Alumina-forming Austenitic Alloys Research Articles

Influence of liquid lead and lead-bismuth eutectic on three alumina forming austenitic (AFA) steels through slow strain rate testing

Liquid metal embrittlement (LME) in three newly developed alumina-forming austenitic (AFA) alloys, two 50 kg batches and one 5-ton heat, was studied in the temperature range 350–600 °C in liquid Pb and 140–600 °C in LBE using slow strain rate testing (SSRT) in a low-oxygen environment. No significant decrease in the engineering strain was observed in either environment. However, the presence of secondary cracks along the length of the specimen and brittle intergranular areas on the fracture surfaces indicates that the AFA alloys do show a minor degree of embrittlement above 570 °C. This appears to be related to grain boundary wetting by Pb/LBE. At temperatures below 570 °C, this wetting effect does not seem to be strong enough to induce LME in the alloys, and their ability to form a sufficiently protective oxide means that they remain unaffected by LME. The results indicate that the AFA alloy group can perform sufficiently well in liquid Pb/LBE environments, and long-term testing should be carried out to determine their viability as candidate materials for use in Pb- and LBE-based cooling systems.

Effect of Ta on selective oxidation behavior and oxide film adhesion of alumina-forming austenitic alloys

The effects of Ta on the selective oxidation behavior and scale adhesion of Fe-45Ni-25Cr-4Al-1.5Nb-0.4C-xTa (x=0.6, 0.9, 1.2, 1.5 wt%) austenitic alloys at high temperature under low oxygen pressure were investigated. The FCC, MC-(Ta, Nb)C, M23C6 and B2-NiAl phases were found in the annealed alloy. Addition of Ta significantly promoted the formation of an external Al2O3 layer and inhibited growth of Cr2O3 and spinel at 850 °C under oxygen pressures of 10−18 atm and 10−24 atm. Similar oxidation behavior was observed in two-step oxidation process (650 °C for 8 hours and 1000 °C for 16 hours). Ta and Nb preferentially segregated at the interface between the oxide scale-matrix as well as grain boundary within the oxide scale during oxidation, and inhibited outward diffusion of Al and Cr. A large oxygen activity gradient was formed in the oxide film, resulting in the formation of columnar grains. Due to a significant decrease in oxygen activity at the front of the reaction, selective oxidation of alumina was promoted. Furthermore, the scratch test technique was used to quantify the adhesion of oxide scales. Peg-like structures formed by Ta and Nb positively influenced the adhesion properties of the oxide scale.

Strength stability at high temperatures for additively manufactured alumina forming austenitic alloy

Several fast-spectrum nuclear reactors designed to generate high power (∼450 MWe) rely on forced convection of media such as supercritical CO2, sodium, or liquid lead to cool the nuclear core, operating at temperatures up to 600 °C. Cost-effective, high-strength Fe-based alumina forming austenitic (AFA) alloys are a promising candidate for the fabrication of critical nuclear components. This study investigated laser powder bed fusion (LPBF) processing of an AFA alloy composition optimized for improved creep resistance. Electron microscopy revealed an elongated grain structure along the build direction with a fine sub-grain cellular structure decorated with (Cr,Fe,Nb)23C6 carbide precipitates at the intercellular boundaries. At temperatures of 20–900 °C, the LPBF alloy's superior tensile properties compared to its arc-melted counterpart and other advanced steels (e.g., SS316) were attributed to the distribution of nano-sized carbide precipitates, whereas the high ductility was attributed to the LPBF alloy's elongated grain structure.

Creep deformation and damage behavior of an alumina-forming austenitic alloy strengthened by intermetallic compounds

Based on Fe–30Ni–15Cr, a new alumina-forming austenitic (AFA) alloy, free of carbon, was designed for potential use in ultra-supercritical generator set. The creep behavior of alloy has been investigated by analyzing the creep constitutive equation, creep damage tolerance factor (λ), microstructure images, and oxide layer structure under different stresses at 750 °C. The creep microstructure and oxide morphology were obtained by electron microscopy. The minimum creep rate (ε˙m) is positively correlated with the stress, and when the stress increases from 120 MPa to 250 MPa, ε˙m increases from 6.26E-3 1/h to 6.34E-4 1/h, almost ten times of the latter. The creep constitutive equation at 750 °C was determined as: ε˙m=9.29×10−11σ3.3. B2–NiAl and L12-Ni3Al were precipitated during creep. The relationship between the misfit of nanoscale L12-Ni3Al phase and γ matrix are perfectly coherent. The L12-Ni3Al phase grows more slowly and remains at the nanoscale size after creep at 750 °C for 869h, which contribute to increase creep strength and reduce the creep rate of the material. However, the coarsened σ-FeCr phase reduced the creep productivity of the alloy. The microstructural degradation caused by precipitate coarsening was found to be responsible for creep failure, as indicated by the creep damage tolerance factor (λ) and creep fracture morphology. The elemental distribution of the oxide layer indicates the absence of a dense alumina protective scale at 750 °C. The extensive presence of oxidation cracks in the matrix suggests that the effect of oxidation cannot be ignored in the analysis of creep life. The fact that σ-FeCr is detrimental to creep damage indicates that the alloy composition needs further optimization.

Characterization of Microstructure and Tensile Properties of a New Alumina-Forming Austenitic Alloy at High Temperatures

Characterization of Microstructure and Tensile Properties of a New Alumina-Forming Austenitic Alloy at High Temperatures

The Impact of Oxidation-Induced Degradation on Materials Used in Hydrogen-Fired Microturbines

Abstract Hydrogen-fueled microturbines are being considered as part of the future green microgrid. However, the use of hydrogen as a fuel presents new challenges for selection and development of suitable high temperature materials for hydrogen combustion. The burning of hydrogen is expected to result in higher operating temperatures and higher than typically observed water vapor contents in exhaust gases versus burning natural gas. In this work, foil specimens of various Fe- and Ni-based alloys were oxidized in air + 10% H2O and air + 60% H2O for up to 5000 h at 700 °C to simulate the exhaust atmosphere of natural gas and hydrogen-fueled microturbines. The impact of alloy composition and water vapor content on the oxidation/volatilization induced loss of wall thickness was experimentally evaluated. Enhanced external oxidation and volatilization of Cr2O3 and Ti-doped Cr2O3 scales were observed in air + 60% H2O compared to air + 10% H2O. No significant impact of the higher water vapor content was observed on Al2O3 scales formed on Fe-based alumina-forming austenitic alloys. Lifetime modeling was employed to predict the combined effects of water vapor content, gas flow rates, temperature, and alloy composition on the oxidation-induced lifetime of the investigated materials.

Mechanisms for high creep resistance in alumina forming austenitic (AFA) alloys

Castable alumina forming austenitic (AFA) alloys have demonstrated superior creep life and oxidation resistance at temperatures exceeding 800⁰C. Despite the success in the applicability of these alloys in extreme environments, there is a limited understanding of the deformation modes and the influence of each alloying element guiding the alloy design strategies that could further enhance the creep strength of these AFA alloys, particularly at temperatures at and above 900⁰C. In this study, we reveal the mechanism underpinning the superior creep performance of castable AFA alloys that involves suppressing primary carbide formation through minor compositional modification. This approach results in a three-fold increase in creep strength at 900⁰C and 50 MPa. By employing integrated characterization techniques, we analyzed the microstructures of two AFA alloys, both before and after the creep process. We discovered that the suppression of primary carbides permits the in-situ clustering of now-available interstitial elements such as C, Si, and O during high-temperature creep. This improved solid solution strengthening and reduced stacking fault energy of the alloy. Moreover, it also enabled controlled secondary carbide formation during testing, further improving the creep resistance. These findings underline the important interplay between alloy composition, microstructure, and creep properties, and offer a promising design strategy for developing economical high-temperature Fe-based alloys suitable for advanced applications.

Precipitates evolution during isothermal aging and its effect on tensile properties for an AFA alloy containing W and B elements

A 20Cr-25Ni-2.5Al alumina-forming austenitic alloy containing W and B elements was aged at 923 K for 5000 h, and the microstructure and tensile properties with different aging time were investigated. NiAl, σ and Laves were observed at grain boundaries (GBs) successively. The matrix was covered by NiAl and Laves with the extension of aging. The evolution of precipitates during aging contributed to the variation of tensile properties. Precipitation of nanosized NbC carbides within grains and σ phase at GBs led to a rapid increase in strength and a decrease in elongation for 500 h aging sample. In the later stage of aging, the coarsening of NiAl and Laves phases, as well as the reduction in dislocation density caused a decline in strength. The coarsening of precipitates upon aging time follows the Ostwald ripening theory. Due to its lower diffusion rate in austenite compared to Mo, W may accelerate the growth of Laves at GBs. Boron was mainly enriched in Laves instead of NiAl, NbC and σ phases after high temperature aging. The addition of W and B improved the precipitation strengthening of Laves, increasing the high temperature tensile strength after long term thermal aging. The difference in tensile properties between room temperature and 923 K is due to the ductile–brittle transition of NiAl. No σ phase was observed within grains even after 5000 h aging and elemental chromium particles occurred around Laves due to boron hindering the growth of σ.

Challenges in computationally designing high temperature Fe-based austenitic alloys: Addressing the role of Ni additions

Alumina-forming austenitic (AFA) alloys are relatively inexpensive high performance materials which combine the creep resistance of low-cost austenitic alloys and the oxidation resistance of expensive alumina forming alloys. However, a fundamental understanding of the role of key alloying elements such as Ni, Cr, Al, Nb, Ti, V, B and C in the experimentally observed oxidation behavior of these alloys is still lacking. The present work is a first in a series of studies aiming to quantitatively describe the role of Ni in promoting or disrupting protective Al2O3 scale formation on AFA alloys. Ternary Fe-Al-xNi model alloys with three different Ni contents were isothermally exposed in an atmosphere with a low partial pressure of oxygen between 800–1000 °C for 24 h to evaluate the role of Ni in the observed internal oxidation behavior. Increasing Ni contents had no impact on the internal oxidation behavior of the alloys. The experimental and theoretical analyses in the present work suggested a negligible effect of the internal oxide precipitates on the inward diffusion of oxygen, typically expected in these systems, while simultaneously highlighting the barriers in the development of reliable models for computation-assisted design of these alloys.

Corrosion of alumina-forming austenitic alloys under the supercritical carbon dioxide-based venus atmospheric surface conditions

Several grades of alumina-forming austenitic (AFA) stainless steel (SS) alloys were probed after exposure to simulated sCO2-based 467 °C Venus surface atmospheric conditions using the NASA Glenn Extreme Environments Rig (GEER) for 30 days. AFA samples were characterized by gravimetric analysis, X-ray diffraction (XRD), and electron microscopy. All AFA samples formed multilayered scales in which the outer layer is mainly magnetite (Fe3O4) and nickel sulfides (Ni3S4, Ni4S3), and the inner layer is mainly chromia containing aluminum, (Cr,Al)2O3. The most resistant of the AFA alloys exhibited specific weight gain corrosion resistance similar to SS304, which formed an inner (Fe3−xCrxO4) spinel.

Investigation of isothermal continuous hot deformation behavior of the new as-cast alumina-forming austenitic alloy

Multi-pass isothermal continuous hot deformation experiments were designed for homogenization and recrystallization in the as-cast alumina-forming austenite (AFA) heat-resistant alloy. Flow curves and microstructure characterization were used to study the grain evolution with different deformation parameters in each pass. In particular, attention was paid to the dendrites and segregated phase. Under different hot deformation conditions, the alloys show significant work-hardening characteristics, which result from the strengthening effect of the primary segregated MC-type carbide. The increase in deformation temperature contributes significantly to dynamic recrystallization (DRX), and the main mechanism of DRX is discontinuous DRX (DDRX). The continuous DRX (CDRX) is less occurring. Static recrystallization (SRX) occurs during the isothermal holding time between deformation passes. SRX becomes more and more apparent with temperature and the holding time. The increase in the deformation passes leads to a gradual disappearance of the alloy dendrites and an increase in the recrystallization volume fraction. Meanwhile, the segregated phase gradually decomposes into particles and partially dissolves. However, the segregated phase particles eventually show band-like distribution. SRX grains around the segregated phase will exhibit similar orientations due to particle-stimulated nucleation (PSN) mechanisms. Until the grain size increases to the point that it can escape from particle restraint, it will transform to random orientation. The above study confirms that multi-pass isothermal continuous hot deformation can improve the as-cast microstructure and guide practical industrial production.

Role of Cr Content in Microstructure, Creep, and Oxidation Resistance of Alumina-Forming Austenitic Alloys at 850–900 °C

Creep-rupture properties and oxidation behavior of a series of alumina-forming austenitic (AFA) alloys with variations of Cr contents, based on Fe-(13.5-18)Cr-25Ni-4Al-1.5Nb-0.1C in weight percent, have been evaluated at 850–900 °C. The study investigates material responses in the properties and microstructure through compositional modifications in AFA alloys, targeting performance optimization of alloys under high-temperature, corrosive industrial environments. The creep-rupture life of the alloys at 850 °C and 30MPa monotonically decreased with increasing Cr content, which was correlated with changes in secondary phase volume fractions, such as the reduction in B2-NiAl + Laves-Fe2Nb and increase in Sigma-FeCr with Cr content. The oxidation test at 900 °C in a water-vapor containing environment revealed a range of Cr content from 13.9 to 15.7 wt.%, enabling the formation of stable, protective external alumina scale as well as preventing internal oxidation/nitridation for up to total 7000 h exposure. On the other hand, the alloys with >16.7 wt.% Cr formed Sigma precipitates, which caused a reduction in not only Cr but also Al in the austenite matrix, resulting in less oxidation resistance than other alloys. The findings will guide the further optimization of material performance in the AFA alloy series.

Creep Behavior and Phase Equilibria in Model Precipitate Strengthened Alumina-Forming Austenitic Alloys

Creep-rupture behavior and microstructural response in alumina-forming austenitic (AFA) alloys with two different precipitation strengthening mechanisms, “Laves-phase + M23C6 carbide” and “coherent L12 γ′-Ni3(Al,Ti),” were explored as “model” cases of multi-phase, multi-scale heat-resistant AFA alloys for 650–750°C use. These alloys will be used to guide and verify computational alloy design and life-prediction modeling under an on-going eXtremeMAT project through the Office of Fossil Energy and Carbon Management, US Department of Energy. Computational thermodynamics were used to design and predict the amounts of strengthening and deteriorating secondary phases at 750°C. Creep-rupture lives of the alloys tested at 750°C and 100 MPa were in a range of 4000–9000 h, and the microstructure at the gage/grip after creep-rupture testing was compared with isothermally aged alloys for 1500 h, as well as the calculated phases. Detailed microstructure characterization includes phase identification, volume fraction measurement, and compositional analysis, which were correlated with the creep-rupture properties. High-temperature oxidation resistance was also screened and compared with commercial, chromia-forming heat-resistant steels. These model alloys also provide the basis for further design and optimization of next generation AFA alloys with improved creep resistance.

Investigation of Isothermal Continuous Hot Deformation Behavior of the New As-Cast Alumina-Forming Austenitic Alloy

Investigation of Isothermal Continuous Hot Deformation Behavior of the New As-Cast Alumina-Forming Austenitic Alloy

Compatibility of Alumina-Forming Austenitic Steels in Static and Flowing Pb

Alumina-forming austenitic (AFA) steels were evaluated for the lead fast reactor application in static and flowing high-purity Pb. Two AFA composition ranges with different Ni, Mn, Al, Cr and Nb contents were targeted, and Zr foil was used in an attempt to getter O in some capsules. Based on two rounds of capsule testing at 500–800°C, two AFA compositions were selected for evaluation in flowing Pb with a peak temperature of 650°C in a thermal convection loop made of pre-oxidized FeCrAlMo tubing. After exposure for 1000 h, the mass losses for both AFA alloys were generally low, suggesting good compatibility under these conditions without preoxidation.

Development of Alumina-Forming Austenitic Alloys for Solid Oxide Fuel Cell Balance of Plant Components

Solid oxide fuel cells (SOFC) are of great interest for cleaner energy production. As the technology moves to more widespread commercial adoption, balance of plant (BOP) components such as heat exchangers and other structural components are becoming increasingly important [1]. These components operate at ~600-950°C in the presence of aggressive species such as water vapor. Currently used chromia-forming Fe-based stainless steels and Ni-based alloys suffer from accelerated oxidation under these operation conditions due to enhanced internal oxidation and Cr oxy-hydroxide volatility [1,2]. Further, the volatilization of Cr species from BOP alloys results in Cr poisoning of the SOFC stack and a concomitant reduction in performance [1]. The use of alumina-forming alloys in SOFC BOP applications is a promising approach to reducing Cr contamination of the SOFC stack [3]. Alumina scales offer 1 to 2 orders of magnitude lower oxide growth rate than chromia and are far more stable in water vapor. Most alumina-forming alloys also make use of Cr alloying additions to promote the formation of the protective alumina; however, studies suggest > 10x reduction in Cr release can be achieved despite the presence of Cr in these alumina-forming alloys [3].Alumina-forming austenitic (AFA) alloys are a new class of stainless steels that offer the potential for improved oxidation resistance via an alumina scale combined with good creep resistance due to their austenitic structure, in a lower cost Fe-based alloy formulation [4]. The alloy design compromises to achieve both creep resistance and alumina formation result in a loss of protective oxidation and a transition to internal oxidation of Al with increasing temperature, typically in the range of ~750-900°C for wrought 20-25Ni mass % based AFA alloys depending on composition [5]. (A 35Ni-based cast grade of AFA alloy capable protective alumina formation to 1100-1200°C is available but is not suitable for SOFC BOP components) [6].The goal of the present work was to develop a lower cost, 25 Ni mass % grade AFA alloy capable of long-term SOFC BOP use at ~800-950°C in water vapor containing environments. The composition range of interest was based on Fe-25Ni-(13-18)Cr-4Al-(1-2.5)Nb-(0.5-2)Mn-(0.15-0.5)Si-(0-2)W-(0-2)Mo-(0-1)Cu-(0-0.1)Ti-(0.1)V-(0.05-0.2)Zr-(0.05-0.2)Hf-(0-0.1)Y-(0.02-0.2)C-(0.005-0.015)B mass %. The effort focused on systematic variation of C, Cr, and Nb levels ± Hf, Y, and Zr, which impact oxidation resistance, manufacturability, and cost. Key findings to achieve oxidation resistance at ≥850-900°C in air with 10% H2O without loss of creep resistance included: Protective alumina formation was enhanced by use of Zr additions, rather than previously demonstrated, more costly additions of Hf and Y.Levels of Nb as low as 1.5 mass % were sufficient to achieve protective alumina formation, as compared to previously used 2.5 mass % Nb levels, which reduces cost and aids manufacturability.The Cr addition levels were critical to pushing protective alumina formation past 900°C, with the critical transition occurring between ~14.5 and 16 mass % Cr.Protective alumina formation was achieved with lower than expected C addition levels, as low as 0.02 to 0.03 mass % C, which aids manufacturability to thin product forms by reducing primary coarse NbC formation. Details of the alloy design strategy and oxidation behavior out to 10,000 h of exposure at 900-1000°C in air with 10% H2O will be presented. Insights for the oxidation mechanism relative to Cr evaporation behavior will also be discussed.References L. Zhou, J. H. Mason, W. Li, and X. Liu, Renewable and Sustainable Energy Reviews Volume 134, December 2020, 110320 S.R.J. Saunders, M. Monteiro, F. Rizzo, Progress in Materials Science, 53 (2008) 775-837.M. Stanislowski, E. Wessel, T. Markus, L. Singheiser, W.J. Quadakkers, Solid State Ionics 179 (2008) 2406–2415.Y. Yamamoto, M.P. Brady, M.L. Santella, H. Bei, P.J. Maziasz, B.A. Pint, Metallurgical and Materials Transactions A, 42 (2011) 922-931.M.P. Brady, K.A. Unocic, M.J. Lance, M.L. Santella, Y. Yamamoto, L.R. Walker, Oxidation of Metals, 75 (2011) 337-357M. P. Brady, G. Muralidharan, Y. Yamamoto, B.A. Pint, , Oxidation of Metals, 87 (1), pp. 1-10 (2017) Notice: This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Chromium evaporation and oxidation characteristics of alumina-forming austenitic stainless steels for balance of plant applications in solid oxide fuel cells

The chromium (Cr) evaporation behavior of several different types of iron (Fe)-based AFA alloys and benchmark Cr2O3-forming Fe-based 310 and Ni-based 625 alloys was investigated for 500 h exposures at 800 °C to 900 °C in air with 10% H2O. The Cr evaporation rates from alumina-forming austenitic (AFA) alloys were ~5 to 35 times lower than that of the Cr2O3-forming alloys depending on alloy and temperature. The Cr evaporation behavior was correlated with extensive characterization of the chemistry and microstructure of the oxide scales, which also revealed a degree of quartz tube Si contamination during the test. Long-term oxidation kinetics were also assessed at 800 to 1000 °C for up to 10,000 h in air with 10% H2O to provide further guidance for SOFC BOP component alloy selection.

Influence of Sulfur and Water Vapor on High-Temperature Oxidation Resistance of an Alumina-Forming Austenitic Alloy

The oxidation behavior of alumina-forming alloys was studied after oxidation treatments conducted at 900 °C for 24 h in air or in steam. Alloys with sulfur contents ranging from 1 to 82 ppm in wt% were used. The influence of trace sulfur as well as the presence of steam in the oxidizing atmosphere were investigated. Under oxidation conditions, higher sulfur contents led to higher mass gains and the trend was more pronounced in steam than in air. Oxides were identified by Raman spectroscopy: a thin and continuous α-Al2O3 layer was formed at the metal-oxide interface in all cases. The mass gain differences were caused by other oxides formed at the surface of the samples—mainly spinel and Cr2O3—and in the internal oxidation zone too— mainly α-Al2O3 and θ-Al2O3—indicating that the protectiveness of the alumina layer greatly depends on the sulfur content in the base material and the oxidizing atmosphere. In order to explain this phenomenon, oxide structures were analyzed at various scales using scanning electron microscopy, transmission electron microscopy and nanoscale secondary ion mass spectrometry. Sulfur was detected at metal-oxide interfaces and also in the alumina layer in regions enriched with chromium. In addition, we demonstrate that steam oxidation leads to finer alumina grains as compared to air oxidation. Finally, the relationship between oxidation conditions, nanoscaled structural features and oxidation kinetics is discussed.

Uncertainty Quantification of Machine Learning Predicted Creep Property of Alumina-Forming Austenitic Alloys

The development of machine learning (ML) approaches in materials science offers the opportunity to exploit existing engineering and developmental alloy datasets, such as Oak Ridge National Laboratory (ORNL)’s consistently measured creep-rupture dataset for alumina-forming austenitic (AFA) alloys, to accelerate their further development. As a first step toward achieving ML insights for improved alloy design, the potential sources of uncertainty and their impacts on ML output are examined. It is observed that the selection of algorithms and features as well as data sampling significantly affects the performance of ML models, either positively or negatively. The performance of various ML models in predicting the creep properties of AFA alloys is compared, with further evaluation by assessment of a small set of new developmental AFA alloys that were not part of the training dataset. The present study demonstrates that uncertainty quantification (UQ) is essential in materials science for evaluating the performance of ML algorithms with specifically selected feature sets and obtaining a comprehensive understanding of their limitations and the resultant capability of effective prediction in complex materials systems.

The Development of Alumina Forming Austenitic Alloy for Core Application in Advanced Nuclear Reactors

The development of materials for core components which can serve in high temperature corrosive environments for a long service time is crucial to realize high efficiency and high-burnup operation of advanced nuclear reactors. Alumina forming austenitic (AFA) alloy is a kind of promising materials with improved corrosion resistance as well as strength at elevated temperature. The progress on the composition design and characterization of AFA alloys are reviewed in this work for evaluation their potential applications in advanced nuclear reactors. AFA alloys without the addition of carbon have been fabricated. Microstructures were observed by SEM and TEM. Mechanical properties were measured at room temperature and high temperature.