Continuous Al2O3 Layer Research Articles

Effect of Al on oxidation behavior of AlxCrCuFeNi2 (x = 0.2, 0.4, 0.6) high entropy alloys

The effect of Al on the oxidation behavior of AlxCrCuFeNi2 (x = 0.2, 0.4, 0.6) high entropy alloys was investigated at 700 °C in water vapor. The oxidation kinetics of all the alloys followed parabolic behavior, with their oxidation rates decreasing with increasing Al concentration. Thick triplex oxide scales with discontinuous Al2O3 formed on the Al0.2CrCuFeNi2 alloy, while a continuous Al2O3 layer and thinner spinel oxide scale grew on Al0.4CrCuFeNi2 and Al0.6CrCuFeNi2 alloys. The early formation of continuous Al2O3 scale in the initial stage of oxidation contributes to the enhanced oxidation resistance of high Al content alloys.

Oxidation behaviors and compressive strength evolution of DD6 Ni-based single-crystal superalloy at 1100 ℃

In this work, the oxidation behaviors of DD6 Ni-based single-crystal superalloy and its compressive strength evolution after oxidation were investigated. The results showed that the thermal exposure time has a significant effect on the oxidation products, oxidation kinetics, Al elements diffusion and compressive strength evolution of DD6 superalloy. Firstly, the relationship between grain size change of (Ni0.9,Co0.1)O, the thickness of γ′-free zone and the exposure time was approximate power exponential law, while the relationship between weight change of DD6 superalloy and exposure time was approximate parabolic law since the densification of inner continuous Al2O3 layers hindered the diffusion of oxygen ions. Moreover, the black phases of AlN in the γ′-free zone formed during the oxidation process, and its quantity and size increased first and then decreased with the oxidation time extending, since the inner Al2O3 not only blocked the ion but also reacted with AlN. In addition, with the increase of oxidation time, the compressive yield strength of DD6 in [001] direction first decreased rapidly and then gradually stabilized at about 740 MPa.

Effect of Chromium Content on the Oxidation Behavior of a Ta Stabilized γ'-Strengthened Polycrystalline Co-30Ni-10Al-4W-4Ti-2Ta Alloy.

The high-temperature oxidation behaviors of polycrystalline Co-30Ni-10Al-4W-4Ti-2Ta superalloys with Cr contents ranging from 1 to 5 at.% are characterized at 900 °C to provide benchmark data for the alloy design of the CoNi-based superalloys. The mass gain curves for all three alloys exhibit parabolic growth, and the addition of 5Cr at.% is sufficient to decrease the oxidation rate by two orders of magnitude compared to the Cr-free alloy. Furthermore, cross-sectional findings reveal that these three alloys form qualitatively similar oxide scales composed of an outer oxide layer of Co3O4 and CoAl2O4 phase on top of an Al2O3 scale, following the inner oxide layers of Cr2O3, TiO2, and TiTaO4, and internally oxidized Al2O3 precipitate. The alloy forms a chromium-rich oxide scale as the Cr addition increased, and the concentration of Cr in the scale/alloy interface increases, promoting the growth of Cr2O3, while CoAl2O4 and Co3O4 nucleation is inhibited. The results further indicate that Cr has a superior effect on improving the oxidation resistance of CoNi-based alloys and that a higher content of Cr can assist the formation of a continuous Al2O3, Cr2O3, and TiTaO4 layers, which in turn hampers outer Co and Ni, and inward oxygen flux.

Oxidation behaviors and mechanical properties of L12-strengthened high-entropy alloys at 700 ℃

Oxidation behavior and mechanical properties of two L12-strengthened high-entropy superalloys (HESAs), i.e., (Ni2Co2FeCr)92Ti4Al4 and (Ni2Co2FeCr)92Nb4Al4, were systematically investigated at 700 ℃. Our results reveal that the oxidation resistance was significantly improved by the replacement of Ti with Nb with slightly optimized mechanical performance. The substitution of Ti by Nb promotes the formation of continuous Cr2O3, AlNbO4, and Al2O3 layers, and causes a remarkable improvement in oxidation resistance, thus highlighting the greater potential of Nb-bearing HESAs to function as the elevated-temperature structural materials. The underlying mechanisms for the superior mechanical properties and oxidation resistance of the Nb-bearing HESA were carefully discussed.

Influence of Si addition on the microstructure, mechanical and lead-bismuth eutectic corrosion properties of an amorphous AlCrFeMoTiSix high-entropy alloy coating

The effect of the Si addition content ranging from 3.3 to 9.1 at.% on the microstructure, mechanical properties and LBE corrosion behaviour of AlCrFeMoTiSix HEA coatings was investigated. The coatings still maintained an amorphous structure, and their mechanical properties were improved. The Si addition greatly contributed to the enhancement of the LBE corrosion resistance of the coatings, which could be attributed to the formation of a more continuous and protective Cr2O3 and Al2O3 layer. However, the improved efficiency was related to the Si addition content, with an oxidation process model put forwards to well explain this phenomenon.

Preparation of Ti-Al-Si Gradient Coating Based on Silicon Concentration Gradient and Added-Ce

Titanium and titanium alloys have excellent physical properties and process properties and are widely used in the aviation industry, but their high-temperature oxidation resistance is poor, and there is a thermal barrier temperature of 600 °C, which limits their application as high-temperature components. The Self-generated Gradient Hot-dipping Infiltration (SGHDI) method is used to prepare the Ti-Al-Si gradient coating based on the silicon concentration with a compact Ti(Al,Si)3 phase layer, which can effectively improve the high-temperature oxidation resistance of the titanium alloy. Adding cerium can effectively inhibit the generation of the τ2: Ti(AlxSi1−x)2 phase within a certain hot infiltration time so as to form a continuous dense Al2O3 layer to further improve the oxidation resistance of the coating. Studies have found that multiple Ti-Al binary alloy phase layers are formed during the high-temperature oxidation process, which has the effect of isolating oxygen and crack growth, and effectively improving the high-temperature resistance of the coating oxidation performance.

Oxidation resistance at 900 °C of porous Ni-Al-Cr intermetallics synthesized via rapid thermal explosion reaction

Ni-Al-Cr intermetallic compounds are candidate at extreme environment due to their excellent oxidation resistance at high temperature, and the oxidation resistance of Ni-Al-Cr alloy system has been studied extensively. However, the research on oxidation resistance mechanism of porous Ni-Al-Cr intermetallic compounds is still not deep. In our work, porous Ni-Al-Cr intermetallic compounds were synthesized by thermal explosion, and the oxidation resistance of porous Ni-Al-Cr system was mainly investigated. The results indicate that Cr promotes the formation of highly protective continuous Al2O3 layer, increasing the oxidation resistance at 900 °C of porous Ni-Al-Cr. After oxidation at 900 °C for 120 h, the minimum mass gain of Ni-Al-Cr products is only 6.96%.

Reactive air brazing of 3YSZ ceramic to aluminized Crofer22H stainless steel using Ag–CuO fillers

AbstractIn this work, an effective protected Al2O3layer was fabricated first on the surface of Crofer22H by reactive air aluminization to suppress cathodic poisoning in solid oxide fuel cells. Then, the reactive air brazing of aluminized Crofer22H to 3YSZ ceramics was carried out by using Ag–CuO fillers. The effects of CuO content and brazing temperature on the interfacial microstructure and mechanical properties of the joints were studied in detail. The results indicated that maintaining a continuous Al2O3layer was the prerequisite for achieving a reliable joint. And the interfacial reaction gradually became intense with the increase of CuO content, and the continuity of the Al2O3layer was disrupted as it was increased to 6 mol%. The well‐connected joints could be achieved when brazing at 970–1100°C with Ag–2CuO filler. The interfacial microstructure did not obviously change with increasing temperature, and the joint strength was stable at ∼35 MPa. While the interfacial reaction gradually weakened with increasing temperature as brazing with Ag–4CuO filler, the well‐connected joints were obtained when brazing above 1000°C, and the maximum shear strength of ∼44.3 MPa was obtained when brazing at 1050°C for 30 min.

High temperature chloride corrosion behavior of AlFe2.5NiMoNbCr high-entropy alloy

High temperature chloride corrosion tests of AlFe2.5NiMoNbCr high-entropy alloy were carried out comparing with those of Inconel625 and AlFeNiMoNb plate. Uniform dispersion of elements in the Cr depletion zone existed during the corrosion process of AlFe2.5NiMoNbCr high-entropy alloy promoted the formation of continuous Al2O3 layer, and the dendritic Al2O3 improved the adhesion of the corrosion layer through “oxide pegging” mechanism. Observation of metal chloride oxides on the samples sealed for several days at R.T. supplemented the reaction of high temperature chlorine corrosion. Al2O3/Cr2O3 double-layer makes AlFe2.5NiMoNbCr have better potential application value than Inconel625 with a single Cr2O3 layer.

Effect of Sn doping concentration on the oxidation of Al-containing MAX phase (Ti3AlC2) combining simulation with experiment

Sn doping is usually adopted to prepare Ti3AlC2 in mass production because it can reduce the synthesis temperature while increasing the phase purity. However, excessive Sn doping usually deteriorates the oxidation resistance of Ti3AlC2. Therefore, an appropriate Sn doping concentration is a vital issue. In this work, the effect of Sn doping concentration on the oxidation behavior of Ti3AlC2 was systematically investigated by combining theoretical calculations and experimental methods. Density function theory calculations suggest that the oxygen adsorption mechanisms for the (001) surface of Ti3AlC2 with and without Sn doping are similar, and Ti-O bonds are always preferentially formed. The molecular dynamics simulation further indicates that Al atoms have a faster diffusion rate during the oxidation process. Therefore, a continuous Al2O3 layer can form rapidly at high temperature. Nevertheless, when the Sn doping concentration exceeds 10 mol%, the continuity of the Al2O3 layer is destroyed, thereby impairing the oxidation resistance of Ti3AlC2. Furthermore, oxidation experiments verify the above results. The oxidation mechanisms of Ti3AlC2 with different Sn doping concentrations are also proposed.

High-Temperature Oxidation Resistance of Alumina-Forming Austenitic Stainless Steels Optimized by Refractory Metal Alloying

Alumina-forming austenitic stainless steels are known for their superior high-temperature oxidation resistance. Following our previous work that solved the matching of major alloying elements in their specific 16-atom cluster formula, we here focus on the 800 °C air-oxidation resistance of 0.08 wt. % C alloy series satisfying cluster formula [(Al0.89Si0.05NbxTa0.06−x)-(Fe11.7−yNiyMn0.3)]Cr3.0−z(Mo,W)z, x = 0.03 or 0.06, y = 3.0 or 3.2, z = 0.07 or 0.2, to explore the effect of minor alloying elements Mo, Nb, Ta and W. This cluster formula is established particularly based on alloys which were originally developed by Oak Ridge National Laboratory. All samples are graded as complete oxidation resistance level according to Chinese standard HB 5258-2000, as their oxidation rate and oxidation-peeling mass are generally below 0.1 g/m2 × h and 1.0 g/m2, respectively. In alloys without Ta and W, a Cr2O3-type oxide layer is formed on the surface and Al2O3 particles of sizes up to 4 μm are distributed beneath it. In contrast, in Ta/W-containing alloys, a continuous protective Al2O3 layer is formed beneath the outer Cr2O3 layer, which prevents internal oxidation and provides the lowest weight gain. Instead of internal Al2O3 particles, AlN is formed in Ta/W-containing alloys. The W-containing alloy possesses the thinnest internal nitride zone, indicating the good inhibition effect of W on nitrogen diffusion.

Isothermal oxidation behavior of micro-regions in multiphase Ni3Al-based superalloys

The isothermal oxidation behavior of micro-regions in multiphase Ni3Al-based superalloys (high Fe and Cr contents) was investigated at 600 °C in air. The oxide scale is mainly composed of the outer mixed oxide layer (NiO, NiFe2O4 and Al2O3) and the inner single Al2O3 layer. In particular, Cr2O3 is formed in the outer oxide scale of γ' + γ dendrite with enrichment in Cr element. The relative dense continuous Al2O3 layer is formed in the interdendritic region (β phase with high Al content). In addition, γ' envelope (connecting the γ' + γ dendrite and interdendritic β phase) shows preferential oxidation featuring significant cellular bulge (NiO and NiFe2O4 mixed oxides).

Investigation of isothermal oxidation performance of TiAl alloys sintered by different processing methods

The production of TiAl alloys can be a discouraging task owing to their low ductility at room temperature. However, the development of novel methods that are alternative to traditional methods for producing TiAl alloys is a fundamental aspect for structural applications in the automotive and aerospace industries. Resistance sintering method enables the production of TiAl alloys, thanks to a combination of remarkable attributes such as shorter sintering time, fast heating rates. This method is promising under circumstances where the rapid densification, short sintering time and low cost becomes a great advantage. The objective of the following work was to compare the isothermal oxidation performance of TiAl alloys produced by resistance sintering (RS) and pressureless sintering (PS) methods. After oxidation at 900 °C for 200 h, the final weight change of RSed alloy (6.36 mg/cm2) was lower than that of PSed alloy (8.92 mg/cm2). X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) were performed to characterize the oxidized alloys. Analyses results show that the oxidation products were TiO2 and Al2O3 oxides, the oxide scale of both alloys are composed a multilayered structure. The relatively continuous Al2O3 layer formed on the scale of RSed alloy played a major role in enhancing the oxidation resistance of the alloy.

The Effects of Grain Size and Twins Density on High Temperature Oxidation Behavior of Nickel-Based Superalloy GH738.

In the present study, surface treatment techniques such as room temperature machining (RTM) and low temperature burnishing (LTB) processing have been used to improve the microstructure of GH738 superalloy. Nano-grains and nano-twins are obtained on the top surface of RTM and LTB specimens. It is found that although the grain size of RTM and LTB specimens is almost the same, different types of nano-twins have been produced. Moreover, the effect of RTM and LTB processing on high temperature oxidation behavior of nickel-based superalloy GH738 at 700 °C is investigated. The result shows that LTB specimen has the best high temperature oxidation resistance owing to the formation of nano-grains and higher twins density, which induce to form a continuous protective Al2O3 layer at the interface between outer oxide layer and matrix. It is observed that this layer inhibits the inward diffusion of O and outward diffusion of Ti and significantly improves oxidation resistance of LTB specimen. Furthermore, the effects of nano-grains and crystal defects on the diffusion mechanism of elements are clarified during the high temperature oxidation test.

Atomic Layer Deposition onto Thermoplastic Polymeric Nanofibrous Aerogel Templates for Tailored Surface Properties.

Poly(vinyl alcohol-co-ethylene) (EVOH) nanofibrous aerogel (NFA) templates were fabricated through vacuum freeze-drying from EVOH nanofibrous suspensions. Aluminum oxide (Al2O3) layers were deposited onto highly porous templates to form organic-inorganic hybrid aerogels by the atomic layer deposition (ALD) technique. Chemical and physical measurements showed that mechanical properties were improved through ALD. In addition, the surface chemistry of ALD modified aerogels showed a fascinating cyclic change based on the number of ALD deposition cycles. A transition from hydrophilicity to hydrophobicity was observed after a few cycles of ALD coating; however, additional deposition cycles changed the wettability characteristics back to hydrophilicity. This hydrophilic-hydrophobic-hydrophilic variation is shown to be governed by a combination of geometrical and chemical surface properties. Furthermore, the deposited Al2O3 could substantially improve aerogels strength and reduce permanent deformation after cyclic compression. The Young's modulus of aerogels increased from 5.54 to 33.27 kPa, and the maximum stress at 80% strain went up from 31.13 to 176.11 kPa, after 100 cycles of trimethyl-aluminum (TMA)/water ALD. Thermogravimetric analysis (TGA) results confirm that ALD can effectively improve the heat resistance characteristics of polymeric aerogel. The onset temperature and the residual mass increased with increasing numbers of ALD cycles. During pyrolysis, the nanofiber cores were decomposed, and the brittle pure Al2O3 self-supporting nanotube aerogels with the continuous hollow nanotubular network were formed. A coating of continuous thickness Al2O3 layer on individual nanofiber was achieved after 100 ALD cycles. In additional to mechanical strength and physical property changes, the ALD modified aerogel also shows a superhydrophobic and oleophilic surface chemistry, which could potentially be used to remove oils/organic solvents from water. The resultant aerogels exhibit excellent absorption capacity (31-73 g/g) for various liquids, and the material could be reused after distillation or squeezing. A successful scale-up of such materials could provide some insights into the design and development of thermoplastic polymeric NFAs with substantial industrial applications.

Effects of temperature and Ti addition on high-temperature oxidation behaviors of Co-Al-W based superalloys

PurposeDue to the special service environment of superalloys, this paper aims to obtain effects of temperature and Ti addition on high temperature oxidation behavior of Co-Al-W-B alloys.Design/methodology/approachIsothermal oxidation experiment of Co-Al-W-based alloys were carried out at 800°C, 900°C and 1000°C for different times (3, 5, 10, 20, 50 and 100 h) referring to the method of HB5258-2000. Oxidation weight gain curves and oxidation products were detected.FindingsThe results showed that the average oxidation rates of Co-Al-W-B alloy at 800 °C and 900 °C were 0.489 g·m−2·h−1and 0.888 g·m−2·h−1, respectively, which belonged to an antioxidant grade. However, the average oxidation rate at 1000 °C was 2.068 g m−2·h−1, belonging to the secondary oxidation resistance class. In the alloy with Ti addition, dense Ti oxides film were formed at the early oxidation stage and then gradually diffused later, which can increase the oxidation resistance of the alloys to some extent. By analyzing the oxidation products of Co-Al-W-B alloy, it was found that a dense Al2O3layer could be formed when the alloy was oxidized at 800°C. The continuous Al2O3layer would prevent the oxygen from further spreading and make the alloy into the stable oxidation stage. However, only a non-dense Al2O3layer were observed with 900°C oxidation.Originality/valueIt can provide references for the composition design, preparation process optimization and protective coating selection of the γ′ phase strengthened cobalt-base superalloys.

High temperature oxidation of NiCrAlY coated Alloy 625 manufactured by selective laser melting

The high temperature oxidation of fully SLM-processed NiCrAlY coated Alloy 625 parts was investigated between 800 °C and 1000 °C. For comparison, bulk NiCrAlY and bulk Alloy 625 were also fabricated using SLM. Two thin layers of NiCrAlY powder were lasered at the surface of Alloy 625 to form the bi-materials. Bulk NiCrAlY and bi-materials showed an improved oxidation behavior compared to Alloy 625. The oxidation of the NiCrAlY coating results in regions with a dense and continuous external Al2O3 layer and regions composed of a mixture of external Cr2O3 and internal Al2O3. Large EDS maps at the surface and cross-sections of the bi-material highlighted a heterogeneous distribution of constitutive elements of the NiCrAlY coating, resulting in some regions with an Al activity lower than the one required for the formation of a continuous and dense Al2O3 layer (≤10 atomic percent). Low Al activity and high Al activity regions were related to the topography of the SLMed surface and correspond to hill and valley regions, respectively. In addition, cracks, mainly occurring in high Al activity regions, were observed across NiCrAlY specimens. The fabrication of coated but small components with a brittle coating by SLM is not trivial and needs further investigations.

Improvement of isothermal oxidation resistance of a γ′-strengthened Co–Al–W–Mo–Ta–B alloy at 800 °C via doping Ce

Isothermal oxidation resistance, oxide scale evolution and failure mechanism of Ce-doped Co–Al–W–Mo–Ta–B alloy (0.01 at%, 0.05 at%, 0.10 at% and 0.20 at% Ce) exposed at 800 °C were compared. The 0.01Ce and 0.05Ce alloys were consisted of γ/γ′ coherent microstructure, while the κ-Co3W compound precipitated at the grain boundary of the 0.10Ce and 0.20Ce alloys in addition to the γ/γ′ microstructure. The oxidation kinetics curves of the Ce-doped alloys exhibited a parabolic time dependence on the weight gain. With an increasing nominal Ce content, the weight gain of the Co–Al–W–Mo–Ta–B alloys monotonically decreased. An oxide scale composed of a dense and uniform outer Co3O4 + CoO layer, a middle CoAl2O4 and CoWO4 compound layer and an inner Al2O3 layer. The excellent oxidation resistance of 0.2Ce alloy was mainly attributed to a shorter incubation stage for the formation of the continuous and protective Al2O3 layer and the thickest Al2O3 layer during entire oxidation process.

Synthesis of chemically bonded phosphate ceramic coatings with tailored curing behaviour and enhanced oxidation resistance

The objective of this study was to develop chemically bonded phosphate ceramic (CBPC) coatings with relatively low curing temperature, properly prolonged curing time, and enhanced oxidation resistance. The coating was synthesised using aluminium phosphate as binder, surface modified MgOw/MgO as curing agent, and spherical microscaled Al particle as filler. The MgOw/MgO curing agent was surface coated by an Al2O3 layer via sol-gel routes. The Al2O3-coated MgOw/MgO agent facilitated a controllable curing process owing to the sustained-release effect of the surrounding microstructure. The weight loss profiles of all CBPCs with a surface modified curing agent exhibited a quasi-parabolic trend. The curing time decreased with the addition of the curing agent, and the Al2O3-coated MgOw curing agent was more effective owing to the good dispersion and larger particle number/volume percentage. The thermogravimetric and differential scanning calorimetry curves proved that the Al element exhibited similar effect as Mg2+, which could also enhance the curing process via substitution reaction between the basic metal ions with hydrogen in phosphate. Therefore, the addition of Al filler resulted in further curing and densification, exhibiting a decrease in the curing time (30–50 min) and increase in the weight loss (~40%). Proper binder-to-curing agent-to-filler mass (B:C:F) ratio was very important, and the CBPCs with improved surface roughness, hardness, and free of cracks were synthesised at a B:C:F ratio of 10:1:0.05. During oxidation at 800 °C, the Al filler in the CBPCs transformed into a continuous Al2O3 layer, which protected the Ti6Al4V alloy from further oxidation.

The role of Al diffusion behavior in the process of forming a super-reliable Al2O3 protective layer during reactive air aluminization

Reactive air aluminization is attractive as a low-cost, simple method for preparing Al2O3 protective layer for ferritic stainless steel (FSS) components in solid oxide cell stacks. The FSS Crofer 22H is aluminized at various temperatures to obtain different levels of elements interdiffusion. The leading role of Al diffusion behavior in the process of forming the Al2O3 protective layer is investigated in detail. The roughening mechanism of aluminized surface is discussed. The formation process of Al2O3 protective layer can be divided into three stages: liquid-solid interdiffusion (<700 °C), solid-solid interdiffusion (700–900 °C), and epitaxial growth of Al2O3 (>900 °C). The intense interdiffusion between the Al liquid and the steel matrix leads to the formation of Al-containing intermetallic compounds (IMCs) layer in a short time. Increasing temperature promotes the diffusion of Al into the steel matrix, causing the phase transition of IMCs. The surface enrichment of Al suppresses the formation of Fe/Cr-oxides, ensuring the formation of a continuous, dense and crack free Al2O3 layer. The rough surface formed in the first stage basically determines the overall roughness of the sample after aluminization.