High temperature oxidation of Ti−48%Al−2%W intermetallics

Powder metallurgically produced Ti-48% Al-2%W alloys were oxidized between 800 and 1050°C in air. The W-addition was quite effective in providing isothermal and cyclic oxidation resistance. The alloys oxidized parabolically up to 1050°C during isothermal oxidation, with small weight gains. The scales were adherent up to 900°C during cyclic oxidation. Oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. Tungsten was present below the intermediate Al₂O₃ layer. and also at the scale-matrix interface as W-enriched compounds. Below the oxide scale, a Ti₃Al zone containing some W and O existed.     

The Isothermal and Cyclic Oxidation Behavior of a Titanium Aluminide Alloy at Elevated Temperature

The isothermal and cyclic oxidation behavior of Ti-47Al-2Mn-2Nb with 0.8 vol.% TiB₂ particle-reinforced alloy was investigated in air between 700 and 1000 °C. In the study, the kinetics of isothermal and cyclic oxidation were performed by using a continuous thermogravimetric method which permits mass change measurement under oxidation conditions. The oxide scales and substrates were characterized by scanning electron microscopy with energy-dispersive x-ray analysis and x-ray diffraction. At 700 and 800 °C, the alloy showed an excellent oxidation resistance under isothermal and cyclic conditions. After exposure to air above 800 °C, the outer scale of the alloy was dominated by a fast-growing TiO₂ layer. Under the coarse-grained TiO₂ layer was the Al₂O₃-rich scale, which was fine-grained. At 900 and 1000 °C, the extent of oxidation increased clearly. The oxidation rate follows a parabolic law at 700 and 800 °C. However, the alloy, upon isothermal oxidation at 900 °C, can be divided into several stages. During the cyclic oxidation at 900 and 1000 °C, partial scale spallation takes place, leading to a stepwise mass change.     

NiCoCrAlY coatings with and without an Al2O3/Al interlayer on an orthorhombic Ti2AlNb-based alloy: Oxidation and interdiffusion behaviors

The mechanism of oxidation protection of NiCoCrAlY overlay coatings on the orthorhombic Ti₂AlNb-based alloy (O alloy) Ti–22Al–26Nb (at.%) is described. While the bare alloy exhibited poor oxidation resistance at 800 °C, adding NiCoCrAlY coatings significantly improved the oxidation resistance. However, serious interdiffusion between the coatings and the substrate resulted in rapid degradation of the coating system. Several reaction layers were formed at the coating/substrate interface by interdiffusion, and non-protective scales mainly of Cr₂O₃ and TiO₂ were formed due to the degradation of the coating. In order to solve this problem, an Al₂O₃/Al interlayer was sandwiched into the coating system as a diffusion barrier. The isothermal and cyclic oxidation protection of the multilayer coating system on the Ti–22Al–26Nb substrate was evaluated at 800 and 900 °C. The results indicated that the interdiffusion was much suppressed, and the duplex coating system demonstrated improved oxidation resistance on the Ti–22Al–26Nb substrate, with a thin and adherent protective α-Al₂O₃ scale forming on the surface.     

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm². The parabolic rate constant, k_p (mg²/cm⁴·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al₂O₃ layer between the outer TiO₂ layer and the inner (TiO₂-rich, Al₂O₃-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.     

SiO2–Al2O3–glass composite coating on Ti–6Al–4V alloy: Oxidation and interfacial reaction behavior

A SiO₂–Al₂O₃–glass composite coating was prepared on Ti–6Al–4V alloy by air spraying and subsequent firing. The oxidation behavior of the specimens at 800 °C and 900 °C for 100 h was studied. The thermal shock resistance of the coating was tested by heating up to 900 °C and then quenching in water. The composite coating acted as an oxygen migration barrier and exhibited good resistance against high temperature oxidation, thermal shock, and oxygen permeation on the Ti–6Al–4V alloy. Coating/alloy interfacial reaction occurred, forming a Ti₅Si₃/Ti₃Al bilayer structure. A thin Al₂O₃ rich layer formed beneath the composite coating during oxidation at 900 °C.     

Comparisons of high temperature oxidation behavior between reactive-sintered and melted Ti-45at.%Al-1.6at.%Mn intermetallics

In order to investigate high temperature oxidation behavior, isothermal oxidation experiments for Ti-45at.%Al-1.6at.%Mn intermetallics prepared by both reactive sintering and melting were carried out in both oxygen and air environments at temperatures of 900, 1000 and 1100°C. In the oxygen environment, reactive sintered Ti-45at.%Al-1.6at.%Mn was found to have excellent oxidation resistance due to the formation of a continuous A1₂O₃ layer at temperatures up to 1100°C; however, when melted it showed very poor oxidation resistance at and above 1000°C. Weight gains of the melted specimen after 12 hours, especially at the temperature of 1000°C and 1100°C, were nearly 12 times that of the reactive sintered one. In air, the reactive-sintered intermetallics showed a poorer oxidation resistance man did that in the oxygen environment. Especially, the weight gains in air were 6 times larger than the weight gains in oxygen when oxidized at 1100°C for 12 hours.     

Cyclic oxidation behavior of Ni3Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025% B between 900 and 1100°C

A Ni₃Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al₂O₃, NiAl₂O₄ and ZrO₂ were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al₂O₃, ZrO₂)-containing oxides that keyed the oxide scale to the matrix alloy.     

Oxidation Behavior of Graded NiCrAlYRe Coatings at 900, 1000 and 1100 °C

A graded NiCrAlYRe coating was prepared by combining arc ion plating (AIP) with chemical vapor deposition (CVD) aluminizing. Quasi-isothermal oxidation tests of the graded NiCrAlYRe coating and the conventional NiCrAlYRe coating were performed in air at 900, 1000 and 1100 °C for up to 1000, 1000 and 200 h, respectively. The results showed that the graded NiCrAlYRe coating exhibited better long time oxidation resistance than the conventional NiCrAlRe coating. This favorable oxidation behavior was attributed to the rapid formation of a protective α-Al₂O₃ scale and a sufficient Al reservoir. The structures and morphologies of oxide scales varied under different oxidation conditions. θ-Al₂O₃ was observed on both coatings during oxidation at 900 °C, however, the graded coating showed more favorable conditions for θ-Al₂O₃ to grow than the conventional coating. For the graded coating, phase transformation from θ-Al₂O₃ to α-Al₂O₃ resulted in a sharp decrease in the parabolic rate constant k_p between 900 and 1,000 °C.     

Effect of hot-dip aluminizing on the oxidation resistance of Ti–6Al–4V alloy at high temperatures

Hot-dip aluminizing and interdiffusion treatment were used to develop a TiAl₃-rich coating on Ti–6Al–4V alloy. Interrupted oxidation at temperatures from 600 to 900 °C and isothermal oxidation at 700 and 800 °C of the coating were conducted. The coating markedly decreases the oxidation rate in comparison with the alloy at temperatures below 800 °C during the interrupted oxidation. The oxidation kinetics follows parabolic relations at 700 and 800 °C during the isothermal oxidation. A layered structure of Al₂O₃/TiAl₃/TiAl₂/TiAl/alloy from the outside to the inside forms after oxidation at 700 °C without changing the main body of the coating.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

Protection of titanium aluminides by FeCrAlY Coatings

The effectiveness of Fe+15%Cr+5%Al+0.3%Y coatings. produced by sputter ion plating, in improving the oxidation behaviour of γ-2 Ti₃Al at 800°C and γ-TiAl at 900° and 1000°C. during cyclic exposures in air of up to 1000 hours duration, has been studied. The 45-135 μm coatings were tested in both the as-coated condition and following densification by peening. The kinetics of attack and of spallation were followed gravimetrically, while the reaction product compositions were examined by a range of surface analytical techniques. Oxidation protection was afforded by the formation on the coating surface of a γ-Al₂O₃ oxide scale but chemical reactions at the coating-substrate interface with accompanying voidage development, eventually caused the coating to fail mechanically. To surmount this problem the potential of several ceramic interlayers produced by the same coating route, and interposed as diffusion barriers between the FeCrAlY overlayers and the Ti₃Al substrate was examined in 1000 hour oxidation tests at 800°C.     

Oxidation behavior of atomized Fe40Al intermetallics doped with boron and reinforced with alumina fibers

Isothermal oxidation resistance of Fe40 (at.%) Al-based atomized and deposited intermetallic alloys has been evaluated. The alloys included Fe40Al, Fe40Al + 0.1B, and Fe40Al + 0.1B + 10Al₂O₃ at 800, 900, 1000, and 1100 °C. The tests lasted approximately 100 h, although in most cases there was scale spalling. At 800 and 900 °C, the Fe40Al + 0.1B alloy had the lowest weight gain, whereas the Fe40Al alloy had the highest weight gain at 800 °C (0.10 mg/cm²) and the Fe40Al + 0.1B + 10Al₂O₃ alloy was the least oxidation resistant at 900 °C with 0.20 mg/cm². At 1000 °C, the Fe40Al + 0.1B alloy showed the highest weight gain with 0.12 mg/cm² and the Fe40Al alloy the lowest. At 1100 °C, again, as at 900 °C, the Fe40Al alloy was the least resistant, whereas the Fe40Al + 0.1B alloy performed the best, but the three alloys exhibited a paralinear bahavior on the weight-gain curves, indicating the spalling, breaking down, and rehealing of the oxides. This spalling was related to voids formed at the metal-oxide interface.     

超音速大气等离子喷涂TiB2-SiC涂层的抗氧化性及耐熔盐腐蚀性能

采用超音速大气等离子喷涂制备全包覆TiB₂-SiC涂层,研究了TiB₂-SiC涂层在400和800 ℃的氧化性能,并探究其氧化机理。对TiB₂-SiC涂层在900 ℃下的抗铝熔盐腐蚀性能进行研究,并探讨其耐熔盐腐蚀机理。结果表明,超音速大气等离子喷涂制备的TiB₂-SiC涂层具有良好的抗氧化性,在400 ℃的氧化速率常数为1.92×10^-5 mg²·cm^-4·s^-1,在800 ℃的氧化速率常数为1.82×10^-4 mg²·cm^-4·s^-1。超音速大气等离子喷涂制备的TiB₂-SiC涂层在900 ℃下具有良好的抗熔盐腐蚀性能,熔盐腐蚀后TiB₂-SiC涂层都保持致密结构,未发生涂层的开裂及剥落。     

Characterisation and oxidation of LVOF sprayed Al2O3–40%TiO2 coating on superni 601 and superni 718 superalloys at 800 and 900°C

Failure of components due to high temperature oxidation is the major degradation mechanism in boiler and gas turbine industries. Superalloys having superior mechanical properties and creep resistance are used in these applications but lack resistance to oxidation under aggressive environments. Protective coatings are used to improve their oxidation resistance in such applications. In the present investigation, Al₂O₃–40%TiO₂ coating was deposited on superni 718 and superni 601 superalloys by low velocity oxy fuel process. The as sprayed coating was characterised for microhardness, surface roughness, scanning electron microscopy and X-ray diffraction analysis. High temperature oxidation behaviour of Al₂O₃–40%TiO₂ coated and uncoated superni 718 and superni 601 superalloys has been evaluated at the elevated temperatures of 800 and 900°C for total duration of 50 cycles under cyclic conditions. Each cycle consisted of keeping the samples for 1 h at the elevated temperature followed by 20 min cooling in ambient air. Al₂O₃–40TiO₂ coating in the as sprayed condition showed the presence of Al₂O₃–TiO₂, α-Al₂O₃, TiO₂ as the main phases. Al₂O₃–40%TiO₂ coating on superni 718 and superni 601 superalloys has shown a lower oxidation rate as compared to those of uncoated superalloys. However, the oxidation rate of the coating was not steady due to the occurrence of spallation/sputtering at various stages. The coating was found adherent on the substrate superalloys throughout the study.     

High temperature sulfidation and oxidation of sputter-deposited Nb−Al−Si coatings

The sulfidation and oxidation behavior of amorphous 58Nb-38Al-4Si(at%) coating sputter-deposited with d.c. magnetron sputtering was studied between 700 and 900°C under 0.1 atm of pure S₂(g) and 1 atm of air, respectively. The coating approximately followed the parabolic sulfidation and oxidation rate law, and displayed superior resistance to sulfidation and oxidation. The coating sulfidized to Al₂S₃ and NbS₂ which protected the substrate. The coating oxidized to TiO₂, AlNbO₄ and κ-Al₂O₃ which acted as an oxidation barrier. The mechanisms of sulfidation and oxidation of the prepared coating are discussed.     

High-Temperature Oxidation Behavior of Sputtered IN 738 Nanocrystalline Coating

A sputtered nanocrystalline coating of IN 738 alloy was obtained by means of magnetron sputtering. The isothermal oxidation behavior at 800, 900, and 1000°C and the cyclic oxidation behavior at 950°C of the coating were studied in comparison with IN 738 cast alloy. The results indicated that a double external oxide scale was formed on the nanocrystalline coating at 900, 950, and 1000°C without internal oxide and nitride. The scale consisted in an outer mixture of Cr₂O₃, TiO₂, and NiCr₂O₄ and an inner, continuous Al₂O₃ layer, which offered good adhesive and protectiveness. However, at 800°C a continuous Al₂O₃ scale could not be formed during oxidation of nanocrystalline coating and aluminum was still oxidized internally.     

Oxidation kinetics of the pack siliconized TiAl-based alloy and microstructure evolution of the coating

The cyclic oxidation resistance of a two-phase TiAl-based alloy was remarkably improved with the formation of composite coating by siliconization with mixed powder of 15 wt%Si + 85 wt%Al₂O₃. The composite coating consists of a Ti₅Si₃-based inner layer and an Al₂O₃-based outer layer. The cyclic oxidation test at 900 °C showed that increasing the siliconization temperature benefits the oxidation resistance. The higher the siliconization temperature, the stabler the Ti₅Si₃-based layer and the more Al₂O₃ concentrated in the outer layer. Usually, the oxidation curves consist of three regions, the parabolic, the linear and the quadratic. For the specimen that was siliconized at 1250 °C for 2 h, however, only the parabolic region appeared during the whole cyclic oxidation test at 900 °C up to 1000 h. The weight gain is less than 0.3 mg cm⁻² after cyclic oxidation at 900 °C for 1000 h, corresponding to a parabolic oxidation rate constant, K_p ≈ 6.03 × 10⁻⁵ mg²/(cm⁴ h). Such a low oxidation rate is attributed to the composite layer of the specimen with a stable Ti₅Si₃-based layer and a dense Al₂O₃-based layer.     

Evaluation of the Parabolic Rate Constant During Different Types of Oxidation Tests for Spinel Coated Fe–17%Cr Alloy

Oxidation resistance, and the long- term stability of interconnects in solid oxide fuel cells, can be ameliorated by the use of a protective, effective, relatively dense and well adherent, spinel coating. In this study, the pack cementation method was employed to coat cobalt onto AISI 430 ferritic stainless steel. Isothermal oxidation, cyclic oxidation, and oxidation at different temperatures (600–900 °C) were applied to evaluate the parabolic rate constant (k_p). In each test, the coated samples demonstrated lower k_p, indicating that the coating layer was effective. The formation of MnCo₂O₄ and CoCr₂O₄ spinels, during the oxidation, improved the oxidation resistance of AISI 430 ferritic stainless steel.     

Effect of different fillers on oxidation behavior of low-temperature chromizing coating

Three different chromizing coatings were produced on Ni substrate using a conventional pack-cementation method with Al₂O₃, Al₂O₃+CeO₂ and CeO₂ acting as filler, respectively, at a greatly decreased temperature (700 °C). Effects of different fillers on the isothermal and cyclic oxidation resistance of chromizing coating in air at 850 °C were comparably investigated. Microstructure results show that the addition of CeO₂ into the filler significantly retards the grain growth of the chromizing coating. Oxidation results indicate that the chromizing coating using CeO₂ as filler exhibits somewhat increased oxidation resistance than the normal chromizing coating, while the chromizing coating using Al₂O₃+CeO₂ as filler exhibits much better oxidation resistance. The effects of different fillers on the oxidation behaviors were discussed in detail.     

High-Temperature-Oxidation Behavior of Iron–Aluminide Diffusion Coatings

Aluminide diffusion coatings were oxidized in air under atmospheric pressure under isothermal and cyclic conditions. The high-temperature efficiency of the pack-aluminized alloys was tested by comparing their oxidation behavior in the temperature range 800–1080°C. The k _p values deduced from the parabolic plots of weight-gain curves showed that α-Al₂O₃ composed the major phase of the oxide scale on samples oxidized at T > 1000°C. For lower temperatures, transient-alumina phases were observed. The aluminide materials also exhibited excellent resistance to cyclic oxidation at 1000°C. The second aim of this study was to dope the aluminide compounds obtained by a pack-cementation process with yttria, which was introduced by metal-organic chemical-vapor deposition (MOCVD). The beneficial effect of the reactive-element-oxide coating is strongly dependent on its mode of introduction, since the oxidation resistance is drastically increased when the Y₂O₃ coating was applied prior to the aluminization process. When applied after the aluminization, the reactive element gave negative effects on the high-temperature oxidation behavior of the iron aluminides. The oxide morphologies, X-ray diffraction patterns and two-stage experiments helped to understand the oxide-scale-growth mechanisms.