The Initial Oxide Scale Development on a Model FeNiCrAl Alloy at 900 °C in Dry and Humid Atmosphere: A Detailed Investigation

The investigated alumina forming FeNiCrAl model alloy shows protective oxidation behavior in dry and humid environment at 900 °C. Hence, this type of alloy may replace conventional chromia forming austenitic alloys in aggressive oxidizing/reducing environments. A detailed investigation of the oxide scale development reveals a complex initial scale development. Firstly, at alloy grain boundaries, a thin Al rich oxide forms which is replaced by transient alumina platelets in dry and equiaxed α-Al₂O₃ crystallites in humid atmosphere. The scale at alloy grain centers develops via a layered scale of external chromia:Fe/Ni metal inclusions:internal alumina to a layered external spinel:internal alumina scale in dry atmosphere. In humid condition an additional oxide feature appears on the center of large alloy grains i.e. thick oxide protrusions. Despite the initially different phase compositions a continuous protective α-Al₂O₃ scale forms both atmospheres.     

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.     

Alumina Scale Formation on a Powder Metallurgical FeCrAl Alloy (Kanthal APMT) at 900–1,100 °C in Dry O2 and in O2 + H2O

A Rapidly Solidified Powder (RSP) metallurgical FeCrAl alloy, Kanthal APMT, was exposed in dry and humid O₂ for 72 h at 900–1,100 °C. The formed oxide scales were characterized using gravimetry in combination with advanced analysis techniques (SEM, EDX, TEM, XRD, AES and SIMS). The oxide scales were at all exposures composed of two-layered α-Al₂O₃ scales exhibiting a top layer of equiaxed grains and a bottom layer containing elongated grains. A Cr-rich zone, originating in the native oxide present before exposure, separated these two layers. The top α-Al₂O₃ layer is suggested to have formed by transformation of outwardly grown metastable alumina, while the inward-grown bottom α-Al₂O₃ layer had incorporated small Zr-, Hf- and Ti-rich oxide particles present in the alloy matrix. The scale also contained larger Y-rich oxide particles. Furthermore, in the temperature range studied, the presence of water vapour accelerated alloy oxidation somewhat and affected scale morphology.     

Development of Oxide Scale at 1,100 °C on Fe20Cr5Al Alloy Non-Implanted and Yttrium-Implanted

The development of the oxide scale on model Fe20Cr5Al-type alloys unmodified and containing implanted yttrium was studied in oxygen-dominated atmosphere at 1,100 °C for up to 1 h. A two-stage-oxidation exposure was applied with the use of ¹⁸O₂ as a tracer. The choice of the exposure durations ensured the possibility to follow the consecutive stages of scale development. The oxidized samples were characterized using SEM (morphology); PLS (phase composition), and SIMS (elemental distributions). The obtained results are discussed in terms of the mechanism of the development of protective α-Al₂O₃ scale and the effect of the additions on this process taking into account the necessity of distinguishing the mechanism and kinetics of the scale evolution. Similar scale evolution stages were found on both studied materials and in both cases the protective α-Al₂O₃ scale developed rapidly, already after the exposure for 3 min. Implanted yttrium appeared to have a negligible effect on the evolution of the scale. It only slightly retarded the evolution which can be attributed rather to a kinetic effect than to mechanistic one. However, the mechanical failure of the scales via formation of cracks at the asperities of convolutions occurred on the yttrium-implanted alloy but not on the non-implanted one.     

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.     

Transient Oxidation of a γ-Ni–28Cr–11Al Alloy

γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al₂O₃ scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr₂O₃ layer, and an α-Al₂O₃ layer at the oxide/alloy interface. The evolution of such an NiO/Cr₂O₃/Al₂O₃ layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr₂O₃ outer layer and a discontinuous inner layer of Al₂O₃ initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-Al₂O₃, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr₂O₃ and Al₂O₃ to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner α-Al₂O₃ layer.     

Transient of alumina oxide scale on β-NiAl coated on M38G alloy at 950 °C

The phase transformation of alumina formed during oxidation of β-NiAl coating prepared on M38G alloy by pack cementation was investigated. Oxidation experiments were conducted at 950 °C for various times from 2 to 180 min. The phase composition and microstructure of the oxide scales were investigated by using glancing angle XRD, AFM and SEM. The results showed that at the initial oxidation stage needle-like θ-Al₂O₃ was formed and then it covered the sample surface rapidly. The formation of α-Al₂O₃ grains beneath the θ-Al₂O₃ layer was favored by depletion of Al in the β-NiAl coating during oxidation. α-Al₂O₃ preferred growing on the top (ridge) of β-NiAl grains, which resulted in the formation of net-like α-Al₂O₃ inner layer. With increasing time, θ-Al₂O₃ transformed to α-Al₂O₃ gradually. After 180 min oxidation, most of θ-Al₂O₃ grains transformed into α-Al₂O₃. A mechanism of excessive voids' formation at the oxide/coating interface was also proposed in this paper.     

The Transition from Transient Oxide to Protective Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scale on Fe–Cr–Al Alloys During Heating to 1000 °C

The transition behavior of an Al-rich amorphous oxide layer to an external Al₂O₃ layer on Fe–(4, 24)Cr–(6, 10)Al (at.%) alloys was investigated during heating to 1000 °C at a heating rate of 50 °C/min, by means of in situ high-temperature X-ray diffraction measurement and TEM observation. In the alloy containing 6Al, internal amorphous Al₂O₃ was initially developed below the Al-rich amorphous surface layer. The amorphous internal precipitates transformed to be crystalline and grew laterally with time. The internal precipitates subsequently connected with each other to form a continuous α-Al₂O₃ scale. In the case of 10Al alloy, an Al-rich amorphous layer transitioned to a crystalline α-Al₂O₃ layer from the interface between transient/amorphous layers during heating. The Al₂O₃ scale developed on high Al alloys contained Fe and Cr with relatively higher contents, but that formed on low Al alloy contained low Fe and Cr. The effect of Cr on promoting an external Al₂O₃ scale formation was found to be weaker for alloys with higher Al content compared to the alloys with lower Al content, if Al₂O₃ scale was directly transitioned from the amorphous layer.     

Influence of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ni–Al Alloys

Ni–Al coating alloys, which are commonly used in gas turbine engines operating in marine environments, are highly susceptible to hot corrosion attack. The effect of alloy composition and exposure conditions on the development of a protective alumina scale, which is important for the hot corrosion resistance of the alloy, and how they affect the transition of alumina from the θ to the α polymorph have been evaluated. A series of Ni–Al model alloys with a base composition of Ni–36 at.% Al, and 5 at.% additions of Cr, Pt and Si were exposed in dry air and in air–10%H₂O at 900 °C. The presence of water vapor in the gas led to higher oxidation rates and retarded the θ- to the α-Al₂O₃ transformation. The oxidation behavior of the alloys and the alumina polymorph which formed differed depending on the alloying element considered. Additions of Cr accelerated the θ to α transformation, while Pt and Si retarded it.     

Growth Rates of Alumina Scales on Fe–Cr–Al Alloys

The growth rates of alumina scales formed on high-temperature alloys often show a strong deviation from classical parabolic kinetics, and frequently scale growth rates near to a cubic time dependence are observed. This is e.g. the case for most RE doped Fe–Cr–Al-alloys oxidized at temperatures in the range of 1000 to 1300°C. For a number of components made of Fe–Cr–Al alloys, which e.g. prevail in car catalyst carriers, gas burners or hot-gas filters, the detailed knowledge of the rate law governing scaling kinetics is an absolutely necessary requirement for a reliable prediction of the component lifetime and/or the materials application limits. It seems that for ideally gas-tight α-Al₂O₃ scales a near-cubic time dependence is more a rule than an exception, although the frequently used methods for evaluating measured oxidation data might lead to the erroneous conclusion that the scale growth is governed by a transient-oxidation stage followed by parabolic oxidation. Deviations from an ideal, near-cubic time dependence of the oxidation kinetics of the Fe–Cr–Al alloys are caused by scale cracking during thermal cycling and/or local inhomogenities in the alloy and/or the scale.     

Effects of a Steam Pre-treatment on the Formation and Transformation of Alumina Phases on Fe Aluminide Coatings

Several researchers have studied the transformation of metastable aluminas (γ- and θ-) to α-Al₂O₃ but very little is known regarding alumina scales formed under water vapour and their transformation to α-Al₂O₃. Some results have indicated that water vapour increases the oxidation rate of alumina-scale forming coatings but others have found the opposite, that is, that under water vapour the oxidation rates decrease as either transition aluminas do not form or the transformation to α-Al₂O₃ is accelerated. In addition, it was found that χ-Al₂O₃ is the only oxide that forms at the initial stages of oxidation under 100 % steam on Fe–Al coatings at 650 °C. Under these conditions, this oxide is very protective, and slowly transforms onto α-Al₂O₃. A preliminary study of the transformation of χ- to α-Al₂O₃ at 900 °C under laboratory air was carried out. χ-Al₂O₃ was generated by a steam pre-treatment on slurry Fe aluminide coatings deposited on P92.     

Rapid Formation of α-Al2O3 Scale on an Fe–Al Alloy by Pure-Metal Coatings at 900 °C

Rapid formation of an α-Al₂O₃ scale on Fe–50 at.%Al by pure metal thin coatings of Ni, Al, Ti, Cr or Fe was investigated, and the effects of those elements on Al₂O₃-scale evolution were assessed. The oxidation behavior of samples with and without coatings could be divided into two groups: the samples with/without Ni and Al, and those with Ti, Cr and Fe. The mass gains of samples coated with Al and Ni were almost the same as that of non-coated Fe–50 at.%Al alloy. The mass gains of samples coated with Ti, Cr, and Fe were much lower than that of the Fe–50 at.%Al alloy. A stable α-Al₂O₃ scale was found to develop from the beginning of oxidation on the samples coated with Ti, Cr and Fe. However metastable θ-Al₂O₃ remained after long-time oxidation of non-coated and Ni- and Al-coated samples. The direct α-Al₂O₃ scale formation on the samples with Cr or Fe coatings was speculated to be due to sympathetic nucleation of α-Al₂O₃ on the surface of Al-supersaturated Fe₂O₃ for Fe-coated sample, and composition changes from (Cr,Al)₂O₃ to (Al,Cr)₂O₃ for the Cr-coated sample. Initial formation of an oxide having a corundum structure was inferred to provide a nucleation site for precipitation of α-Al₂O₃ without prior formation of a metastable Al₂O₃ scale.     

Early-Stage Oxidation Behavior of Pt-Modified γ′-Ni3Al-Based Alloys with and without Hf Addition

The early-stage oxidation behavior in air of Pt-modified γ′-Ni₃Al-based alloys of composition (in at.%) Ni–22Al–30Pt with and without 0.5Hf was investigated in terms of oxidation kinetics, scale evolution and Al₂O₃ phase transformation. Oxidation exposures included heating to and short-term holds at 1,150 °C. Hafnium addition did not appear to affect microstructural evolution and growth rate of the oxide scales during heating to 1,150 °C; however, it was found that Hf delayed the metastable-to-α-Al₂O₃ phase transformation, thus allowing continued fast growth of oxide scale. After the transient oxidation stage of up to about 10 min (including heating time), Ni-rich metallic particles precipitated in the lower part of the metastable Al₂O₃ layer, due to a decrease in the oxygen potential resulting from scale evolution. The present results indicated that the period of oxide phase transformation was followed by the establishment of steady-state oxidation kinetics. However, the steady-state kinetics were different for the two alloy systems. Specifically, after complete phase transformation to α-Al₂O₃, rapid growth of oxide grains occurred on the Hf-free alloy; whereas, the oxide grain size remained small for the Hf-containing alloy. Such a difference of transformation and subsequent grain-growth behavior greatly affected oxide thickening kinetics.     

A TEM study of the oxide scale development in Ni-Cr-Al alloys

In the present study the isothermal oxidation behaviours of Ni-10Cr-5Al, Ni-20Cr-5Al and Ni-30Cr-5Al alloys were investigated. The alloys were oxidised in air for 50 h at 1000 °C. Analytical transmission electron microscopy was used to characterize the morphology, structure and composition of the oxide scale. The oxide formed adjacent to the alloy was α-Al₂O₃ such that the higher was the Cr content of the alloy the easier was its formation. The Ni-30Cr-5Al alloy formed a complete layer of α-Al₂O₃ in the initial stages of oxidation through ‘oxygen gettering’ by Cr. A decrease in scale thickness and an increase in scale adherence were observed with an increase in Cr content from 10 to 30 wt.%.     

Stability of External α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scales on Alloy 602 CA at 1100–1200 °C

An external ultrathin α-Al₂O₃ scale grown on the Ni-base alloy 602 CA during air oxidation at 800 °C was characterized by means of high-resolution TEM/EDX and electron diffraction. Alloy samples pre-oxidized at 800 °C were subsequently exposed at 1100, 1150 and 1200 °C for up to 100 h. Whereas the external alumina remained stable at 1100 °C, with the increasing exposure temperature, the pre-grown alumina scale tended to break down resulting in an external chromia scale accompanied by internal alumina precipitation. The transition from external to internal Al oxidation was investigated using SEM/EDX/EBSD. The critical Al depletion at the scale-alloy interface during the post-exposure at 1100–1200 °C was modeled using the CALPHAD-based thermodynamic-kinetic approach.     

The Oxidation behavior of ODS iron aluminides

Oxide-dispersed Fe-28at.% Al-2%Cr alloys were produced by a powder metallurgy technique followed by hot extrusion. A variety of stable oxides were added to the base alloy to assess the effect of these dopants on the oxidation behavior at 1200°C in air and O₂. An Al₂O₃ dispersion flattened the α-Al₂O₃ scale, but produced none of the other reactive element effects and had an adverse influence on the long-term oxidation behavior. A Y₂O₃ dispersion improved the alumina scale adhesion relative to a Zr alloy addition at 1200 and 1300°C. However, the Y₂O₃ dispersion was not as effective in improving scale adhesion in Fe₃Al as it is in FeCrAl. This inferior performance is attributed to a larger amount of interfacial void formation on ODS Fe₃Al.     

Phase Transformation Behavior of Al2O3 Scale Formed on Pt-Modified γ′-Ni3Al-Based Alloys With and Without Hf Addition

The allotropic phase transformation behavior of Al₂O₃ scale formed on Ni–22Al–30Pt (in at.%) with and without 0.5Hf was investigated during short-term (i.e., 3?min dwell) cyclic oxidation at 1,150?°C in air. Hafnium addition did not appear to affect the oxidation rate in the early oxidation cycles, but it did delay the phase transformation from the metastable θ-Al₂O₃ structure to the stable α-Al₂O₃. Small dimples, which corresponded to α-Al₂O₃ grains, started to form on the Hf-free alloy after only three oxidation cycles; whereas, no apparent morphological change of the oxide scale surface was observed on the Hf-modified alloy. The transformation to α-Al₂O₃ was found to initiate at scale/alloy interface on the Hf-free alloy, but it initiated at gas/scale interface on the Hf-modified alloy. Depth profiling using glow discharge optical emission spectroscopy revealed that Hf enriched at the scale/alloy interface due to Hf rejection associated with the formation of an Al-depleted γ-layer, which has a low Hf solubility. Higher positive strain energy due to Hf solution in the metastable Al₂O₃ was inferred to be the main contributor to the delayed the transformation.     

Preparation and oxidation resistance of a crack-free Al diffusion coating on Ti22Al26Nb

A crack-free Al diffusion coating has been developed to improve the oxidation resistance of Ti22Al26Nb. It was produced by a two-step method; an Al film was deposited on the substrate alloy by arc ion plating followed by a diffusion process conducted at 873 K in pure Ar to form the Al diffusion coating. The two-step method lowers the temperature required to form the diffusion coating, which dramatically decreases the thermal stress developed in the coating and results in it being crack-free. The oxidation resistance of the non-coated Ti22Al26Nb alloy in isothermal and cyclic tests in air at 1073 K was poor, but the coated specimens possessed excellent oxidation resistance because a protective α-Al₂O₃ scale formed. The life of the Al diffusion coating greatly depends upon the rapid initial formation of a protective Al₂O₃ scale and interdiffusion between coating and substrate. Once the stable Al₂O₃ scale has formed and the composition changes from (Ti, Nb)Al₃ into (Ti, Nb)Al₂, the coating has a long life.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.