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![Composition maps of alumina scale formation as a function of Al and Nb level in (a) air and (b) air with 10 pct water vapor. Compositions above the boundary lines (estimated) exhibit high oxidation resistance after 2000 to 5000 h exposure (20 to 50 9 100 h cyclic oxidation testing) at those conditions due to protective alumina scale formation. [10]](profile/P-Maziasz/publication/225985163/figure/fig5/AS:393734668734504@1470885068384/Composition-maps-of-alumina-scale-formation-as-a-function-of-Al-and-Nb-level-in-a-air.png)
Composition maps of alumina scale formation as a function of Al and Nb level in (a) air and (b) air with 10 pct water vapor. Compositions above the boundary lines (estimated) exhibit high oxidation resistance after 2000 to 5000 h exposure (20 to 50 9 100 h cyclic oxidation testing) at those conditions due to protective alumina scale formation. [10]
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A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use
in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture
lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the...
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Context 1
... unexpected finding in AFA alloys is that oxidation resistance correlates not simply with the level of Al and Cr additions, but also with the level of Nb and Ni additions. Figure 6 shows plots of the Al and Nb contents in AFA alloys relative to alumina scale formation. [10,14] The alloys above the boundary lines exhibit external, protective alumina scale formation (based on 20 to 50 9 100 hour cyclic oxidation testing, total ~2000 to 5000 hours exposure, scale thickness typically on the order of a micron or less) at designated conditions. ...
Context 2
... the lines, the alloys show internal oxidation of Al with Fe-and Cr-rich oxide nodule formation after a certain number of cycles of oxidation test. The relative amounts of Al and Nb needed for the external alumina scale formation in air increase moder- ately from 1073 K to 1173 K (800 °C to 900 °C) ( Figure 6(a)). Increasing the Ni level from 20 to 25 to 26 wt pct significantly decreases the amount of Nb needed for alumina scale formation at 1173 K (900 °C) in air, indicating that the Ni addition is also a key to improving oxidation resistance. ...
Context 3
... the Ni level from 20 to 25 to 26 wt pct significantly decreases the amount of Nb needed for alumina scale formation at 1173 K (900 °C) in air, indicating that the Ni addition is also a key to improving oxidation resistance. In air with 10 vol pct water vapor at 1073 K (800 °C), on the other hand, much higher Nb (2.5 to 3 wt pct) rather than high Al is needed for the protective alumina scale formation (Figure 6(b)). Resistance in oxidizing environments containing high concentrations of water vapor is a key issue not only for fossil-fired steam plants, but also for applications ranging from combustion environments in gas turbines and engines to chemical/petrochemical processing to solid oxide fuel cell heat exchangers. ...
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Citations
... wt%) [23][24][25], whose yield strength reaches 580 MPa at 650 • C [26][27][28]. However, the γ′-Ni 3 (Ti, Al) phase formed in A286 is metastable, which tends to transform into η-Ni 3 Ti phase above 730 • C [29,30]. The alloy also relies on chromia-scale for oxidation resistance [21]. ...
Low cost stainless steels (SS) with high performance, especially with good mechanical properties and oxidation resistance at high temperature, are continuous pursuit for manufacture industry. To achieve good combination of mechanical properties, oxidation resistance and cost, nanoprecipitate strengthened alumina forming austenitic (NSAFA) SS have been developed by removing expensive elements such as Nb and Ta, lowering Ni concentration in Fe-26Ni-15Cr-1.5Mo-(2.8-3.5)Al-(1-2.35)Ti (wt.%) alloys. Microstructure investigation demonstrates that NSAFA SS are strengthened by L12 structured (Ni, Fe)3(Al, Ti) γ′ nanoprecipitates with volume fraction around 25%-40%. Oxidation experiments (750 o C/480h) in air demonstrates that all the NSAFA steels have alumina forming ability. In addition, it is found that Al and Ti are all necessary for γ′ nanoprecipitates, but additional Al and Ti facilitate formation of the Ni2AlTi phase. To ensure high volume fraction of γ′ nanoprecipitates and reduce Ni2AlTi phase, 2.8-3.0 wt.% Al and 1.8-2.35 wt.% Ti are preferred. Benefit from the high volume fraction of γ′ nanoprecipitates, yield strength of NSAFA steels is much higher than that of Fe-base superalloy (e.g., A286) and can compare to some Ni-base superalloys (e.g., Hayners 282 & Inconel 740H).
... Its high-temperature and high-carbon working environment places strict requirements on the oxidation resistance and carburization resistance requirements for equipment materials [1,2]. Stainless steel and Fe-Ni-Cr centrifugal casting alloys are commonly used, with their oxidation resistance effectively enhanced by the rapid formation of a surface Cr 2 O 3 oxygen barrier layer [3], which obstructs the flow of oxygen into the substrate, and the formation of a low-oxygen site at the oxide-metal interface, thus reducing the solubility of O [4,5]. If the working temperature of the material is excessively high, Cr 2 O 3 will continue to oxidize to form volatile CrO 3 [6,7]. ...
... steels with 1.5 wt.% and 2.5 wt.% Nb additions exhibited excellent oxidation resistance in air at 1173 K [21,22]. This was attributed to the fact that Nb promotes the formation of an AlNi phase and the continuous formation of Al 2 O 3 layers [3,13,16]. If more Nb is added to the alloy, the formation of Nb 2 O 5 in the oxide layer is detrimental to the oxidation resistance of the alloy [23]. ...
To investigate the impact of Al and Nb elements on the formation of a protective oxide layer on the surface of Fe-35Ni-20Cr-xAl-yNb (x = 0, 2, 4, 6 wt.%; y = 0, 1, 2 wt.%) alloys, their oxidation behavior was examined at 1000 °C, 10−17 atm. and 10−25 atm. oxygen pressure, and the oxidation mechanism was analyzed by Factsage and Pandat calculations. Enhancing the Al content at 10−17 atm. inhibited the generation of FeCr2O4 on the alloy surface and increased the Al content in the M2O3 layer. When the Al content exceeded 6 wt.%, the oxide film partially peeled off. It was found that the addition of Nb increased the activity of Cr and Al and decreased the activity of Ni and Fe and promoted the formation of Al2O3, and the appearance of Nb2O5 in the subsurface layer increased the density of the oxide film. In addition, under an oxygen pressure of 10−25 atm., the only protective layer on the surface of the alloy comprised of Al2O3. The experimental results demonstrated that the Fe-35Ni-20Cr-4Al-2Nb alloy generated a continuous and dense Al2O3 protective film, and the reduction in oxygen pressure and the addition of Nb elements were favorable for selective external oxidation of Al2O3.
... Jozaghi et al. [16] visualized the third-element effect and emphasized that no such effect was observed at overly low Al and Cr contents. An Al content of 2.5 wt% is sufficient for forming a passive oxide with at least 16 wt% Cr upon exposure to high-temperature steam [21], which explains why the AFA alloys prepared by Nie (14Cr-3Al) [8] and Sun (15Cr-5Al) [7] do not form a distinct alumina scale when exposed to SCW. If Si [22], Nb [12,23], or Cu [24] are introduced into AFA alloys, they can also supply the third-element effect (TEE) and, thus, facilitate alumina scale formation. ...
... The appearance of the δ-ferrite in the 3.5Al alloy might be explained by the Schaeffler diagrams [37], as shown in Fig. 1i, which presents the Cr equivalent (Cr eq wt% = Cr + Mo + 1.5Nb + 3Al) against Ni equivalent (Ni eq wt% = Ni + 0.87Mn + 30 C) values of the examined alloys. Notably, with the increasing Al content, the increased Cr eq favors the δ-ferrite formation [21,40]. According to the prediction in Fig. 1i, δ-ferrite would be formed only in AFA alloys with an Al content higher than 3.0 wt%, which agrees Fig. 1j shows that the average grain size was close to 56 μm for alloys with the Al content below 3.5 wt% and dropped to about 30 μm for the 3.5Al alloy, suggesting the occurrence of grain refinement with high Al content. ...
... Yamamoto et al. suggested that a minimum Al content of 2.4 wt% was required to maintain the formation of a continuous protective alumina scale [6,21]. Brady et al. reported that 2.5 wt% Al and 16 wt% Cr were necessary [64], as Cr can serve as the third element to promote the formation of Al 2 O 3 [65]. ...
... However, the performance of these alloys relies on the formation of a protective surface layer composed of Cr 2 O 3 , which starts to degrade in harsh conditions at temperatures above 650 • C [2,3] or in the presence of water vapor [4]. In recent years, a novel type of stainless steel known as alumina -forming austenitic (AFA) steel, containing Al, has been developed at Oak Ridge National Laboratory (ORNL) [5][6][7][8][9][10]. AFA steel demonstrates exceptional oxidation resistance in comparison to conventional austenitic steels due to the addition of an appropriate amount of Al, which facilitates the formation of an effective Al 2 O 3 film [8,11]. ...
... In recent years, a novel type of stainless steel known as alumina -forming austenitic (AFA) steel, containing Al, has been developed at Oak Ridge National Laboratory (ORNL) [5][6][7][8][9][10]. AFA steel demonstrates exceptional oxidation resistance in comparison to conventional austenitic steels due to the addition of an appropriate amount of Al, which facilitates the formation of an effective Al 2 O 3 film [8,11]. Such a feature renders AFA steel highly attractive for applications requiring both high creep strength and excellent oxidation resistance, including superheater / reheater tubes. ...
... The significant improvement in creep strength of Fe-20Cr-30Ni-2Nb steel through B addition [25,26] is primarily attributed to its influence on the distribution of the Fe2Nb-type Laves phase [17,18,25], which dominates the precipitation behavior at high temperatures. However, AFA alloys exhibit various precipitates [8], making the impact of B on precipitation more intricate. Although limited studies have reported on the influence of B on precipitation behavior and tensile strength of AFA alloys [27], there lacks related research regarding the preferential distribution behavior of B and its influence on creep performance. ...
... Moreover, the inclusion of Al and Si helps form a continuous protective oxidation layer, which improves the corrosion resistance ability at high temperature [18][19][20]. However, there is evidence that a certain amount of alloying is beneficial due to the synergetic effects of the alloy elements [3,9,11,17,[21][22][23]. ...
... However, with the increase of Al, the corrosion potential decreased and the corrosion current density increased, indicating a slight deterioration in corrosion resistance. Generally, Al is profitable to form a stable and dense oxide film on the steel surface, leading to enhanced corrosion resistance [19,21]. Nonetheless, contrary to expectations, the experiment results obtained the opposite conclusion. ...
The effects of alloy elements (Cr, Mn, Cu, Si, Al, Ni, and Ce) on the anti-corrosion behavior and soft magnetic properties of ferritic stainless steel were studied. The results show that the increasing content of Cr, Mn, Cu, and Si ameliorate anti-corrosion behavior, and among the alloy elements, Cu is the most effective to enhance corrosion resistivity. Conversely, the anti-corrosion performance deteriorates with increasing Al and Ce content, despite still maintaining good corrosion resistance overall. Furthermore, in regard to the soft magnetic properties, the regulatory mechanism is more complex and the addition of Cu, Si, and Ce can improve the soft magnetic properties, while their content should be limited. The augmentation of Cr deteriorates the soft magnetic properties, while the increase of Al proves advantageous. Meanwhile, Ni is effective to improve the soft magnetic properties; however, it does not significantly enhance the corrosion resistance, and considering its cost, it is not a necessary element to add. With the optimal composition, the alloy exhibits superior anti-corrosion behavior and softer magnetic properties compared to the commercial grade steel, which can contribute to expanding the applications of ferritic stainless steel.
... Alumina-forming austenitic (AFA) steel, developed by Oak Ridge National Laboratory (ORNL), is a kind of advanced stainless steel with excellent oxidation properties and high temperature creep resistance [1][2][3][4][5]. Protective Al 2 O 3 oxide film is formed on the surface of AFA alloys at 873-1123 K [4], making it more stable in harsh environment such as water vapor compared with traditional stainless steels with Cr 2 O 3 layer (such as Super304H and TP347HF) [6]. ...
... Alumina-forming austenitic (AFA) steel, developed by Oak Ridge National Laboratory (ORNL), is a kind of advanced stainless steel with excellent oxidation properties and high temperature creep resistance [1][2][3][4][5]. Protective Al 2 O 3 oxide film is formed on the surface of AFA alloys at 873-1123 K [4], making it more stable in harsh environment such as water vapor compared with traditional stainless steels with Cr 2 O 3 layer (such as Super304H and TP347HF) [6]. AFA alloy can be widely used in ultrasupercritical steamplants, chemical plants and other industries [7], and is also expected to serve as an advanced reactor cladding material [8]. ...
... For (9-12)% Cr martensitic steels, boron is enriched at GBs to form M 23 (CB) 6 carbides, which can effectively reduce the coarsening of M 23 C 6 [15,16]. However, M 23 C 6 is not precipitated for most AFA alloys due to the stabilization of Nb or Ti [4]. Korean researchers used W to replace Mo to form a slowly coarsened Laves phase in AFA. ...
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.
... In fact, the alumina content is the key component for the formation of the Al-rich oxide layer. The Nb content is as important as the Al element for α-Al 2 O 3 [15]. Previous studies have shown that higher Nb content than Al is required for the protective layer [15,16]. ...
... The Nb content is as important as the Al element for α-Al 2 O 3 [15]. Previous studies have shown that higher Nb content than Al is required for the protective layer [15,16]. Yamamoto [15] confirmed that the increased Nb content (between 0.6 and 1 wt.%) was suspected to be a key factor for the formation of the external oxide layer. ...
... Previous studies have shown that higher Nb content than Al is required for the protective layer [15,16]. Yamamoto [15] confirmed that the increased Nb content (between 0.6 and 1 wt.%) was suspected to be a key factor for the formation of the external oxide layer. In addition, AFA-Nb steel exhibited improved oxidation resistance after oxidation at 1173 K in air, which was attributed to the formation of the B2-NiAl phase by Nb addition from the studies of Brady [17]. ...
The corrosion behavior of alumina-forming austenitic (AFA) stainless steels with different Nb additions in a supercritical carbon dioxide environment at 500 °C, 600 °C, and 20 MPa was investigated. The steels with low Nb content were found to have a novel structure with a double oxide as an outer Cr2O3 oxide film and an inner Al2O3 oxide layer with discontinuous Fe-rich spinels on the outer surface and a transition layer consisting of Cr spinels and γ’-Ni3Al phases randomly distributed under the oxide layer. Oxidation resistance was improved by accelerating diffusion through refined grain boundaries after the addition of 0.6 wt.% Nb. However, the corrosion resistance decreased significantly at higher Nb content due to the formation of continuous thick outer Fe-rich nodules on the surface and an internal oxide zone, and Fe2(Mo, Nb) laves phases were also detected, which prevented the outward diffusion of Al ions and promoted the formation of cracks within the oxide layer, resulting in unfavorable effects on oxidation. After exposure at 500 °C, fewer spinels and thinner oxide scales were found. The specific mechanism was discussed.
... M(C, N) (M = Nb, V, Ti or Ta) has a fcc structure and a coherent relationship with the austenitic matrix (γ) (Zhao et al., 2018). The nano-scale M(C,N) precipitated during aging is mainly distributed in the grain and not easy to coarsening, which is considered to be one of the main sources of creep resistance of advanced stainless steels (Sourmail, 2001;Yamamoto et al., 2011). The primary Nb(CN) at grain boundaries is not stable (Ecob et al., 1987) and is usually transformed into a nickel-niobium-silicide phase (Nb 6 Ni 16 Si 7 , G phase) after aging. ...
In this paper, the performance degradation behavior of fuel cladding material 20Cr25NiNb for the British Advanced Gas Reactor (AGR) is reviewed in detail, which is a strong guideline for the material selection of supercritical carbon dioxide cooled reactors. The degradation behavior during in-core service mainly includes high-temperature creep, thermal aging and mechanical property degradation caused by neutron irradiation (fission gas products, helium embrittlement and irradiation sensitization) and CO2 (oxidation and carburizing). Long-term service in AGR leads to coarsening of the second phase and precipitation of harmful phases such as σ, leading to performance degradation of the cladding. A point that should require special attention is that intergranular stress corrosion cracking (IGSCC) and intergranular attack (IGA) problems occur during wet storage of spent fuel.
... The strengthening through fine precipitates like carbides and intermetallics (IM), such as Fe 2 M type Laves-phases, is a well proven approach for iron-based heat resistant austenites. The addition of aluminum improves the oxidation resistance by forming a stable Al 2 O 3 protective layer [7][8][9][10] . Alloys combining these two approaches are known as alumina-forming austenitic (AFA) steels and have compositions with wt.% ranges of Fe , Ni , Cr (12)(13)(14)(15)(16)(17)(18)(19), Al (2.5-4), Nb (0.6-3) together with minor additions of C, Ti, Si, or B [11][12][13] . ...
... AFA Steels matrixes have typically Al solubilities ≤ 2 wt.%. Further Al would lead to B2 formation together with Ni [9] . Besides promoting Al oxide layer formation, higher Al content also reduces the alloys density. ...
... According to the work of Ma et al. [62] , HEAs from the Al-Co-Cr-Fe-Ni system composed of an FCC matrix and ∼15% volume fraction of NiAl B2 precipitates would present differences from compressive to tensile yield strength of around 40 MPa. Considering this, A4Z3 and A9Z2, can be assumed as comparable to AFA steels which typically range from 300 to 800 MPa tensile strength at RT, depending on composition and treatment condition [9] . Highly thermomechanically processed HEAs of similar composition have reported yield strength of 860 MPa [14] and some AFA steels ≥ 1000 MPa [12] . ...
Laser powder bed fusion (LPBF) was used to manufacture two high entropy alloys (HEAs) within the Al-Co-Cr-Fe-Ni-Zr system. The selected compositional ranges were similar to alumina forming austenitic (AFA) steels but omitting interstitials and replacing Nb with Zr as the Laves forming element. The Fe-rich austenitic HEAs were prepared with varying Al and Zr content to evaluate the influence on the presence, size, and distribution of the intermetallic (IM) precipitates. The LPBF process combined with a single (950°C 6h) heat treatment formed large, elongated grains with a fine dispersion of multiple different nano-sized IM phases. Synchrotron high energy x-ray diffraction (HEXRD) revealed the cubic M23Zr6 as the main Zr-rich IM phase, stabilized by the multi-element mixture (M=Co, Fe, Ni) and the high colling rates of the LPBF process. Further HEXRD in-situ compression was performed from room temperature to 900°C to evaluate the phase stability, thermal expansion, and the strength contribution of the austenitic matrix and the M23Zr6 and NiAl B2 IM phases. The evolution of lattice strain and full width at half maximum of the reflexes was tracked using a statistical model, enabling quantitative analysis along the deformation.
... In addition, Al shows a higher diffusion rate compared with Ni and Cr in both γ and γ′ below 900 °C, indicating a fast Al outward diffusion occurring in the early stages, which favors Al-rich oxides formation. On the other hand, the B2 phase ( NiAl) [104] and γ′ (Ni3Al) [105] are considered Al reservoirs and benefit the formation of Al2O3. Zhai et al [97] revealed that the capable to increase spallation resistance, where a smooth oxide layer was observed; however, surface heat treatment benefits oxide growth and leads to severe spallation when q is 50J/mm. ...
Additive manufacturing (AM) fabricated oxide dispersion strengthened (ODS) alloys are given high expectations for critical structural components such as the first stage turbine blade for their excellent creep strength and oxidation resistance compared to superalloys. However, the powder feedstock processing is still an open question since current state-of-the-art processes are not capable of achieving ultrafine strengthening elements such as Y2O3 in powder which leads to agglomeration issues in as-consolidated alloys. In this research, the oxidation behavior and stability of ultrafine oxide in AM-printed alloys using mechanically alloyed powders were evaluated at 1100 oC. In addition, a novel powder processing method was developed based on the mechanism of strengthening element (i.e.Y2O3) evolution in powder precursor and as-consolidated alloys with the benefit of grain refinement and improved mechanical property and oxidation resistance.
In this study, it was found that 1) the size of Y2O3 in powder precursor is of importance to achieving ultrafine Y-rich nanoparticles in as-consolidated alloys, both plate-like and spherical nanoparticles formed using mechanically alloyed powder due to non-uniform convection of molten pool, 2) nanoparticles stabilized under the protection of Al2O3 at 1100 oC and spallation of oxide layer occurs in oxide mixture formed in early oxidation stages, 3) high energy mixing of gas atomized alloy powder and Y2O3 can generate ultrafine nanoparticles in powder and as-printed alloys attributed to the functional composite layer formation with surface deformation in metallic powder, 4) ultrafine Y2O3 and coarser Y-Al-rich particles (<10 nm) were observed in AM-printed IN718, 5) exceptional strength and ductility of AM-printed ODS IN718 alloy are attributed to fine structure and oxides strengthening effects, 6) adding 1 wt.% Y2O3 can effectively refine the sub-grain of SS316L alloy with improved Vickers hardness (HV1) from 177 to 240.
The output of this research demonstrates the mechanism of Y2O3 evolution in powder and AM process, and provides one way to cost-effective fabricate ultrafine oxide dispersed alloys with fine grain structure and improved mechanical properties.
