Oxidation of γ-Ni−Al−Si alloys at 1073 K

The formation and development of oxides in Ni–4Al and Ni–4Al–xSi (at.%, x=1, 3, 5) alloys at 5–9×10^–6 and 1 atm oxygen pressure at 1073 K have been studied. The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl₂O₄ and Ni₂SiO₄) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni–Al–Si alloys. In Ni–4Al alloy where no complete intermediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher oxidation rate than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO.     

Improved Oxidation Resistance of a New Aluminum-Containing Austenitic Stainless Steel at 800 °C in Air

The oxidation resistance of austenitic stainless steels modified with various aluminum contents was investigated. The weight gain per unit area is in parabolic relation to oxidation time, and the oxidation rate significantly decreases with increased aluminum content. Outer layer oxides of austenitic stainless steel transform from Cr₂O₃ to a composite oxide layer comprising Cr and Al, and more dense Al-containing oxides formed with increasing the added Al contents. Since the diffusion of element Al is enhanced and the diffusion of element Cr is inhibited, the oxides enriched in Al dramatically contribute to the improved oxidation resistance of austenitic stainless steels at high temperature. The possible oxidation mechanisms are also proposed based on microstructural observations.     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

Influence of Moisture on the oxidation of γ-TiAl

The influence of water vapour on the oxidation of Ti-50 at.% Al was studied at 900°C. Thick, well-adherent oxide scales were formed consisting of an outer TiO₂ layer and an inner heterogeneous mixture of TiO₂ and Al₂O₃. The interface between these layers is marked by large pores and Al₂O₃ particles embedded in TiO₂. No compact Al₂O₃ barrier layer was observed. The oxidation leads to formation of a two phase, oxygen enriched subsurface zone, which is Al-depleted by inner oxidation of Al to Al₂O₃. The oxidation kinetics were followed by continuous thermogravimetry. Exposure in moist oxygen leads to an accelerated attack compared with oxidation in dry oxygen. In moist oxygen the rate is linear after a short transition period. The oxidation rate is influenced by water partial pressure and oxygen partial pressure.     

High Temperature Oxidation of Hot-Dip Aluminized T92 Steels

The T92 steel plate was hot-dip aluminized, and oxidized in order to characterize the high-temperature oxidation behavior of hot-dip aluminized T92 steel. The coating consisted of Al-rich topcoat with scattered Al₃Fe grains, Al₃Fe-rich upper alloy layer with scattered (Al, Al₅Fe₂, AlFe)-grains, and Al₅Fe₂–rich lower alloy layer with scattered (Al₅Fe₂, AlFe)-grains. Oxidation at 800 °C for 20 h formed (α-Al₂O₃ scale)/(AlFe layer)/(AlFe₃ layer)/(α-Fe(Al) layer), while oxidation at 900 °C for 20 h formed (α-Al₂O₃ scale plus some Fe₂O₃)/(AlFe layer)/(AlFe₃ layer)/(α-Fe(Al) layer) from the surface. During oxidation, outward migration of all substrate elements, inward diffusion of oxygen, and back and forth diffusion of Al occurred according to concentration gradients. Also, diffusion transformed and broadened AlFe and AlFe₃ layers dissolved with some oxygen and substrate alloying elements. Hot-dip aluminizing improved the high-temperature oxidation resistance of T92 steel through preferential oxidation of Al at the surface.     

Oxidation behavior of Ti–46Al–8Nb alloy with boron and carbon addition under thermal cycling conditions

Oxidation behavior of Ti–46Al–8Nb (in at.%) alloy with boron and carbon addition under thermal cycling conditions was investigated. Oxidation of Ti–46Al–8Nb, Ti–46Al–8Nb–1B and Ti–46Al–8Nb–1B-0.25C (in at.%) alloys was carried out at 1073 K in laboratory air for 42 cycles (1 cycle, 24 h), 1008 h in total. The mass loss rates of Ti–46Al–8Nb and Ti–46Al–8Nb–1B measured during the oxidation were similar. The best oxidation resistance was found for Ti–46Al–8Nb–1B-0.25C alloy with the smallest mass change. XRD and SEM-EDS investigations showed that in all cases, the oxide scales compositions were substantially similar. The scale consisted of an outer layer built of Al₂O₃ with the presence of some amounts of TiO₂, an intermediate layer of the scale consisting of TiO₂, an inner layer composed of oxides and nitrides. Additionally, niobium rich particles at the scale/substrate interface were present. The oxidation mechanism of Ti–46Al–8Nb was studied via two-stage isothermal oxidation (24 h in ¹⁶O₂ followed by 24 h in ¹⁸O₂) at 1073 K combined with secondary neutral mass spectroscopy (SNMS). These results indicate that the oxidation mechanism depends on a mixed diffusion process, consisting of outward transport of cations and simultaneous oxygen inward transport.     

Establishment and Breakdown of α-Al2O3 Scales on Ni-Al Alloys at High Temperatures

Abstract

The isothermal oxidation behaviour of Ni-7% Al and Ni-12·5% Al in 1atm oxygen at 800, 1000 and 1200°c has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis and scanning electron microscopy. The ease of formation of an initially complete, protective, external α-Al₂O₃ scale is greater the higher the alloy aluminium content and the higher the temperature. Once developed, this external α-Al₂O₃ scale tends to fail mechanically in localised regions, the subsequent oxidation behaviour depending upon the composition of the exposed alloy. Under conditions favouring breakaway, Ni-7% Al yields rapid oxidation rates as NiO-rich nodules are formed on the exposed alloy substrate, although the rate declines later as a healing α-Al₂O₃ layer develops at their bases. If the α-Al₂O₃ fails on Ni-12·5% Al, the aluminium content of the revealed alloy is still high enough to ensure rapidproduction of a new external α-Al₂O₃ layer.     

Superior long-term oxidation resistance of Ni–Al coated TiAl alloys

A nickel aluminide coating, developed on γ-TiAl alloy by electroplating a Ni film followed by a high Al activity pack cementation, has a duplex layer structure with an outer δ-Ni₂Al₃ layer and an inner TiAl₃/TiAl₂/TiNiAl₂ layer. The coated γ-TiAl was oxidized in air for up to 36,000 ks (10,000 h) under thermal cycling between room temperature and 1173 K. A protective Al₂O₃ scale formed with little oxide exfoliation and the average oxidation amount was 37 g/m² after the 36,000 ks oxidation. During oxidation at 1173 K the outer δ-Ni₂Al₃ changed to β-NiAl with voids and then to TiNiAl₂, and the inner TiAl₃/TiAl₂/TiNiAl₂ layers to TiAl₂ and TiNiAl₂ layers and then to TiAl₂ and τ₃ layers. The voids in the outer layer were formed by the phase transformation from the δ-Ni₂Al₃ to β-NiAl during oxidation. It was found that after the 36,000 ks oxidation the higher Al contents in the inner layers were better retained than that in the outer layer.     

High-Temperature Scaling of Cu–Al and Cu–Cr–Al Alloys: An Example of a Non-Classical Third-Element Effect

The oxidation of three Cu–xCr–2Al and three Cu–xCr–4Al alloys (x ≅ 0,4,8 at.%) has been investigated at 800°C in 1 atm O₂. Oxidation of a binary Cu–Al alloy containing 2.2 at.% Al produced external scales composed mainly of copper oxides with small amounts of Al-rich oxide in the inner region, while the internal oxidation of Al was almost absent. The addition of 3.9 at.% Cr to this alloy was able to decrease the oxidation rate but was insufficient to prevent the oxidation of copper. Conversely, addition of 8.1 at.% Cr to the same binary alloy promoted the rather fast formation of a protective Al₂O₃ layer in contact with the metal substrate, with a simultaneous large decrease in the oxidation rate, producing a form of third-element effect. On the contrary, all the Cu–xCr–4Al alloys formed an internal Al₂O₃ layer after an initial stage during which all the alloy components were oxidized, so that the only effect of the presence of chromium was to decrease the duration of the fast initial stage. The third-element effect due to chromium additions to Cu–2Al is related to a transition from the formation of external scales composed of mixtures of Cu and Al oxides to the external growth of Al₂O₃–rich scales as a consequence of a thermodynamic destabilization of copper oxides associated with the formation of solid solutions between Al₂O₃ and Cr₂O₃.     

Effect of a Yttrium Coating on the Oxidation Behavior of Ni3Al (II)

A Y-coated Ni₃Al with a postheattreatment shows much better oxidation resistance than aY-coated Ni₃Al without a postheat treatment.In order to explain the effect of postheat treatment atlow oxygen pressure, postheat treatment after Y-ion plating wasperformed in flowing hydrogen as a function of time.During these treatments, the Y-coated layer was modifiedinto a (Y, Al)O-type oxide by reaction betweenY₂O₃ and Al₂O₃. The thicknessof the modified Y-layer was related to oxygen pressureand time. The Y-modified layer formed by postheattreatment acts as a barrier to the transport of oxygen.The fine (Y, Al)O-type oxide can easily relieve growth stress bypermitting easy plastic deformation, and the (Y,Al)O-type oxide layer can absorb the thermal stressdeveloped in the Al₂O₃ layer. Thetensile stress generated by the difference in thermal-expansioncoefficients of the (Y, Al)O-type oxide and theAl₂O₃ layer compensates the largecompressive stress generated by the difference inthermal-expansion coefficients of the Al₂O₃ layer andthe Ni₃Al alloy.     

Changes of an Outer β-NiAl and Inner α-Cr Coating on Ni–40 at% Cr Alloy during Oxidation at 1373K in Air

Formation mechanisms of a coating with a duplex layer, outer β-NiAl(Cr) and inner α-Cr(Ni) layer structure on a Ni–40.2 at% Cr alloy were proposed and change in the coating structure was investigated during high temperature oxidation. The Ni–40.2 at% Cr alloy was electro-plated with about 12μm Ni followed by a high Al activity pack cementation at 1073K to form a coated layer with an outer δ-Ni₂Al₃ and an inner layer containing Al more than 70at% which grew with an inward diffusion of Al. The coated Ni–40.2at% Cr alloy was oxidized at 1373K in air for up to 2592ks. It was found that at the initial stage of oxidation the as-coated layer structure changed to a two-layer, outer β-NiAl(Cr) and inner α-Cr(Ni), structure. Al contents in the α-Cr(Ni) layer was less than 0.3at%. With long term oxidation an intermediate γ-Ni(Cr, Al) layer formed between the outer and inner layers, whereas the inner α-Cr(Ni) layer became thinner and then disappeared after the 2592ks oxidation at 1373K. Coating processes and changes in the coating structure during high temperature oxidation were discussed based on diffusion and composition paths plotted on a Ni–Cr–Al phase diagram     

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.     

Effect of yttrium coating on the oxidation behavior of Ni3Al

Yttrium-coated Ni₃Al with post heat treatment has shown good high-temperature oxidation resistance. To understand the effect of the Y-coating and post heat treatment on the oxidation resistance of Ni₃Al, the specimens were coated with Y by an ion-plating method, and heat treatment was performed at low oxygen level before or after the Y-coating was applied. Performance of the Y-coated Ni₃Al was evaluated by isothermal and cyclic oxidation tests. A simple deposition of Y on Ni₃Al did not change the oxidation kinetics, but the post heat treatment after Y-ion plating significantly decreased the oxidation rate of Ni₃Al. The scale formed on Y-coated Ni₃Al with post heat treatment after Y-ion plating showed a fine and dense structure which was grain refined by the presence of a (Y, Al) O-type oxide in the scale. The coated Y layer becomes a Y-Al compound during heat treatment. The presence of the (Y, Al)O-type oxide in grain boundaries or the lattice of Al₂O₃ modify the diffusion rate of Al and oxygen, and the oxide microstructure during oxidation. Improvement of cyclic-oxidation resistance of Ni₃Al by the Y-coating occurs because the presence of (Y, Al)O-type oxide develops fine-grain oxides which can easily relieve the growth stress.     

Oxidation resistance and mechanical characterization of silicide coatings on the Nb-18Ti-14Si-9Al alloy

The Nb-Si alloys are attractive candidate for more advanced aircraft engines, however their oxidation resistances are poor. In this work, silicide coatings were prepared on the Nb-18Ti-14Si-9Al substrate, and we present the concern of this Nb-Si alloy with high Al content, and focused the modification effect of Al on NbSi₂ coatings. It is found that composition of the substrate alloy have an essential effect on coatings, which is composed of (Nb,Ti)Si₂ outer layer and (Nb,Ti)Si₂ + (Nb,Ti)₃Si₅Al₂ inner layer. Underneath inner layer, NbAl₃ is formed and surrounded by Nb₅Si₃. Beyond fracture toughness test, the coating still preserved the integrity and tightly adhered to substrate, no cracks nucleated between substrate and the coating. After oxidation at 1250 °C for 50 h, the mass gain of substrate and silicide coating is 398.85 mg/cm² and 2.34 mg/cm² respectively. The excellent oxidation resistance of the coating is proved to benefit from modification effects of high Al in the substrate.     

A new technique to study the stages of oxidation of metals using oxidation induced changes in the shape of secondary ion energy spectra

A new technique to study the oxidation process in metals based on the yield variation with oxygen dose of secondary ions whose ejection mechanisms are identified with distinctly different dynamical sputtering processes is presented. When applied to a polycrystalline copper sample the method identifies four possible oxidation stages, from adsorption into the surface layer for doses up to about 10⁴L, followed by a rapid reconstruction to a Cu₂O structure at 10⁴L, followed by a slow growth of bulk oxide up to 10⁴L and a further physisorption of oxygen or transition to a surface CuO structure at higher doses. The technique provides a significant increase in the versatility and usefulness of SIMS equipment incorporating appropriate energy analysis facilities.     

Influence of niobium additions on high-temperature-oxidation behavior of Ti3Al alloys and coatings

In this study, the oxidation properties of Ti₃Al+Nb bulk alloys, as well as IMI 829 alloy, coated with a Ti₃Al+Nb layer, have been considered. Model alloys have been prepared, with 5–25 at.% niobium contents; 50-m-thick Ti₃Al+10 at.% Nb coatings have also been deposited on IMI 829 by triode sputtering. Bulk alloys and coated substrates have been exposed to cyclic and isothermal oxidation in air between 700 and 800°C. Niobium additions generally caused the oxidation rate of Ti₃Al to decrease significantly. In all cases rutile is the main oxide formed. It is believed that the ability of niobium to dissolve in the rutile lattice, and therefore to lower the oxygen diffusion rate through the oxide layer, is a contribution to the observed oxidation resistance enhancement. The formation of niobium oxide has also been envisaged for this matter.     

Accelerated Corrosion of Fe–xCr–10Al Alloys Containing 0–20 at.% Cr Induced by Sulfur and Chlorine in a Reducing Atmosphere at 600 °C

The corrosion behavior of five Fe–xCr–Al alloys with a constant Al content of 10 at.% and Cr contents ranging from 0 at.% to 20 at.% was examined at 600 °C in a H₂–HCl–H₂S–CO₂ gas mixture providing 3.7 × 10⁻²² atm O₂, 2.4 × 10⁻¹⁴ atm Cl₂ and 3.9 × 10⁻⁹ atm S₂. All the alloys formed duplex scales containing an outermost layer of iron oxide plus an inner layer composed of mixtures of the oxides of all the alloy components. Besides, a region of internal attack of Al or Al + Cr, whose depth decreased with increasing Cr content, formed in all the alloys. The simultaneous presence of chlorine and sulfur in the gas mixture significantly accelerated the corrosion of all the alloys with respect to their oxidation in a simpler H₂–CO₂ mixture providing the same oxygen pressure, by forming thick and cracked scales. The effect was particularly large for the high-Cr alloys due to their inability to form external protective alumina scales in the present gas mixture.     

High-temperature oxidation of Al-deposited stainless-steel foils

The oxidation resistance of Al-deposited Fe–Cr–Al foils containing small amounts of La and Ce was assessed by a cyclic oxidation test with temperature varying between room temperature and 1323 K to 1423 K in static air. (1) The Al content of Fe–Cr–Al–La, Ce foils can be increased by depositing an Al layer from the vapor phase. The deposition of a 1-m-thick Al layer on both sides of the 50-m-thick foil is equivalent to a 1.5 mass% increase in the Al content. The deposited Al diffuses into the foil during heat treatment. The uniform distribution of Al is obtained by heating at 1273 K for 18 ks. (2) After the initial transition stage the oxidation follows the parabolic law until breakaway sets in. The scale consists mainly of -Al₂O₃ during the parabolic period. (3) The increase in the Al content by more than 5 mass% by the Al-deposition remarkably improves high-temperature oxidation resistance (smaller parabolic rate constant and longer protection time). (4) The Al-deposited foils have better oxidation resistance than the conventional foils with the same contents of Al and rare-earth elements. This is attributable to the different nature of the initially formed oxide on the Al-deposited foil. (5) The so-called rare-earth element effect was also observed for the Al-deposited foils. Predominant diffusion of oxygen through the Al₂O₃ scale and vacancy-sink mechanism are applicable to the present results.     

Competitive Effect of Water Vapor and Oxygen on the Oxidation of Fe–5 wt.% Al Alloy at 1073 K

The effect of oxygen on the oxidation of Fe–5wt.% Al alloy was investigated at 1073 K in N2–12.2 vol.% H₂O, O₂–12.2 vol.% H₂O, and N₂–O₂–12.2 vol.% H₂O with various amounts of oxygen. The results showed S-shaped oxidation curves that consisted of three stages: slow-incubation, rapid transition, and relatively slow oxidation. The amount of oxidation increased with increasing oxygen contents up to 0.9 vol.% O₂ and then rapidly decreased. On the oxygen-rich side, a slow incubation oxidation stage was observed and its duration increased with increasing oxygen content. The extent of oxidation decreased gradually with decreasing oxygen content from the critical value and the incubation period disappeared. In the transient period, Fe₂O₃ was formed on the lean oxygen-content side and elongated voids were formed in the outer Fe₃O₄ and FeO layer. It was suggested that the differences in the morphology of Fe₂O₃ formed on the surface affected by the dissociation and gas-transport process due to differences in oxygen partial pressure at the gas–scale interface.     

XPS study of the initial stages of oxidation of α2-Ti3Al and γ-TiAl intermetallic alloys

The initial stages of oxidation of α₂-Ti₃Al and γ-TiAl alloys at 650 °C under low oxygen pressure are shown to be characterized by three stages using XPS. Stage I is a pre-oxidation stage characterized by the adsorption and absorption of oxygen species. When the subsurface is saturated with dissolved oxygen (16 and 2 at.% on α₂-Ti₃Al and γ-TiAl, respectively), the selective oxidation of the alloy leads to the nucleation and growth of ultrathin (∼0.5 and ∼1.2 nm on α₂-Ti₃Al and γ-TiAl, respectively) alumina layers (stage II). The growth kinetics of the alumina layers is limited by the transport of Al in the alloy, leading to Al-depletion in the metallic phase underneath the oxide. When a critical concentration is reached (Ti₈₂Al₁₈ and Ti₇₅Al₂₅ on α₂-Ti₃Al and γ-TiAl, respectively), titanium oxidation occurs (stage III). Ti(III) and Ti(IV) oxide particles are formed at the surface of the still growing alumina layer.