Metastable θ-Al2O3 Research Articles

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.

Influence of Pt on Oxidation Behavior of CoNiCrAlY Alloy

The microstructure, hardness and isothermal oxidation behavior of CoNiCrAlY alloy modified by addition of 3 wt% and 5 wt% Pt were investigated using scanning electron microscopy, Vickers hardness tester, X-ray diffraction and energy dispersive spectroscopy analysis. The results reveal that Pt has a significant influence on the microstructure of the experimental alloy by formation of β-(Ni,Pt)Al phase. Addition of Pt can slightly increase the hardness of the experimental alloys. Pt will accelerate the phase transformation of alumina from metastable θ-Al2O3 to stable αAl2O3, which is the main reason for the lowering oxidation rate and the formation of a protective oxide scale on the Pt-containing alloys. Furthermore, adding of Pt can improve the Al transport, which suppresses the consumption of βphase and eliminates the internal oxidation.

Internal stresses and textures of nanostructured alumina scales growing on polycrystalline Fe3Al alloy

The evolution of internal stresses in oxide scales growing on polycrystalline Fe3Al alloy in atmospheric air at 700 °C was determined using in situ energy-dispersive synchrotron X-ray diffraction. Ex situ texture analyses were performed after 5 h of oxidation at 700 °C. Under these conditions, the oxide-scale thickness, as determined by X-ray photoelectron spectroscopy, lies between 80 and 100 nm. The main phase present in the oxide scales is α-Al2O3, with minor quantities of metastable θ-Al2O3 detected in the first minutes of oxidation, as well as α-Fe2O3. α-Al2O3 grows with a weak (0001) fiber texture in the normal direction. During the initial stages of oxidation the scale develops, increasing levels of compressive stresses which later evolve to a steady state condition situated around −300 MPa.

Oxidation of TiAl3 and L12 Coatings on Ti-45Al-5Nb Alloy at 1173K

Oxidation behavior of TiAl3 and L12 coated Ti-45at%Al-5at%Nb alloys at 1173K in air was investigated using mass gain measurement, field emission scanning electron microscopy, transmission electron microscopy, electron-probe microanalysis, glow discharge optical electron spectroscopy, and glancing angle X-ray diffraction analysis. The TiAl3 formed meta-stable alumina θ-Al2O3, while the oxide scale on the L12 consisted of a duplex structure with an outer rutile TiO2 and an inner α-Al2O3. During the very initial stages of oxidation both Ti and Al in the L12 coating could be oxidized, and then the faster diffusing Ti goes out to form an outer, continuous Ti-rich oxide which covers the slow growing Al2O3. In case of the TiAl3 an outer, continuous TiO2 layer was not observed because of the smaller amount of Ti in the TiAl3. It could be concluded that the outer, Ti-rich oxide enhanced a phase transformation of Al2O3 from θ to α. To elucidate the Ti effect, Ti-vapor treated TiAl3 and Ni-50at%Al were oxidized at 1173K in air, and showed formation of an α-Al2O3, in contrast to θ-Al2O3 on their bare alloys.

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.

On the Microstructure of the Initial Oxide Grown by Controlled Annealing and Oxidation on a NiCoCrAlY Bond Coating

Different pre-annealing and pre-oxidation treatments were conducted on a dual phase γ+β Ni–21Co–18Cr–22Al–0.2Y (at.%) bond coating for 1 hr at 1373 K (i) with or without a native oxide upon heating, (ii) in two different atmospheres upon heating, and (iii) under various oxygen partial pressures (pO2) in the range of 0.1–105 Pa during oxidation. The chemical composition, structure, morphology and phase constitution of the resulting oxide layers were investigated using a range of analytical techniques. It is found that the exclusive formation of a continuous α-Al2O3 layer without the simultaneous formation of NiAl2O4 spinel was promoted for oxidation at low pO2. The formation of metastable θ-Al2O3 was suppressed for a low fraction of the β phase, coupled with a high fraction of segregated Y at the initial bond coat surface. Initial Y segregation and incorporation of Y2O3 and Y3Al5O12 within the developing oxide layer was promoted in the absence of a native oxide and for heating in an inert atmosphere. The development of protrusions (i.e. pegs) at the oxide/coating interface, as a result of the incorporation of internal Y2O3 precipitates by the inward growing oxide layer, was most pronounced upon heating in an inert atmosphere, followed by oxidation at an intermediate pO2.

Abnormal High Growth Rates of Metastable Aluminas on FeCrAl Alloys

Experimental evidence in high temperature oxidation of alumina-forming alloys has accumulated that the overall growth kinetics of the oxide scale are slower for 1000°C, where the stable α-Al2O3 phase predominates, than for 900°C where metastable γ-Al2O3 and/or θ-Al2O3 polymorphs predominate. This intriguing behaviour has been unanimously related to the substantial presence of twin boundaries and the cation vacancy network intrinsic to the metastable aluminas allowing faster diffusion than in the nearly close packed corundum structure. This paper shows that this abnormal growth rate accompanying the presence of stable alumina polymorphs in platelets or needle-like morphology is rather due to the formation of a corundum-alumina-rich compact layer from an outer metastable layer by the concomitant sintering at the intersection vertices of the platelets and secondary recrystallization in these platelets. These phenomena are illustrated from oxidation tests performed on thin FeCrAl foils in both a conventional muffle furnace (designed by AET) and thermogravimetric analysis furnace (TGA) over the temperature range of 800–1300°C using field emission scanning electron microscope (FEG-SEM), transmission electron microscope (TEM), electron probe microanalysis (EPMA), atomic force microscope (AFM), grazing incidence X-ray diffraction (GIXRD) and image analysis (IA) techniques.

TEM analysis of the growth of oxide scales at different temperatures in FeAl grade 3 intermetallic alloy

Upon the isothermal oxidation of an ODS FeAl Grade 3 intermetallic alloy, a structured oxide scale is developed between 800 and 950°C. TEM studies have revealed a three-layered structure with a top nanoequiaxed alumina, a central alumina and a bottom Fe-Al spinel. At these temperatures, aluminium outward diffusion does not seem to be suppressed neither by the spinel sub-layer nor by the yttria present in the superalloy. However, the formation of metastable θ-Al2O3 seems to be significantly hindered. On the contrary, at 1000°C, the spinel phase no longer forms but a columnar alumina layer topped up with a nanoequiaxed structure. Yttria seems then to segregate at the scale/alloy interface and to randomly coarsen to produce an Y-Al oxide phase.

Combined laser and flame surface coating of refractory ceramics: phase and microstructural characteristics

Hybrid flame spraying, combined with CO2 laser irradiation, has been developed to improve the structural integrity and performance of alumina coatings deposited on an alumina-based refractory substrate. The alumina coatings, over 400 μm in thickness, were prepared using various process parameters. The phases and microstructures of the coatings were investigated with X-ray diffraction, optical microscopy, scanning electron microscopy and an image analysis method. It was found that, with the hybrid spraying, the extent of densification was increased and the β-Al2O3 content of the coatings was decreased, with increase in laser power level and decrease in workpiece traverse velocity. A larger laser beam size could decrease and, even, eliminate the intermediate metastable γ-Al2O3, δ-Al2O3 and θ-Al2O3 phases in the corresponding coating. The above results suggest that the β-Al2O3 content of the coatings should reflect the degree of melting of the starting alumina powder. A larger laser beam would be useful for decreasing the cooling and solidifying rates of the melted alumina.

Effect of Nickel Aluminide Microcrystalline Coating on High-Temperature Oxidation Behavior of NiAl-33.5%Cr-0.5%Zr Alloy

Abstract The effect of nickel aluminide (NiAl) microcrystalline coating prepared by magnetron-sputtering on the high-temperature oxidation behavior of the NiAl-33.5Cr-0.5Zr (at%) alloy was investigated in static air from 1,000°C to 1,150°C. The additions of Cr and Zr changed the single β phase structure into the multiphase structure (β-NiAl, α-Cr, and Heusler [Ni2AlZr] phase). The NiAl-33.5Cr-0.5Zr alloy formed a nonprotective Al2O3 + Cr2O3 + ZrO2 (aluminum oxide + chromium oxide + zirconium dioxide) mixed scale and exhibited relatively large weight gains. The large weight gains were attributed to extensive internal oxidation. The sputtered NiAl microcrystalline coating remarkably improved the oxidation resistance compared to the NiAl-33.5Cr-0.5Zr alloy due to the formation of a continuous Al2O3 scale from 1,000°C to 1,150°C. The metastable θ-Al2O3 → α-Al2O3 transformation was identified by x-ray diffraction (XRD) results and a change in the morphology of scales also occurred with the transformation.

Oxidation kinetics and surface chemistry of an Fe-Cr-Al-Y alloy medium made of 12-μm diameter fibers at elevated temperatures in dry air

Fiber media composed of Fe–Cr–Al–Y alloy are being used increasingly as materials for high-temperature applications for their excellent oxidation resistance. The oxidation kinetics of Fe–Cr–Al–Y alloy fiber medium as a heat-resistant material for high-temperature applications was studied in dry air at 1073, 1188, 1255, and 1318 K. The oxidation process followed the parabolic kinetic law. The alumina-scale growth was found to be influenced by short-circuit diffusion and the presence of stresses related to oxide-scale growth. The surface of the oxide scale formed on the fiber medium was analyzed using X-ray photoelectron spectroscopy, which revealed that the outer surface of the oxide scale formed on the fiber medium composed of 12-μm diameter Fe–Cr–Al–Y alloy fibers, consisted of θ-Al2O3, α-Al2O3, and Cr-oxide. The metastable θ-Al2O3 subsequently partially transformed into the more stable α-phase following a time-temperature-transformation relationship. The surface morphology and the cross section of the oxide scale formed on the fiber medium in the temperature range 1073–1318 K in dry air, have been studied by scanning-electron spectroscopy (SEM) and focused-ion beam, respectively.

Oxidation Of AI-Cu-Fe Quasicrystals

AbstractThe oxidation behavior of i-A163Cu25Fe12 at 800°C in air was investigated by means of TGA, XRD, SEM and TEM. In the beginning a homogeneous oxide layer is formed by the subsequent growth of metastable γ-Al2O3 and Θ-Al2O3. Nucleation of the thermodynamical stable α-Al2O3 occurs at the interface oxide/quasicrystal. The following growth of α-Al2O3 through the oxide layer leads to the formation of oxide nodules. The high growth rate of the α-Al2O3 can be explained by the incorporation of copper ions. The oxidation resistance of the quasicrystal is insufficient at high temperatures, because no protective oxide layer is formed. The high temperature oxidation behavior of Al-Cu-Fe quasicrystal and the aluminides β-FeAl and β-NiAl is compared regarding the oxidation rate, the oxide phases and the concentration changes in the material due to selective oxidation of aluminum.

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 θ-Al2O3 in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable θ-Al2O3 to stable α-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 α-Al2O3 was observed beneath a thin and fine-grain outer layer of α-Al3O3. The oxidation mechanism of pure Ni3Al is described.

Thermal barrier coating life prediction model

A Co32Ni21Cr8Al0.6Y (wt.%) alloy with and without doping 3 wt.% platinum, or co-doping 3 wt.% platinum and 0.1 wt.% dysprosium was produced by arc melting. The hardness of both base alloy and composition-modified alloy was measured by using a Vickers hardness tester. Isothermal oxidation tests at 1000 °C in static air atmosphere were conducted to assess the isothermal oxidation behavior of the alloys. The microstructure and composition of the tested alloys before and after oxidation were investigated by means of X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) and back scatter detector. Results showed that platinum had significant influence on microstructure of the tested alloy by the formation of β-(Ni,Pt)Al phase. Addition of 3 wt.% platinum could slightly increase the hardness of the tested alloy. Platinum accelerated phase transformation of alumina from metastable θ-Al2O3 to stable α-Al2O3 and suppressed the consumption of β-phase. Co-doping both 3 wt.% platinum and 0.1 wt.% dysprosium induced the fastest transformation of θ- to α- alumina and the formation of a fine-grained oxide scales. The most effective reduction of oxidation rate was achieved by the Pt-Dy co-doping effects.