The influence of platinum on the maintenance of α-Al2O3 as a protective scale

The influence of externally located platinum on the isothermal stability of -Al₂O₃ scales formed at high temperatures has been examined. It has been observed that a nickel-base alloy forms an external scale of -Al₂O₃ during oxidation at 1200°C, but this scale breaks down isothermally, enabling a faster-growing Cr₂O₃-rich scale to develop. However, in the presence of platinum metal alongside the specimen in the furnace hot zone, the breakdown of the -Al₂O₃ scale is postponed for a substantial period of time. It appears that platinum, as the volatile species PtO₂, is incorporated into the growing -Al₂O₃ scale where it either influences the stress relief mechanism at temperature or reduces oxidation growth stress generation and thus significantly enhances the isothermal stability of the scale.     

Oxidation properties of Fe-5Cr-4Al (wt.%) alloys in oxygen at temperatures 1000°C–1320°C

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al₂O₃ in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al₂O₃ film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al₂O₃, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al₂O₃ scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al₂O₃ grains playing an essential role during growth of this protective oxide layer.     

Development,growth, and adhesion of Al2O3 on platinum-aluminum alloys

The development, growth, and adhesion of -Al₂O₃ scales on platinum-aluminum alloys containing between 0.5 and 6 wt.% aluminum have been studied at temperatures in the interval between 1000 and 1450° C. The morphologies and microstructures of the -Al₂O₃ scales were found to be influenced by the temperature, oxygen pressure, and the microstructures of the alloys. The oxidation rates of the alloys appeared to be controlled by transport of oxygen along grain boundaries in the -Al₂O₃ scales. The -Al₂O₃ scales adhered to the platinum-aluminum substrates even after extensive periods of cyclic oxidation. The good adhesion of the -Al₂O₃ may result from mechanical keying of the oxide to the alloys due to the development of irregular oxide-alloy interfaces.This work was supported by the U.S. Army Research Office, Durham, under Contract Number DAHCO 4 73 C 0021.     

Oxidation Behavior of a Single-Crystal Ni-Base Superalloy in Air—II: At 1000, 1100, and 1150°C

The oxidation behavior of a single-crystal (SC) Ni-base superalloy was studied over the temperature range from 1000–1150°C and analysed by TGA, XRD, EDAX, and SEM. The results indicated that the SC Ni-base superalloy exhibited parabolic oxidation kinetics, which were controlled by the growth of the inner -Al₂O₃ layer. A mixed scale formed on the SC Ni-base superalloy after prolonged oxidation. The scale consisted of an outer layer of spinel, a sublayer of mainly -Al₂O₃ with small amount of spinel adjoined by a very thin and even discontinuous layer of CrTaO₄-rich oxide, and an inner -Al₂O₃ layer. The inner -Al₂O₃ layer provided good protection. No internal oxides or nitrides were observed below the inner -Al₂O₃ layer after 1000 hr at 1000°C, and after 200 hr at 1100 and 1150°C.     

The early stages of development of α-Al2O3 scales on Fe-Cr-Al and Fe-Cr-Al-Y Alloys at high temperature

The development ofthe oxides on Fe-14%Cr-4%Al, Fe-27%Cr-4%Al, and similar alloys containing 0.008% Y, 0.023% Y, and 0.8% Y has been investigated during the early stages of oxidation in 1 atm oxygen at 1000 and 1200°C. In all cases, a steady-state -Al₂O₃layer is established rapidly, after some initial formation of transient oxides rich in iron and chromium. For the yttrium-free alloys the steady-state situation is achieved more rapidly for the higher chromium-containing alloy and at the higher temperature. The amount of transient oxide formed is also determined by the specimen surface topography since the development of the -Al₂O₃ layer is less rapid at the base of alloy asperities than at a flat alloy-oxide interface. Following establishment of the complete -Al₂O₃layer, the oxide develops a convoluted oxide morphology at temperature, due to high compressive growth stresses in the oxide. These arise following reaction between oxygen ions diffusing inward down the oxide grain boundaries and aluminum ions diffusing outward through the bulk oxide. This results in lateral growth of the oxide and plastic deformation and movement of the alloy in a direction parallel to the alloy-oxide interface. The addition of yttrium to the alloys promotes the selective oxidation of aluminum. Also, the yttrium is incorporated into the growing oxide where it changes the mechanism of growth, reducing the production of the high compressive growth stresses and thus the development of the convoluted oxide morphology.     

On the transient oxidation of a Ni-15Cr-6Al alloy

Stages in the development of a protective -Al₂O₃ scale on a Ni-15Cr-6Al (wt.%) alloy have been examined. It is shown that prior to the formation of a continuous -Al₂O₃ layer, a transient stage of oxidation occurs that consists of a rapid uptake of oxygen with conversion of a thin surface layer of alloy to predominantly spinel and the subsequent development of a discrete layer of Cr₂O₃. It is also shown that during the transient period of oxidation metastable phases of aluminum oxide are formed which transform to -Al₂O₃ upon incorporation into the external oxide scale.     

Beneficial Effects of Rhenium Additions on the Cyclic-Oxidation Resistance of β-NiAl + α-Cr Alloys

This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior of-NiAl + -Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in the phase and causing the interdendritic -Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the -Cr precipitates fromthe subsurface region of the alloy. The maintenance of -Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl₂O₃-rich scale.     

TEM Study of the Effect of Y on the Scale Microstructures of Cr2O3- and Al2O3-Forming Alloys

This study was undertaken to investigate and compare the effects of a yttrium addition on the oxide scale development of -Cr₂O₃- and -Al₂O₃-forming alloys under isothermal oxidation conditions. The alloys had a nominal composition (in wt.%) of Ni–30Cr, Ni–30Cr–0.5Y, Ni–30Cr–5Al, and Ni–30Cr– 5–Al–0.5Y. They were oxidized in air for 50 hr at 1000°C. The scale microstructures were characterized using cross-sectional transmission-electron microscopy combined with energy-dispersive X-ray spectroscopy. It was observed that the scale thickness decreases and the scale adherence increases due to the Y addition. The growth direction of -Cr₂O₃ scale changes from predominately outward to inward while countercurrent diffusion within -Al₂O₃ is replaced by inward diffusion due to Y modification. It is considered that the ability of Y to scavenge sulfur from the alloy and its segregation to the oxide grain boundaries primarily account for most of its beneficial effects.     

High-temperature oxidation behavior of a Ni-Cr-Al-Fe-Y alloy

A study was conducted to examine the isothermal oxidation behavior of a wrought Ni-Cr-Al-Fe-Y alloy in air at temperatures in the range of 950–1150°C. Oxidation kinetics were determined from weight-change measurements. Analytical electron microscopy, scanning electron microscopy, and x-ray diffraction were used to characterize the morphology, structure, and composition of the oxide scale. Overall oxidation of the alloy was found to follow parabolic kinetics. Under steady-state conditions, the oxidation reaction appeared to be controlled by diffusional transport in an adherent Y-modified -Al₂O₃ scale with an activation energy of about 400 KJ/mol. Yttrium was found to preferentially segregate to grain boundaries of -Al₂O₃ which maintained a fine columnar grain structure about 0.05–0.2 m in size. Based upon the results obtained, it was suggested that the role of Y was to promote the formation of a thin layer of -Al₂O₃ with improved mechanical strength.     

Improvement in the Oxidation Resistance of an Al-Deposited Fe–Cr–Al Foil by Preoxidation

The oxidation kinetics of conventional Fe–20Cr–5Al (in mass %) foil, Al-deposited foil and Al-deposited and preoxidized foil was studied at 1373 K in air. All the foils were 50-m thick and contained minor additions of rare-earth elements. The oxide scales were observed with SEM and TEM combined with EDS and were characterized with X-ray diffractometry and electron diffraction. The deposition of Al onto the foil from the vapor phase improves oxidation resistance. The details regarding this matter were reported elsewhere. The combination of the Al deposition and the subsequent preoxidation at 1173 K for 90 ks in air further increases the oxidation resistance, i.e., the smallest parabolic rate constant among the three kinds of foils, and excellent scale adherence. Preoxidation enhances the growth of -Al₂O₃, which transforms to -Al₂O₃ during subsequent oxidation. However, such -Al₂O₃ grains are much larger than those formed on the conventional foil of similar chemical composition. Small closed voids and small spinel-type, oxide particles appear in -Al₂O₃ grains with the progress of oxidation. The former is explained in terms of the volume decrease accompanying the phase transformation and the latter by the low solubility of Fe in -Al₂O₃.     

The influence of aluminum additions on the oxidation of Co-Cr alloys at 1000 and 1200°C

The isothermal oxidation of Co-Cr-Al alloys, containing 10–30 % Cr and 1 or 4.5% Al in 1 atm flowing oxygen at 1000 and 1200°C has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis, and scanning electron microscopy. The addition of 1 % Al to Co-10% Cr and Co-15% Cr has little effect on the over-all oxidation rate, although there is increased internal oxidation and the outer-inner scale thickness ratio is decreased. The oxidation rate is controlled largely by Co²⁺ ion diffusion out through the entire scale, with oxygen gas transport across voids, spinel blocking effects and doping in the inner layer probably playing subsidiary roles. With Co-15 %-Cr-1%Al, limited healing by Cr₂O₃ increases progressively with time at the alloy-oxide interface. An addition of 1 % Al to Co-30 %Cr assists the formation of an initially protective Cr₂O₃-rich surface layer by internally oxidizing, thereby allowing more of the chromium to diffuse to the surface and form an external scale. This Cr₂O₃ layer tends to lift and crack open, enabling CoO-rich scales to form on the exposed alloy. Co-15%Cr-4.5% Al produces a protective -Al₂O₃ layer on certain surface regions, sometimes with an overlying Cr₂O₃ layer and internal -Al₂O₃ particles in the underlying alloy. In other regions, rapidly growing CoO-rich nodules develop from the outset, or after early lifting and fracture of the -Al₂O₃ scale. Generally, the presence of 28% Cr and 4.5% Al is sufficient to ensure an external scale of -Al₂O₃, the chromium acting as an oxygen getter. If such scale fractures, healing is very rapid.     

Oxidation Behavior and Breakdown of an Aluminide Coating on DZ40M Alloy at High Temperatures

DZ40M alloy is a new Co-base superalloy, which is suitable for the blade material of gas turbines. In this paper, isothermal oxidation of an aluminide coating on this alloy was examined at 900–1100°C in air. It was observed that the weight gain at lower temperatures (900 and 1000°C) was greater than that at the higher temperature (1050°C), which was due to the formation of both -Al₂O₃ and -Al₂O₃ at 900 and 1000°C but only -Al₂O₃ at 1050 and 1100°C.     

The oxidation of iron-aluminum alloys

This paper briefly summarizes studies related to the oxidation of Fe-Al alloys. Emphasis is placed on oxide phase composition, morphology, and the development of protective -Al₂O₃ scales on oxidation-resistant Fe-Al alloys both with and without ternary additions.     

The effect of various oxide dispersions on the phase composition and morphology of Al2O3 scales grown on β-NiAl

A series of oxide-dispersed-NiAl alloys were oxidized in order to explore the effect of various cation dopants on the - phase transformation in the Al₂O₃ scale and the effect of phase composition on the scale microstructure. Larger ions such as Y, Zr, La, and Hf appeared to slow the- to-Al₂O₃ phase transformation, while a smaller ion, Ti, appeared to accelerate the transformation.     

18O/SIMS characterization of the growth mechanism of doped and undoped α-Al2O3

Sequential oxidation experiments at 1200°C and 1500°C using¹⁶O and >95%¹⁸O-enriched environments were conducted on undoped and Y- and Zr-doped -NiAl and FeCrAl alloys. After oxidation, samples were analyzed by SIMS sputter depth profiling. At 1200°C, a clear pattern was established where the undoped -Al₂O₃ was found to grow by the simultaneous transport of both Al and O. Zr-doped -Al₂O₃ was found to grow mainly by the inward transport of oxygen. The profiles in all cases indicate O diffusion primarily by shortcircuit pathways. Results at 1500°C (only on -NiAl) indicated a similar behavior but were less conclusive. Y and Zr were found to segregate to the oxide grain boundaries at 1200°C and 1500°C. The segregation of these dopants is believed to impede the cation transport in the -Al₂O₃ scale and thereby change the oxidation mechanism.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. II: Oxidation Kinetics and Morphology at 1000°C

The oxidation behavior of nickel-matrix/aluminum-particle composite coat–ings was studied using thermogravimetric (TG) analysis and long-term furnace exposure in air at 1000°C. The coatings were applied by the composite-electrodeposition technique and vacuum heat treated for 3 hr at 825°C prior to oxidation testing. The heat-treated coatings consisted of a two-phase mixture of (Ni)+ (Ni₃Al). During short-term exposure at 1000°C, a thin -Al₂O₃ layer developed below a matrix of spinel NiAl₂O₄, with -Al₂O₃ needles at the outer oxide surface. After 100 hr of oxidation, remnants of -Al₂O₃ are present with spinel at the surface and an inner layer of -Al₂O₃. After 1000–2000 hr, a relatively thick layer of -Al₂O₃ is found below a thin, outer spinel layer. Oxidation kinetics are controlled by the slow growth of the inner Al₂O₃ layer at short-term and intermediate exposures. At long times, an increase in mass gain is found due to oxidation at the coating–substrate interface and enhanced scale formation possibly in areas of reduced Al content. Ternary Si additions to Ni–Al composite coatings were found to have little effect on oxidation performance. Comparison of coatings with bulk Ni–Al alloys showed that low Al -alloys exhibit a healing Al₂O₃ layer after transient Ni-rich oxide growth. Higher Al alloys display Al₂O₃-controlled kinetics with low mass gain during TG analysis.     

The Effect of Yttrium on the Growth Process and Microstructure of α-Al2O3 on FeCrAl

The oxidation behavior of undoped and Y-dopedhigh-purity FeCrAl at 1200°C was investigated. Thescale-growth processes were studied using two-stage¹⁶O/¹⁸O experiments andhigh-resolution imaging SIMS. SEM, TEM, and scanning transmission electronmicroscopy-electron-dispersive X-ray spectrometry(STEM-EDS) studies were performed in order to determinethe microstructure of the oxide and the chemistry of the metal-oxide interface. An equiaxed-Al₂O₃ scale was found togrow by simultaneous outward Al and inward oxygendiffusion on the undoped FeCrAl. In contrast, a columnar-Al₂O₃ grain structure on the Y-doped FeCrAl was produced by inwardoxygen diffusion; outward Al diffusion was suppressed.Y was shown to segregate to-Al₂O₃ grain boundaries andto the metal-oxide interface.     

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.     

Oxidation Behavior of a Sputtered Ni–5Cr–5Al Nanocrystalline Coating

A NiO-forming Ni–5Cr–5Al (at.%) alloy has been developed anddeposited as a sputtered nanocrystalline coating. The oxide formation andoxidation behavior of this coating have been studied at 1000°C inair. The oxidation rate markedly decreased with time and the oxidationkinetics obeyed the fourth power law. Complex oxide scales, consisting ofNiO, NiAl₂O₄ and -Al₂O₃,were formed during 200 hr oxidation. The outer oxide layer consisted of NiOand NiAl₂O₄ and an inner oxide layer of-Al₂O₃. The sputtered Ni–5Cr–5Alnanocrystalline coating showed good oxidation resistance due to theformation of an -Al₂O₃ inner layer andexcellent adhesion of the complex oxide scales.     

The Morphology of Thermally Grown α-Al2O3 Scales on Fe-Cr-Al Alloys

The morphology and growth kinetics of protective-Al₂O₃ scales formed duringhigh-temperature oxidation of the Fe-Cr-Al andFe-Cr-Al-Y alloys were studied. Scanning electronmicroscopy observations were focused on the oxidewrinkling phenomenonand formation of interfacial cavities in the course ofoxidation of the yttrium-free alloy. In this paper,emphasis is also placed on the effects of surface preparation and yttrium additions on the scaleand scalealloy interface structures. Several features ofalumina morphology, which have not received sufficientattention previously, are discussed in the context of existing models; some criticalissues are outlined.