Early stages of the oxidation of a FeCrAlRE alloy (Kanthal AF) at 900 °C: A detailed microstructural investigation

Early stages of the evolution of Al₂O₃ scales formed on a FeCrAlRE alloy (Kanthal AF) have been investigated by analytical TEM. The samples were oxidized isothermally at 900 °C in dry O₂ or O₂ + 40% H₂O for 1 h or 24 h. All oxide scales exhibited a two-layered structure, with a continuous inward growing α-Al₂O₃ inner layer and an outward growing outer layer. After 1 h, the outer oxide layer consisted of γ-Al₂O₃ in both environments. After 24 h exposure in dry O₂, the γ-Al₂O₃ in the outer oxide layer was partly transformed to α-Al₂O₃ and spinel oxide (Mg_1−xFe_xAl₂O₄). In contrast, the γ-Al₂O₃ in the outer layer was not transformed after 24 h in O₂ + 40% H₂O, showing that water vapour stabilizes γ-Al₂O₃. All oxide scales contained a Cr-rich band, a product of the initial oxidation. The inner α-Al₂O₃ layer is suggested to nucleate on Cr₂O₃ or Cr_2−xFe_xO₃ in the initial oxide.     

Cyclic oxidation behavior and thermal barrier effect of YSZ-(Al2O3/YAG) double-layer TBCs prepared by the composite sol-gel method

Novel YSZ (6 wt.% yttria partially stabilized zirconia)-(Al₂O₃/YAG) (alumina-yttrium aluminum garnet, Y₃Al₅O₁₂) double-layer ceramic coatings were fabricated using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies. The thin Al₂O₃/YAG layer had good adherence with substrate and thick YSZ top layer, which presented the structure of micro-sized YAG particles embedded in nano-sized α-Al₂O₃ film. Cyclic oxidation tests at 1000 °C indicated that they possessed superior properties to resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1000 °C and 1100 °C indicate that the 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. These beneficial effects should be mainly attributed to that, the oxidation rate of thermal grown oxides (TGO) scale is decreased by the “sealing effect” of α-Al₂O₃, the “reactive element effect”, and the reduced thermal stresses by means of nano/micro composite structure. This double-layer coating can be considered as a promising TBC.     

Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al₂S₃ or Cr₂S₃ layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al³⁺ ions into NbS₂ and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al₂S₃ layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr₂O₃ layer in the oxide scales is formed on Cr-rich Cr-Nb and Cr-Ta alloys due to outward diffusion of Cr³⁺ ions. In contrast, continuous alumina layer cannot be formed on the Al-Nb and Al-Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.     

Tribological properties of titanium aluminides coatings produced on pure Ti by laser surface alloying

Titanium aluminides coatings were in-situ synthesized on a pure Ti substrate with a preplaced Al powder layer by laser surface alloying. The friction and wear properties of the titanium aluminides coatings at different normal loads and sliding speeds were investigated. It was found that the hardness of the titanium aluminides coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating. Friction and wear tests revealed that, at a given sliding speed of 0.10 m/s, the wear volume of pure Ti and the titanium aluminum coatings all increased with increasing normal load. At a given normal load of 2 N, for pure Ti, its wear volume increased with increasing sliding speed; for the titanium aluminides coatings, the wear volume of Ti₃Al coating and TiAl coating first increased and then decreased, while the wear volume of TiAl₃ coating first decreased and then increased with increasing sliding speed. In addition, the friction coefficients of pure Ti and the titanium aluminides coating decreased drastically with increasing sliding speed. Under the same dry sliding test conditions, the wear resistance of the titanium aluminium coatings was in the following order: Ti₃Al coating > TiAl coating > TiAl₃ coating.     

Oxidation resistance of TiAl3-Al composite coating on orthorhombic Ti2AlNb based alloy

The TiAl₃-Al composite coating on orthorhombic Ti₂AlNb based alloy was prepared by cold spray. Oxidation in air at 950 °C indicated that the bare alloy exhibited poor oxidation resistance due to the formation of TiO₂/AlNbO₄ mixture and intended to scale off at the TiO₂ rich zone. A nitride layer about 2 µm was formed under the oxide layer. The oxygen invaded deeply into the alloy and caused severe microhardness enhancement in the near surface region. The TiAl₃-Al composite coating exhibited parabolic oxidation kinetics and showed no sign of degradation after oxidized up to 1098 h at 950 °C in air under quasi-isothermal condition. No scaling of the coating was observed after oxidized at 950 °C up to the tested 150 cycles. The major oxide in the oxidized coating was Al₂O₃. The AlTi₂N, TiAl and small amount of TiO₂ were also observed in the oxidized coating. The EPMA and microhardness tests showed that inward oxygen diffusion was prevented by the interlayer, which was formed between the composite coating and the substrate during heat-treatment. Microstructure analyses demonstrated that the interlayer play a major role in protecting the substrate alloy from high temperature oxidation and interstitial embrittlement.     

High temperature corrosion of hot-dip aluminized steel in Ar/1%SO<Subscript>2</Subscript> gas

Carbon steels were hot-dip aluminized in Al or Al-1at%Si baths, and corroded in Ar/1%SO₂ gas at 700-800 °C for up to 50 h. The aluminized layers consisted of not only an outer Al(Fe) topcoat that had interdispersed needle-like Al₃Fe particles but also an inner Al-Fe alloy layer that consisted of an outer Al₃Fe layer and an inner Al₅Fe₂ layer. The Si addition in the bath made the Al(Fe) topcoat thin and nonuniform, smoothened the tongue-like interface between the Al-Fe alloy layer and the substrate, and increased the microhardness of the aluminized layer. The aluminized steels exhibited good corrosion resistance by forming thin α-Al₂O₃ scales, along with a minor amount of iron oxides on the surface. The interdiffusion that occurred during heating made the aluminized layer thick and diffuse, resulting in the formation of Al₅Fe₂, AlFe and AlFe₃ layers. It also smoothened the tongue-like interface, and decreased the microhardness of the aluminized layer. The non-aluminized steel formed thick, nonadherent, nonprotective (Fe₃O₄, FeS)-mixed scales.     

Oxidation resistance of aluminized coatings on a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Although a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy has good high-temperature mechanical properties, it does not have adequate oxidation resistance for prolonged exposure to high surface temperatures. Thus the oxidation behavior of several aluminized coating systems on this alloy in flowing air at temperatures of 900 to 1100°C under isothermal and thermal cycling conditions has been investigated. Attempts to produce an oxidation-resistant system by direct aluminizing have not been successful since removal of carbide fibers results in a porous coating which gives little protection to the alloy. The deposition of a layer of nickel or a Ni-20%Co-10%Cr-4%Al alloy on the eutectic prior to aluminizing gives improved isothermal oxidation resistance for prolonged exposure to high surface temperatures. Thus the eutectic alloy substrate occur during thermal cycling. A more successful system has been produced by depositing a thin layer of platinum on the eutectic alloy prior to aluminizing. Protective -Al₂O₃ scales are formed and maintained during isothermal and thermal cycling oxidation at 900 and 1000°C. Similar scales are developed at 1100° C although these do break down during thermal cycling. However, surface -Al₂O₃ scales are able to re-form rapidly, thereby preventing excessive oxidation of the coating.     

High temperature cyclic oxidation of aluminide layers on titanium

Aluminide layers containing TiAl₃ or TiAl on the surface were formed on titanium samples using the pack-cementation process. Cyclic oxidation (1 hr at the desired temperature and 20 min at room temperature) was carried out over the temperature range 850–1000°C. TiAl surfaces showed poor oxidation resistance compared to TiAl₃. The oxide morphology showed a peculiar protrusion formation on TiAl₃ surfaces at lower temperatures. A cross-section of the oxidized sample showed discrete Al₂O₃ and TiO₂ layers.     

Effect of Ternary Elements on the Oxidation Behavior of Aluminized TiAl Alloys

The effects of ternary elements added to TiAl on the coating layer formed by the pack-aluminizing process was studied with respect to oxidation resistance and mechanical properties. All the TiAl specimens, with various amounts of Nb, Cr, Fe, and V, were pack aluminized under identical conditions using a high-activity process. Among the ternary alloying elements tested, Nb showed the best property of the TiAl₃ coating layer formed on the surface and, consequently, the best oxidation resistance. The TiAl₃ coating layer becomes thicker and has a finer grain size as the content of Nb or Cr is increased. Microhardness tests revealed that the addition of Nb or Cr improved the toughness of the coating layer and thus improved the cracking resistance. Cyclic oxidation tests showed that the TiAl₃ coating layer formed on the TiAl alloy has better oxidation resistance with increasing Nb content. The ductility and oxidation resistance of the TiAl₃ coating layers improved with Nb addition, which contributes to the grain refinement of TiAl₃. The Nb present in the TiAl₃ coating layer inhibits grain growth by the solute-drag effect and retards inward diffusion of Al to the TiAl matrix by forming (Nb, Ti)Al₃ precipitates during high-temperature oxidation.     

The improvement of high temperature oxidation of Ti–50Al by sputtering Al film and subsequent interdiffusion treatment

The high temperature oxidation resistance of Ti–50Al can be improved by sputtering an Al film and subsequent interdiffusion treatment at 600 °C for 24 h in high vacuum. In these conditions, a TiAl₃ layer is formed on the surface, which exhibits good adhesion with Ti–50Al substrate and provides high oxidation resistance. Cyclic and isothermal oxidation tests show that the Ti–50Al with 3–5 μm Al film can dramatically reduce the oxidation at 900 °C in air, at which the parabolic oxidation rate constant K_p of specimen with 5 μm Al film is only about 1/15,000 of that of bare Ti–50Al. XRD and SEM results indicate that the TiAl₃ layer can promote the formation of a protective Al₂O₃ scale on the surface as well as react with γ-TiAl to form TiAl₂ during the oxidation. Simultaneously, layers of Al₂O₃/TiAl₂/Al-enriched γ-TiAl/Ti–50Al are also formed on specimens. The TiAl₂ layer thickness will decrease gradually with increasing the oxidation time. After oxidation at 900 °C for 300 h, there is a clearly discontinuous thin layer of Ti₃₇Al₅₃O₁₀ compound observed in between Al₂O₃ and TiAl₂.     

Microstructure and high temperature oxidation behavior of hot-dip aluminized coating on high silicon ductile iron

High silicon ductile iron was coated by hot-dipping into an Al molten bath. The oxidation behavior of the aluminized alloy and the bare substrate was studied in air at 750 °C. The results showed that the coating layers consisted of three layers, in the sequence of Al, Fe-Al intermetallic and Si pile-up layers from the external topcoat to the substrate. The intermetallic layer was composed of outer FeAl₃ and inner Fe₂Al₅. The outer rod-shaped FeAl₃ dispersed in the aluminum topcoat, while the inner tongue-like Fe₂Al₅ formed in the metallic layer becoming the major phase in the aluminide coating layer. Those three layers of aluminum, Fe₂Al₅ and silicon pile-up layer exhibited hardness of HV 50, HV 1100 and HV 450, respectively. In this study, when the as-coated specimens were examined, Fe-Al-Si compounds could not be found. But the silicon pile-up at the interface between the substrate and the Fe-Al intermetallic layer could be seen in all the as-coated specimens. The graphite nodules were noticed in the substrate. The presence of graphite nodules in the substrate might be markers of hot-dipping. After hot-dipping in Al all the specimens contained graphite nodules in the aluminide layer.The oxidized graphite nodules resulted in cracks propagating in aluminide coating. Even though graphite nodules meant the existence of crack in the aluminide coating, the high temperature oxidation experiments indicated that the aluminide coating could prevent the oxidation of substrate effectively even at 750 °C.     

Preparation and oxidation behavior of NiCrAlYSi coating on a cobalt-base superalloy K40S

Arc ion plating had been employed on a cobalt-base superalloy K40S to deposit a NiCrAlYSi coating to improve its oxidation resistance at 1323-1423 K in air. The K40S superalloy had poor oxidation resistance because a non-protective and easy spalling surface oxides scale mixed of Cr₂O₃ and CoCr₂O₄ was formed on its surface. After coated with NiCrAlYSi coating, a dense and protective α-Al₂O₃ scale was formed on the coating and excellently improved its oxidation resistance. Inter-diffusion obviously occurred between the coating and the substrate K40S superalloy in oxidation process, which resulted from Co atoms in K40S outwards diffused. A richen Cr and W carbides inter-diffusion layer was formed, which could acted as a diffusion barrier that barred Al atoms in coating inwards diffusion. Though the NiCrAlYSi changed into NiCoCrAlYSi during oxidation process, it still possessed a good oxidation resistance and had a considerable long-term life.     

Oxidation Behavior of Ti--50Al Intermetallics with Thin TiAl3 Film at 1000°C

Isothermal oxidation tests at 1000°C in air indicate that the Ti--50Al alloy with about 8 m TiAl₃ layer on the surface can resist the oxidation for 10 hr. From the FESEM and EPMA/EDS results, the rapid oxidation behavior is attributed to the formation of oxide nodules through the protective Al₂O₃ and TiAl₂ layers on the outer surface. Upon increasing the oxidation time at 1000°C, the size and the number of oxide nodules increase. After 3 hr of oxidation at 1000°C, a laminated layer is formed in between the oxide nodule and substrate, which consists of two nearly parallel phases. The EDS results suggest that these two phases are Ti--Al--O compounds. After 20 hr oxidation, the oxidation nodules and laminated layers disappear and a complex oxide scale is formed which is similar to the bare Ti--50Al oxidized at 1000°C.     

Oxidation and Sulfidation Behavior of AlTiN-Coated Ti–46.7Al–1.9W–0.5Si Intermetallic with CrN and NbN Diffusion Barriers at 850°C

Attempts have been made to improve the high-temperature corrosion behavior of an intermetallic alloy, Ti–46.7Al–1.9W–0.5Si, in an H₂/H₂S/H₂O atmosphere at 850°C using AlTiN coating with and without CrN and NbN diffusion barriers. The oxidation and sulfidation behavior of the uncoated Ti–46.7Al–1.9W–0.5Si alloy followed protective kinetics with a parabolic rate constant of 6×10^–11 g²/cm⁴/s. A multi-layered scale developed: an outer rutile (TiO₂) layer, a continuous layer of -Al₂O₃ beneath the rutile layer, and an inner TiS layer, in which pure W was scattered. Fast outward diffusion of Ti within the substrate resulted in the formation of a zone of high concentration of aluminum (TiAl₃ and TiAl₂) between the scale and substrate.The use of an AlTiN coating greatly increased the oxidation and sulfidation resistance of Ti–46.7Al–1.9W–0.5Si. The use of NbN and CrN diffusion barriers further enhanced its corrosion resistance. The protection of the double-layer coatings persisted even after 240 hr exposure. However the mismatch of thermal expansion coefficients between the coating and substrate led to the development of cracks in some locations within the coatings. A 2.5 m thick AlTiN coating on the Ti–46.7Al–1.9W–0.5Si substrate with an embedded defect was modeled using the general finite element (FE) program ABAQUS. The modeling results showed rapid mode I failure of the coating at a temperature of 774°C. The through-fracture of the nitride film caused the nitride coating to shrink back leading to delamination around the crack in the nitride coating. The cracks formed acted as diffusion paths, for the ingress of oxygen and sulfur species and the outward diffusion of substrate elements, which resulted in the formation of nodular corrosion products with similar morphologies and microstructures to the uncoated alloy in those locations where cracks developed.     

Microstructure and cyclic oxidation behavior of hot dip aluminized coating on Ni-base superalloy Inconel 718

Ni-base superalloy In-718 was coated by hot-dipping into a molten bath containing Al-7wt.%Si. Cyclic oxidation experiments on bare substrate and aluminized alloy were conducted at 1100 °C, covering 240 cycles in static air. After hot dip treatment the coating layers consisted of two phases Al and FeAlSi were detected in the external topcoat to the aluminide/alloy substrate. After oxidation testing, a continuous alumina scale was detected on the surface of the aluminide layer. This coating shows better cyclic oxidation resistance for In-718 alloy than untreated substrate. Cr₂O₃ was found to be the primary oxide phase in the oxidation of bare In-718 alloy. The inward diffusion of Al in the aluminide layer was restricted by the interdiffusion zone. The NiAl phase constituent of the aluminide layer was similar through all of the testing. Only the γ phase could be found below the coating surface and in the subsurface region as aluminum was lost to form the oxide.     

Pack Cementation Coatings for High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450???m each, the titanized coating consisted of two layers with a thickness of 100???m each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200???m each. The XRD investigation of the coatings showed that the aluminized coating consisted of Al₂O₃, AlCr₂, FeAl, and Fe₃Al phases; the titanized layers contained TiO₂, Ni₃Ti, FeNi, and Fe₂TiO₅ phases; and the two-step coating contained AlNi, Ti₃Al, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050?°C for 100?h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones.     

Preparation of TiAl<Subscript>3</Subscript>-Al Composite Coating by Cold Spray and Its High Temperature Oxidation Behavior

A novel TiAl₃-Al coating was prepared by cold spray for high temperature protection of titanium aluminum-based alloy. The substrate alloy was orthorhombic-Ti-22Al-26Nb (at.%). The composite coating was mainly composed of TiAl₃ embedded in the matrix of residual aluminum. An interlayer about 10 μm was formed between the coating and the substrate. The oxidation test indicated that this composite coating was very effective in improving the high-temperature oxidation resistance of the substrate alloy at 950 °C in the tested 150 cycles without any sign of degradation. The microstructure analysis of the oxidized composite coating showed that an Al₂O₃ scale with a complex structure can be formed outside the interlayer during oxidation and no oxides beneath the interlayer were detected, which indicated that the complex continuous Al₂O₃ and the interlayer provide the protection of the substrate at high-temperature oxidation condition.     

Effect of lamellar microstructure on oxidation kinetics of Fe3Al sintered by hot isostatic pressing

The present paper focuses on the investigation of the relationship between microstructure of Fe₃Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe₃Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe₃Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al₂O₃ stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al₂O₃. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe₃Al samples are supposed to be the main factors that determine observed peculiarities after Fe₃Al oxidation at 900 °C and 1000 °C.     

High-temperature oxidation behavior of pure Ni3Al

The high-temperature oxidation behaviour of pure Ni₃Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni₃Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni₃Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni₃Al was oxidized at 1000°C, Al₂O₃ was present as -Al₂O₃ in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable -Al₂O₃ to stable -Al₂O₃ was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of -Al₂O₃ was observed beneath a thin and fine-grain outer layer of -Al₃O₃. The oxidation mechanism of pure Ni₃Al is described.     

Influence of pre-oxidation on the hot corrosion of Ti3SiC2 in the mixture of Na2SO4-NaCl melts

Polycrystalline Ti₃SiC₂ suffered from serious hot corrosion attack in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C. In order to improve the hot corrosion resistance of this material, pre-oxidation treatment was conducted at 1200 °C in air for 2 h. A duplex oxide scale with an outer layer of TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed during the pre-oxidation. Because the outer oxide layer of the pre-oxidation treated specimens could inhibit hot corrosion process, they exhibited good hot corrosion resistance in the mixture of 75wt.%Na₂SO₄ + 25wt.%NaCl melts at 850 °C for 50 h. However, during the hot corrosion the outer layer of TiO₂ would degrade gradually. Once the outer layer damaged, the hot corrosion rate increased sharply, the corrosion behavior was similar to Ti₃SiC₂ corroded under the same conditions. The microstructure and phase compositions of the hot corrosion samples were investigated by SEM/EDS and XRD.