposition during nitriding of Fe-Al alloys.

High-energy x-ray diffraction microscopy study of deformation microstructures in neutron-irradiated polycrystalline Fe-9%Cr

In this study, we used far-field high-energy x-ray diffraction microscopy (FF-HEDM) to measure in 3D the spatial distributions of the sizes, orientations, and residual strains of grains and subgrains formed by room-temperature tensile deformation in a neutron-irradiated (450 °C, 0.01dpa) Fe-9Cr specimen and its unirradiated counterpart. We found that neutron irradiation under this condition alone had no effect on the grain size distribution. After deformation, grains fragmented into subgrains in both unirradiated and irradiated specimens: the irradiated specimen contained a few large subgrains which co-existed with many smaller subgrains, while the unirradiated specimen contained small subgrains with a relatively uniform size distribution. Prior to deformation, the irradiated specimen had higher residual strain spread compared to its unirradiated counterpart, while after deformation to the maximum uniform elongation, the strain distributions among subgrains were similar between the unirradiated and irradiated specimens. The FF-HEDM measurements provide new insight into the effects of neutron irradiation on the mechanical response of Fe-Cr ferritic alloys.

Sealing performance and chemical compatibility of SrO–La2O3–Al2O3–SiO2 glasses with bare and coated ferritic alloy

Sealing performance and chemical compatibility of SrO–La2O3–Al2O3–SiO2 glass sealants with bare and Mn–Co spinel coated SUS430 ferritic alloys are investigated. The low–SrO glasses (≤40mol%) show good wettability and adherence to the bare SUS430 alloy due to their lower viscosity and negligible reactions with the alloy. The high–SrO glass (e.g. 45mol%) exhibits higher viscosity and more serious reactions with the bare SUS430 alloy. The formation of SrCrO4 with higher CTE causes obvious delamination and dramatic oxidation of the alloy at the glass/alloy interface. In contrast, Mn–Co spinel coating has good compatibility with both SrO–La2O3–Al2O3–SiO2 glasses and Fe–Cr ferritic alloy. Moreover, the Mn–Co spinel coating on the alloy can effectively restrain the diffusion of Fe and Cr and the reactions between the glasses and the alloy. Due to the ameliorated interfacial microstructure, sealing performance can be improved by applying the Mn–Co spinel coating on SUS430 alloy, especially for the bonded couple with high–SrO glass.

Modeling radiation induced segregation in iron–chromium alloys

Radiation induced segregation in ferritic Fe–Cr alloys is studied by Atomistic Kinetic Monte Carlo simulations that include diffusion of chemical species by vacancy and interstitial migration, recombination, and elimination at sinks. The parameters of the diffusion model are fitted to DFT calculations. Transport coefficients that control the coupling between diffusion of defects and chemical species are measured in dilute and concentrated alloys. Radiation induced segregation near grain boundaries is directly simulated with this model. We find that the diffusion of vacancies toward sinks leads to a Cr depletion. Meanwhile, the diffusion of self-interstitials causes an enrichment of Cr in the vicinity of sinks. For concentrations lower than 15%Cr, we predict that sinks will be enriched with Cr for temperatures lower than a threshold. When the temperature is above this threshold value, the sinks will be depleted in Cr. These results are compared to previous experimental studies and models. Cases of radiation induced precipitation and radiation accelerated precipitation are considered.

New observations of electron domains in Fe–Cr–Ni–Mo duplex alloy

Electron domains were discovered in body-centred-cubic (bcc) ferritic Fe–Cr alloys in the 1990s for the first time. In this paper, it is reported that electron domains may also exist in the face-centred-cubic (fcc) austenite phase, and they can cross the boundary between the bcc ferrite and the fcc austenite.

Effects of Grit Blasting and Annealing on the High-Temperature Oxidation Behavior of Austenitic and Ferritic Fe-Cr Alloys

Grit blasting (corundum) of an austenitic AISI 304 stainless steel (18Cr-8Ni) and of a low-alloy SA213 T22 ferritic steel (2.25Cr-1Mo) followed by annealing in argon resulted in enhanced outward diffusion of Cr, Mn, and Fe. Whereas 3 bar of blasting pressure allowed to grow more Cr2O3 and MnxCr3−xO4 spinel-rich scales, higher pressures gave rise to Fe2O3-enriched layers and were therefore disregarded. The effect of annealing pre-oxidation treatment on the isothermal oxidation resistance was subsequently evaluated for 48 h for both steels and the results were compared with their polished counterparts. The change of oxidation kinetics of the pre-oxidized 18Cr-8Ni samples at 850 °C was ascribed to the growth of a duplex Cr2O3/MnxCr3−xO4 scale that remained adherent to the substrate. Such a positive effect was less marked when considering the oxidation kinetics of the 2.25Cr-1Mo steel but a more compact and thinner FexCr3−xO4 subscale grew at 650 °C compared to that of the polished samples. It appeared that the beneficial effect is very sensitive to the experimental blasting conditions. The input of Raman micro-spectroscopy was shown to be of ground importance in the precise identification of multiple oxide phases grown under the different conditions investigated in this study.

Oxidation Behavior of Fe-Cr Ferritic Alloy for SOFC Interconnects ZMG232G10 in Air and H2/H2O

ZMGTM232G10 is an improved Fe-Cr ferritic alloy for SOFC interconnects which has better oxidation resistance than ZMG232L. Oxidation behavior of ZMG232G10 was investigated in air as well as H2/H2O atmosphere at 850 oC. ZMG232G10 showed almost same oxidation weight gain in both atmospheres. Oxidation behavior of some commercial Fe-Cr ferritic stainless steels was also investigated. 430 type stainless steel indicated a large amount of oxidation weight gain in air although it indicated a small amount of oxidation weight gain in H2/H2O. Oxide layers formed on the surface of ZMG232G10 were different from those of commercial stainless steels.

Effect of pre-oxidation on the oxidation resistance of spinel-coated Fe–Cr ferritic alloy for solid oxide fuel cell applications

Low-temperature-sintered MnCo2O4–MnO2 coatings have been prepared on pre-oxidized SUS430 ferritic alloy by slurry coating. The effect of pre-oxidation treatment before slurry coating is then investigated. Microstructural and electrical characterizations show that 25 h of pre-oxidation at 800 °C could significantly improve the oxidation resistance of the coated alloy and effectively inhibit the increase in area-specific electrical resistance during long-term oxidation. These effects can be explained by the interfacial reactions between the coating and the pre-oxidized alloy during sintering and oxidation tests. Furthermore, this study suggests that the dense reaction layer at the coating-alloy interface could be the key to improving the oxidation resistance of metallic interconnects with low-temperature-sintered spinel coatings.

Radiation-induced strengthening and absorption of dislocation loops in ferritic Fe–Cr alloys: the role of Cr segregation

The understanding of radiation-induced strengthening in ferritic FeCr-based steels remains an essential issue in the assessment of materials for fusion and fission reactors. Both early and recent experimental works on Fe–Cr alloys reveal Cr segregation on radiation-induced nanostructural features (mainly dislocation loops), whose impact on the modification of the mechanical response of the material might be key for explaining quantitatively the radiation-induced strengthening in these alloys. In this work, we use molecular dynamics to study systematically the interaction of dislocations with 1/2〈111〉 and 〈100〉 loops in all possible orientations, both enriched by Cr atoms and undecorated, for different temperatures, loop sizes and dislocation velocities. The configurations of the enriched loops have been obtained using a non-rigid lattice Monte Carlo method. The study reveals that Cr segregation influences the interaction mechanisms with both 1/2〈111〉 and 〈100〉 loops. The overall effect of Cr enrichment is to penalize the mobility of intrinsically glissile 1/2〈111〉 loops, modifying the reaction mechanisms as a result. The following three most important effects associated with Cr enrichment have been revealed: (i) absence of dynamic drag; (ii) suppression of complete absorption; (iii) enhanced strength of small dislocation loops (2 nm and smaller). Overall the effect of the Cr enrichment is therefore to increase the unpinning stress, so experimentally ‘invisible’ nanostructural features may also contribute to radiation-induced strengthening. The reasons for the modification of the mechanisms are explained and the impact of the loading conditions is discussed.

Development of low-temperature sintered Mn–Co spinel coatings on Fe–Cr ferritic alloys for solid oxide fuel cell interconnect applications

To reduce the sintering temperature of spinel coatings to a safe temperature for Fe–Cr ferritic alloys, an MnCo2O4–MnO2 bicomponent coating has been prepared on SUS430 ferritic alloy by slurry coating, followed by air sintering at a temperature of 800 °C. The resultant coating is stable during cyclic oxidation tests and is effective at improving the oxidation resistance of SUS430 alloy. The area-specific resistance of coated samples remains nearly constant during long-term oxidation at 800 °C and is less than 6 mΩ cm2 even after oxidation for 1000 h. These results indicate that the low-temperature sintered MnCo2O4–MnO2 coating is a promising protective material for Fe–Cr ferrite alloys, especially SUS430, which are seriously oxidized at temperatures ≥850 °C.

Cr segregation on dislocation loops enhances hardening in ferritic Fe–Cr alloys

The effect of chromium on iron hardening via segregation on dislocation loops was studied by atomic scale computer modeling. A combination of Monte Carlo and molecular dynamics techniques together with the recently determined Fe–Cr interatomic potentials fitted to ab initio data was used to investigate Cr segregation on ½〈111〉 interstitial dislocation loops and its impact on the interaction with moving dislocations. The Monte Carlo results reveal that Cr atoms segregate to the loop tensile strain region and dissolve well above the temperature corresponding to the solubility limit. The molecular dynamics results demonstrated that local micro-chemical changes near the loop reduce its mobility and increase the strength. The stress to move a dislocation through the array of Cr “decorated” loops increases due to modification of the dislocation–loop interaction mechanism. A possible explanation for a number of experimental observations being dependent on the radiation dose and for Cr concentration effects on the yield stress is given on the basis of the modeling results.