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.     

Effect of Oxide Growth Strain in Residual Stresses for the Deflection Test of Single Surface Oxidation of Alloys

Residual stresses developed in FeCrAlY and Ni₈₀Cr₂₀ alloys have been predicted considering growth strain and creep strain in oxide layer and creep strain in alloy or metal. Such stresses, a net compressive stress developed in oxide scales and a net tensile stress developed in alloy strip, produce deflection of a single surface oxidized specimen during high temperature isothermal oxidation. Stresses generated in these alloys and oxide scales were compared with creep deflections. Introducing oxide growth strain in the oxide scales increase the oxide stress value during the initial oxidation stage, during which creep analysis lacks prediction. Oxide stress reaches maximum value at certain oxidation time in the initial oxidation stage. After that oxidation time relaxation of oxide stress occurs considerably in later oxidation stage.     

Improved Cr2O3 adhesion by Ce ion implantation in the presence of interfacial sulfur segregation

As-polished and preoxidized Ni–20Cr alloys were Ce-implanted with a dosage of 1 × 10¹⁷ ions/cm², then subsequently oxidized at 1050 °C in air. The oxide adhesion and the extent of sulfur segregation at the oxide–alloy interface were determined, respectively, using tensile pull testing and scanning Auger microscopy with an in situ scratch device. The critical load for oxide failure was the lowest on the unimplanted Ni–20Cr, and was slightly higher on those with implantation made into a preformed oxide. Oxides that formed directly on Ce-implanted Ni–20Cr never failed under the pull test, which showed the strongest scale adhesion; however, similar amounts of interfacial sulfur, which segregated from the alloy during oxidation, were found at all interfaces. Ce additions were also found to reduce the oxidation rate and affect the extent of voids at the scale–alloy interface. It is suggested that the change in the oxide growth mechanism reduces the number of interfacial voids and, unlike Al₂O₃, these factors are more important for Cr₂O₃ scale adhesion than sulfur segregation to the scale–alloy interface.     

The oxidation of nickel—tungsten alloys

The oxidation behaviour of NiW alloys and NiWCr alloys containing up to 40 wt%W has been studied in the temperature range 900–1200°C. The parabolic rate constant for oxidation increases with increasing tungsten content in the alloy. Addition of 10 or 15%Cr causes a significant reduction in the oxidation rate.In the Ni—7·5W alloy, spherical internal oxide particle of WO₃ are formed within the alloy, whereas as the tungsten content is increased the tendency to internal oxidation diminishes but the alloy/scale interface develops a highly irregular morphology. The roughened alloy/scale interface is less marked at the higher oxidation temperatures, and also when chromium is present in the alloy. The morphology of the interface is probably related to the relatively low interdiffusion coefficient in NiW alloys.     

TEM studies of oxidized NiAl and Ni3Al cross sections

Cross sections of oxide scale/(Ni-Al) intermetallics were prepared by a new method and studied using primarily transmission electron microscopy (TEM). The cross sections were prepared by encasing an oxidized metal specimen sandwich in a low-melting-temperature zinc alloy. Observations of oxidized zirconium-doped -NiAl cross sections revealed crystallographic voids beneath an adherent Al₂O₃ scale. The oxide-metal interface was incoherent, but a high dislocation density in the metal near the interface suggested that a large tensile stress was induced by the attached oxide scale. A duplex Al₂O₃-NiAl₂O₄ scale formed on zirconium-doped and zirconium/boron-doped -Ni₃Al alloys. Additional results are presented involving oxidation mechanisms and oxide-metal interface structures.     

The mechanism of oxidation of dilute NiAl alloys at 800–1200°C

The oxidation behaviour of dilute NiAl alloys at 800–1200°C in flowing oxygen at 1 atm pressure has been studied using kinetic measurements, optical and scanning electron microscopy and electron probe micro-analysis. The oxidation rates of Ni0.5 to 4%Al alloys are greater than the corresponding values for nickel at 1000 and 1200°C, but less at 800°C. The increased rates at the higher temperatures are largely due to increases in the total cation vacancy concentration in the scale, although internal oxide formation can make a significant contribution to the oxidation rate. The decreased rates at 800°C are almost certainly due to a build-up of Al₂O₃ particles at the oxide/alloy interface. The roles played in the oxidation processes by doping, internal oxidation, blocking effects in the oxide, dissociation of NiO and gaseous transport of oxygen within the scale are considered in detail and related to the oxidation rates of the various alloys.     

Determination of Oxygen Diffusion Along Nickel/Zirconia Phase Boundaries by Internal Oxidation

Internal oxidation tests with nickel alloys that contained up to 8 at.% zirconium were carried out. All alloys were two-phase consisting of γ-Ni and the intermetallic phase Ni₅Zr. Their behavior under low oxygen partial pressures in the range of 800–1,000?°C could not be described by the Wagnerian analysis. Oxygen diffusivity along the interface nickel/monoclinic zirconia plays an important role for the rate of internal oxidation. The early stages of internal oxidation show the in situ mode where diffusion of the less noble element zirconium cannot diffuse in the matrix and is oxidized instantly. Later in the process the mode shifts from in situ towards the diffusive mode as zirconium has the possibility to diffuse. This change could also be observed as the size of the oxide particles varied with ongoing oxidation. A method for the determination of the oxygen diffusivity in nickel/monoclinic zirconia phase boundaries is presented.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

Kinetics of high temperature oxidation of chromium rich HfC reinforced cobalt based alloys

Mostly known to improve the high temperature oxidation resistance of superalloys, hafnium may also form carbides. Several per cents of Hf allow developing a dense carbide network to mechanically strengthen alloys. Here, the high temperature oxidation behaviour of three HfC containing cobalt alloys was characterised at all steps of a thermogravimetry test: heating, isothermal stage and cooling, compared with two Co–Cr–C model alloys. The five alloys were heated in synthetic air, maintained at 1200°C during 50 h and then cooled. The mass gains were plotted versus time or according to (m×dm/dt?=?Kp–m×Kv) to specify the isothermal kinetic constants, or versus temperature to determine how oxidation acts during heating and oxide spallation occurs during cooling. Compared to the ternary alloys, the oxidation of the HfC reinforced alloys starts earlier but leads to lower mass gains during heating, the isothermal oxidation is faster and oxide spallation occurs later.     

Oxidation of Powder Metallurgy (PM) Ti–48%Al–2%Cr–2%Nb–(0–1%)W Alloys Between 800 and 1000°C in Air

Three powder metallurgy (PM) TiAl alloys with a fully lamellar structure were oxidized isothermally and cyclically between 800 and 1000°C in air in order to find the effect of W on the oxidation behavior of Ti–48Al–2Cr–2Nb alloys. The alloys oxidized parabolically during isothermal oxidation. Tungsten improved the isothermal and cyclic oxidation resistance. The oxide scales consisted primarily of an outer TiO₂ layer, an intermediate Al₂O₃-rich layer, and an inner TiO₂-rich layer. The alloying elements of Cr, Nb, and W tended to segregate in the lower part of the scale owing to their thermodynamic nobility. In the vicinity of the scale/matrix interface, TiN and Ti₂AlN coexisted.     

The High temperature oxidation of iron-chromium alloys containing 0.1 wt. % cerium or cerium oxide

The effects of 0.1 wt% Ce in both metal and oxide form on the high temperature oxidation of Fe-Cr (Cr = 10, 12, 14, 16, 18, 20) have been investigated at 1000°C in a 0.1315 bar O₂/He mixture at a total pressure of 1 bar. The presence of Ce and CeO₂ markedly affects the oxidation characteristics of the tested alloys, causing rapid initial coverage with protective oxide, decreased overall rates of oxidation, modifications in scale morphologies and enhanced scale adhesion. There is a possibility of a “trade-off” between the amount of Cr in the Fe-Cr materials and the presence of Ce of CeO₂, since such additions have been shown to decrease the amount of Cr necessary to form a protective Cr₂O₃ scale on the alloy surface. The mechanisms by which the Ce or CeO₂ improve the oxidation behaviour of Fe-Cr alloys are discussed.     

Effect of Sulfur Partial Pressures on Oxidation Behavior of Fe–Ni–Cr Alloys

Fe–Ni–Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050?°C with varied sulfur partial pressures. The oxide-scale growth on Fe–Ni–Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCr₂O₄ spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe–Ni–Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed.     

A Study on Effect of Reactive and Rare Earth Element Additions on the Oxidation Behavior of Mo–Si–B System

Mo–9Si–8B–1Ti, Mo–9Si–8B–1.8Ti, Mo–9Si–8B–0.2La and Mo–9Si–8B–0.4La₂O₃ (at.%) alloys were prepared using mechanical alloying followed by hot isostatic pressing and field assisted sintering. XRD, SEM and EBSD analysis confirmed the formation of Mo solid solution, A15 and T2 phases in the alloys. Isothermal oxidation behavior of the specimens was studied in the temperature range from 750 to 1,300 °C for up to 100 h. Both the Ti and La containing alloys showed superior oxidation behavior compared to unalloyed Mo–Si–B at 900 °C at the initial periods of oxidation. Ti-added alloys suffered higher rate of weight loss at higher temperatures (1,000–1,300 °C) due to the formation of non-protective low viscosity SiO₂-TiO₂-B₂O₃ scale. La-alloyed Mo–Si–B showed superior oxidation resistance at intermediate temperatures (900 °C) as well as at higher temperatures. Enrichment of La at the oxide/alloy interface was found to be the reason for improved oxidation behavior of La-alloyed Mo–Si–B. Amongst the four materials studied, the La₂O₃ containing alloy showed the best oxidation resistance at 900 °C.     

The oxidation of Fe-Cr alloys containing an oxide dispersion or reactive metal additions

The oxidation of an iron-16% chromium alloy containing a dispersion of yttria particles and of iron-16 to 18% chromium alloys containing small additions of yttrium or zirconium has been studied at 1100 and 1200°C in 100 Torr oxygen. The yttria-containing alloy exhibited the excellent oxidation resistance usually associated with oxide-dispersion-containing alloys, having a thin, adherent, virtually iron-free scale which resisted the breakaway rapid oxidation behavior commonly found in Fe-Cr alloys in this composition range. Of the alloying additions intended to form a fine oxide dispersion during oxidation, only zirconium affected the oxidation behavior in a beneficial way, the scale on the yttrium-containing alloy being possibly less protective than that on the equivalent binary alloy.Supported by Battelle, Columbus Laboratories, Columbus, Ohio.     

Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C

It is known that the oxide scale formed on TiAl alloys is generally composed of a mixture of alumina (Al₂O₃) and titania (TiO₂). The presence of niobium changes the activities of Ti and Al and influences the kinetics of oxidation and oxide layer composition. In this work, the Ti–46Al–8Nb alloy was subjected to cyclic oxidation in air at 700 °C (for 2 and 24 h) and 800 °C (for 300 h). Scale composition was analyzed by means of different techniques including X-ray photoelectron spectroscopy, X-ray diffraction and secondary ion mass spectroscopy. The scale consisted of several layers. The outer layer was built of alumina (amorphous or with very fine grains), whereas the inner layer – mainly of titania. After a longer exposure at a higher temperature (800 °C), niobium-rich precipitates and aluminum oxide grains were detected near to the alloy/scale interface and titanium nitride was found in the inner parts of the scale. Oxidation mechanism was studied by two-stage oxidation method using oxygen-18 and oxygen-16 isotopes combined with SIMS analyses. The distribution of oxygen isotopes over the alloy/scale interface indicated mixed inward/outward diffusion at the of reacting species. The experiments using Au markers showed that after longer oxidation time the inward diffusion was a predominant transport process.     

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.     

Feinstrukturuntersuchung des Zunders von Fe-Cr-Al-Heizleiterlegierungen mit Zr-Zusatz

X-ray diffraction analyses of the oxide scales of Fe-Cr-Al heat-resisting alloys with Zr-additions X-ray diffraction analyses of oxide scales spalled during cyclic oxidation of Fe-Cr-Al heat resisting alloys with Zr-additions were performed in the temperature range of up to 1300°C. The addition of 0,2% Zr causes the formation of ZrO₂ beside the predominant Al₂O₃ in the oxide scale exhibiting both monoclinic and tetragonal structures. By means of high-temperature X-ray diffractometry it was demonstrated that the zirconium oxide shows different transformation behaviour with respect to the investigated alloy. Thermal expansion characteristics of the aluminum oxide are not influenced by the Zr-additions being important for the adherence of the scale to the alloy.     

Einfluß von HCl und Cl2 auf die Hochtemperaturkorrosion des 2 1/4 Cr 1 Mo-Stahls in Atmosphären mit hohen Sauerstoffdrücken

Influence of HCl and Cl₂ on high temperature corrosion of 2 1/4Cr 1 Mo Steel in atmospheres with high oxygen pressures The oxidation of the 2 1/4 Cr 1 Mo steel was investigated at 773 K in oxidizing He-O₂-HCl atmospheres. The addition of HCl to He-O₂ atmospheres leads to accelerated oxidation rates. Below porous and cracked oxide scales condensed chlorides are formed. At low HCl pressures 0–1000 vppm the “active oxidation” is determining the corrosion process; i.e. oxidation of evaporating chlorides within the oxide scale. For higher HCl contents 1000–3000 vppm the corrosion behaviour changes to paralinear; i.e. simultaneous parabolic oxide growth and linear mass loss by chloride evaporation.     

High temperature alloy chloridation at 850°C

The resistance of eight alloys against chloridation was tested at 850°C in Ar/Cl₂ (2.5% Cl₂) for 15 min. Pre‐oxidation treatments were performed for 1 h and 8 h at 850°C in order to produce a thin, adherent and protective oxide scale able to improve the chloridation behaviour of the tested materials. The chloridised sample morphologies were compared to the morphologies observed on the non pre‐oxidised samples. The alloys containing a large amount of iron did not exhibit any chloridation resistance, even after pre‐oxidation, and were severely damaged. The nickel based alloys gave interesting results but were also attacked by chloride, probably by the “active oxidation” mechanism. The duration of the pre‐oxidation treatment plays an important role, since the 8 h pre‐oxidation appears more beneficial than the 1 h pre‐oxidation, to delay the chloridation, probably because of the best quality of the oxide layer grown during 8 h. For the nickel based materials, the effects of chloride appear less severe than for the iron‐based alloys, but are not stopped. The “active oxidation” mechanism is proposed to be responsible for the degradation of the tested materials.     

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.