The effects of alloy microstructure refinement on the short-term thermal oxidation of NiCoCrAlY alloys

Three γ + β NiCoCrAlY alloys (a cast alloy, a laser-surface-melted (LSM) alloy, and a coating as deposited by electron beam-physical vapor deposition (EB-PVD)) with similar average composition (Ni-20Co-19Cr-24Al-0.2Y in at. pct), but with different microstructures prior to oxidation, were oxidized for 0.5 and 1 hours at 1373 K in an Ar 20 vol pct O₂ atmosphere (i.e., at a partial oxygen pressure of 20 kPa). It was found that on the alloy with β precipitates larger than 20 μm, the oxide layer was nonuniform in thickness, and had a laterally inhomogeneous composition and phase constitution. In this case, the oxide layer developed on top of the γ phase was thicker than that formed on top of the β phase and consisted of a NiCr₂O₄/Cr₂O₃ outer and an α-Al₂O₃ inner layer. For the thinner oxide formed on top of the β phase, the outer layer was constituted of a Cr and Co containing NiAl₂O₄ spinel and the inner layer also consisted of α-Al₂O₃. For the alloys with β precipitates smaller than 3 μm, a uniform and laterally homogeneous oxide formed, consisting of a Cr and Co containing NiAl₂O₄ outer layer on top of an α-Al₂O₃ inner layer. After oxidation, Y was distributed as numerous, small precipitates within the oxide layer for a homogeneous Y distribution prior to oxidation, or as a few, very large pegs along the γ/β phase boundaries of the alloy for an inhomogeneous Y distribution prior to oxidation. The performance of the alloys upon thermal cycling was improved for smaller β precipitates and for a more homogeneous Y distribution in the alloy prior to oxidation.     

Effects of preoxidation on the nucleation and growth behavior of chemically vapor-deposited α-Al2O3 on a single-crystal Ni-based superalloy

A chemical vapor deposition (CVD) procedure was developed for preparing a high-quality α-Al₂O₃ coating layer on the surface of a single-crystal Ni-based superalloy using AlCl₃, CO₂ and H₂ as precursors. A critical part of this procedure was a short-time preoxidation step (1 min) with CO₂ and H₂ in the CVD chamber, prior to introducing the AlCl₃ precursor. Without this preoxidation step, extensive whisker formation was observed on the alloy surface. Characterization results showed that the preoxidation step resulted in the formation of a continuous oxide layer (∼50 nm) on the alloy surface. The outer part of this layer (∼20 nm) appeared to contain mixed oxides, whereas the inner part (∼30 nm) mainly consisted of α-Al₂O₃ grains with θ-Al₂O₃ as a minor phase. We observed that the nucleation of α-Al₂O₃ in the preoxidized layer was promoted by (1) rapid heating (10 seconds) of the alloy surface to the temperature region where α-Al₂O₃ was expected to nucleate; (2) the low oxygen pressure environment of the preoxidation step, which kept the rate of oxidation low; and (3) contamination of the reactor chamber with HfCl₄. The preoxidized layer served as an effective diffusion barrier for mitigating the interaction with some of the alloying elements such as Co and Cr with the CVD precursors and eliminating whisker formation on the alloy surface.     

Effects of preoxidation on the nucleation and growth behavior of chemically vapor-deposited α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> on a single-crystal Ni-based superalloy

A chemical vapor deposition (CVD) procedure was developed for preparing a high-quality α-Al₂O₃ coating layer on the surface of a single-crystal Ni-based superalloy using AlCl₃, CO₂ and H₂ as precursors. A critical part of this procedure was a short-time preoxidation step (1 min) with CO₂ and H₂ in the CVD chamber, prior to introducing the AlCl₃ precursor. Without this preoxidation step, extensive whisker formation was observed on the alloy surface. Characterization results showed that the preoxidation step resulted in the formation of a continuous oxide layer (∼50 nm) on the alloy surface. The outer part of this layer (∼20 nm) appeared to contain mixed oxides, whereas the inner part (∼30 nm) mainly consisted of α-Al₂O₃ grains with θ-Al₂O₃ as a minor phase. We observed that the nucleation of α-Al₂O₃ in the preoxidized layer was promoted by (1) rapid heating (10 seconds) of the alloy surface to the temperature region where α-Al₂O₃ was expected to nucleate; (2) the low oxygen pressure environment of the preoxidation step, which kept the rate of oxidation low; and (3) contamination of the reactor chamber with HfCl₄. The preoxidized layer served as an effective diffusion barrier for mitigating the interaction with some of the alloying elements such as Co and Cr with the CVD precursors and eliminating whisker formation on the alloy surface. L.M. HE, formerly Doctoral Candidate, Dept. of Chemical, Biomedical, and Material Engineering, Stevens Institute of Technology, Hoboken, NJ 07030 Y.-F. SU, formerly Doctoral Candidate     

The codeposition of alumina and titania with copper

The codeposition of Al₂O₃ and TiO₂ from the acid copper electrolyte was investigated with respect to solution pH, particle concentration in the electrolyte, addition agents and the nature of the crystalline phase of the oxide particles. The incorporation of oxides was found possible without the need of addition agents, provided the oxide particles are of the favorable crystalline form as α-Al₂O₃ and rutile TiO₂. A technique is reported whereby the γ-Al₂O₃ particles are partially transformed to the α-form making the oxide suitable for codeposition.     

Effects of an α-Al2O3 thin film on the oxidation behavior of a single-crystal Ni-based superalloy

A ∼ 150-nm-thick coating layer consisting of α-Al₂O₃ as the major phase with a minute amount of θ-Al₂O₃ was deposited on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD). Within 0.5 hours of oxidation at 1150 °C, the resulting thermally grown oxide (TGO) formed on the coated alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became established. After 50 hours, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on the uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al₂O₃ layer suggested that the transient oxidation of the alloy surface was considerably reduced. Also, the CVD-Al₂O₃ layer significantly reduced the growth rate of the TGO and improved its spallation resistance, while slowing the internal oxidation of Ta-rich areas that were present in the superalloy as-casting defects. These results demonstrated that this thin α-Al₂O₃ coating could be used as a means of favorably altering the TGO morphology and growth kinetics for no bond coat thermal barrier coating (TBC) applications.     

Effects of an α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> thin film on the oxidation behavior of a single-crystal Ni-based superalloy

A ∼150-nm-thick coating layer consisting of α-Al₂O₃ as the major phase with a minute amount of Φ-Al₂O₃ was deposited on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD). Within 0.5 hours of oxidation at 1150°C, the resulting thermally grown oxide (TGO) formed on the coated alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became established. After 50 hours, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on the uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al₂O₃ layer suggested that the transient oxidation of the alloy surface was considerably reduced. Also, the CVD-Al₂O₃ layer significantly reduced the growth rate of the TGO and improved its spallation resistance, while slowing the internal oxidation of Ta-rich areas that were present in the superalloy as-casting defects. These results demonstrated that this thin α-Al₂O₃ coating could be used as a means of favorably altering the TGO morphology and growth kinetics for no bond coat thermal barrier coating (TBC) applications. Y.-F. SU, formerly Doctoral Candidate     

Isothermal oxidation of TiAl alloy

Isothermal oxidation behavior of Ti-48.6 at. pct Al alloy was studied in pure dry oxygen over the temperature range 850 °C to 1000 °C. The oxidation was essentially parabolic at all temperatures with significant increase in the rate at 1000 °C. Effective activation energy of 404 kJ/mol was deduced. The oxidation products were a mixture of TiO₂ (rutile) and α-Al₂O₃ at all temperatures. An external protective layer of alumina was not observed on this alloy at any of the temperatures studied. A layered structure of oxides was formed on the alloy at 1000 °C.     

The oxidation behavior of some Ni-Cr-Al alloys at high temperatures

Oxidation of ternary Ni-Cr-Al alloys containing different Cr/Al ratios has been studied in the temperature range 800° to 1300°C. Most of the studies were performed in 1 atm oxygen or air, but the oxygen pressure dependence for one of the alloys was also investigated. The experimental methods included thermogravimetric measurements of oxidation rates and studies on reacted specimens by means of X-ray diffraction, metallographic techniques, electron microprobe analysis, and electron microscopy. In general, the oxidation rates decrease faster with time than that for an ideal parabolic behavior. The major reaction products were NiO, Cr₂O₃,α-Al₂O₃, and Ni(Cr,Al)₂O₄. The relative amounts of these were a function of composition, temperature, oxygen pressure, and reaction time. The Ni-9Cr-6Al alloy has the best oxidation resistance due to the formation ofα-Al₂O₃ at all temperatures investigated. The oxidation mechanism of the alloy is discussed.     

Oxidation of TiAl intermetallic

The oxidation kinetics of TiAl intermetallic at 500–900 °C in air is studied using a gravimetric method, and the phase composition of the scale is studied using an x-ray phase analysis. At t > 600 °C, the kinetics of oxidation is described by a parabolic equation. The oxides TiO₂ (rutile), γ-Al₂O₃, α-Al₂O₃, Ti₂O₃ are found in the scale. It is shown that at the first stage the γ-Al₂O₃ and low-titanium oxides form on the sample surface at t < 70 °C. At t ≥ 850 °C, the Ti₂O₃ forms on the external surface of the scale, TiAl₃ is found in the sublayer at the alloy/scale interface. It is shown that at t ≤ 800 °C the process is controlled by oxygen diffusion. At t > 800 °C, the oxidation mechanism changes: counterdiffusion of titanium ions through interstitial sites in TiO₂ lattice occurs.     

Structure of transient oxides formed on nicrai alloys

The structure of transient scales formed on pure, Y-doped, and Zr-doped NiCrAI alloys was examined by transmission electron microscopy. Oxidation for 0.1 hour in 1100 °C air produced many of the features observed in mature α-Al₂O₃ scales, but on a much finer degree: randomly oriented 0.1 to 0.2 μm grains, dispersed porosity decreasing in size and amount toward the oxide-metal interface, and indications of strain and deformation. Other layers in the scale contained structures which preceded the random α-Al₂O₃ layer: γ-Al₂O₃, α-(Al, Cr)₂O₃, or Ni(Al, Cr)₂O₄ oxides which were composed of 0.1 μm subgrains having nearly the same crystallographic orientation. These layers were densely populated with internal precipitates and Moiré patterns. The underlying metal structures showed evidence of plastic flow (dislocations) due to growth stresses in the oxide and recovery of these interface dislocations into low energy networks. The formation of coherent layers of aluminum-depleted phases indicated the selective removal of aluminum, even for these very short times.     

Nucleation of solid aluminum on inclusion particles injected into Al-Si-Fe alloys

Systematic inoculation experiments were carried out to study the influence of various inclusions on the nucleation of the α-Al phase in Al-Si-Fe alloys at different cooling rates. The results showed that in dilute alloys, containing less than 1.5 pct Si+Fe, almost all the inclusion types have high percentages of occurrence within the α-Al phase, indicating that nucleation can be promoted on the surface of such inclusions. In a hypoeutectic Al-Si alloy containing 6.3 pct Si, the inclusion particles of MgO, TiB₂, TiC, α-Al₂O₃, and SiC become mostly inactive nucleants and are pushed to the interdendritic regions because of the dominating poisoning effect of Si. The current results were used successfully to explain the efficiency differences between the commercial grain refiners in the hypoeutectic Al-Si alloys. Silicon is observed to preferentially segregate to the liquid-Al/inclusion interfaces so as to lower the free energy of such interfaces. A theoretical analysis of the poisoning effect of Si showed that Si segregation to the liquid/nucleant interface alters the interfacial energy balance so that the catalytic efficiency of the nucleant particles is dramatically reduced. Careful analysis showed that the poisoning effect of Si in the hypoeutectic alloy is overcome when the nucleant particles have active surface characteristics, as represented by the high catalytic potencies of γ-Al₂O₃, CaO, and Al₄C₃ particles in nucleating the α-Al phase of the hypoeutectic Al-Si alloy. Although some inclusions have comparable or higher occurrence levels than TiB₂ in the α-Al phase, they cannot be used as efficient nucleants because of either their poor wettability with liquid aluminum or their chemical reactivity, which can change the alloy chemistry.     

Evolution and coarsening of Al2O3 dispersoids at 500 °C to 600 °C in a mechanically alloyed Al-Ti-O based material

The formation and coarsening of Al₂O₃ dispersoids have been investigated at 500 °C, 550 °C, and 600 °C in a mechanically alloyed (MA) extrusion of composition Al-0.35wt pct Li-1wt pct Mg-0.25wt pct C-10vol pct TiO₂ for times up to 1500 hours. In the as-extruded condition, the dispersed phases included Al₃Ti, Al₄C₃, MgO, cubic TiO (C-TiO), monoclinic TiO (M-TiO), TiO₂, and a small amount of Al₂O₃. However, numerous Al₂O₃ dispersoids (various polymorphs: η, γ, α, and δ) with “block-shaped” morphology were formed after heat treatment due to reduction of C-TiO, M-TiO, and TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed conclusively the transformation of these phases to additional Al₂O₃ and Al₃Ti. High resolution TEM showed that the α-Al₂O₃ dispersoids exhibited some lattice matching with the α-Al matrix. Coalescence of the block-shaped Al₂O₃ dispersoids occurred after heat treatment, and Al₄C₃ also became attached to them. The length and width of the block-shaped Al₂O₃ dispersoids increased by a factor of ∼1.55 between 340 and 1500 hours at 600 °C.     

Measurement and modeling of Alloy-Spinel-Corundum equilibrium in the Ni-Mn-Al-0 system at 1873 K

The oxygen content of liquid Ni-Mn alloy equilibrated with spinel solid solution, (Ni,Mn)O. (1 +x)A1₂O₃, and α-Al₂O₃ has been measured by suction sampling and inert gas fusion analysis. The corresponding oxygen potential of the three-phase system has been determined with a solid state cell incorporating (Y₂O₃)ThO₂ as the solid electrolyte and Cr + Cr₂O₃ as the reference electrode. The equilibrium composition of the spinel phase formed at the interface of the alloy and alumina crucible was obtained using EPMA. The experimental data are compared with a thermodynamic model based on the free energies of formation of end-member spinels, free energy of solution of oxygen in liquid nickel, interaction parameters, and the activities in liquid Ni-Mn alloy and spinel solid solution. Mixing properties of the spinel solid solution are derived from a cation distribution model. The computational results agree with the experimental data on oxygen concentration, potential, and composition of the spinel phase.     

Relative performance of alumina coatings prepared by micro arc oxidation and detonation gun spray on AA 6063 under plain fatigue and fretting fatigue loading

The present study compares the performance of alumina coatings prepared by two different methods (micro arc oxidation (MAO) and detonation gun (D-gun) spray) on AA 6063 (Al alloy) fatigue test samples under plain fatigue and fretting fatigue loading. While MAO coating had comparable proportions of γ-Al₂O₃ and α-Al₂O₃, D-gun sprayed coating contained γ-Al₂O₃ with minimal quantities of α-Al₂O₃. MAO coating was relatively harder than D-gun sprayed coating. As both types of coated samples were ground, they exhibited almost the same surface roughness. D-gun sprayed alumina coated samples exhibited slightly higher magnitude of surface residual compressive stress compared with MAO coated specimens. Both types of coated samples experienced almost the same friction force. D-gun spray coated samples exhibited superior plain fatigue and fretting fatigue lives compared with MAO coated specimens. This may be attributed to layered structure of the D-gun sprayed coating.     

Oxide formation at the initial stages of oxidation of a eutectic Pb-Bi alloy

The results of electron diffraction study of the oxidation of a eutectic Pb-Bi alloy during heating at various partial oxygen pressures in the gas phase are presented. It is revealed that only the oxide phases of lead form at the initial stages of oxidation, which occurs from α-PbO₂ through intermediate oxides nPbO₂ · mPbO and Pb₃O₄ to the β-PbO modification.     

Riction properties of composite materials of the system TiN - Ain. II. Effect of oxide films on friction and wear of a ceramic material — Steel system

Friction and wear are studied for materials of the system TiN — AlN preliminary oxidized at 800–1100°C. It was established that thin oxide films containing Al₂TiO₅ and α-Al₂O₃, that promote a decrease in frictional wear, form on the surface of composite materials of the system TiN — AlN. Our assumptions are confirmed that the improvement in tribological properties of TiN — AlN composites is caused by forming oxide screening layers that prevent direct contact between the ceramics and steel counter-body. At high rates (V=16 m/sec) and pressure (P=2.0 MPa) the oxide films form more rapidly. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 121–124, January–February, 2000.     

Synthesis of Nanocrystalline α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from Nanocrystalline Boehmite Derived from High Energy Ball Milling of Gibbiste

Dry milling of gibbsite has been carried out for 5 h in planetary ball mill to study the effect of mechanical activation on α-Al₂O₃ formation. Gibbsite undergoes phase transformation during milling and has resulted nanocrystalline boehmite after 5 h of milling. The average crystallite size and the BET surface area of the nanocrystalline boehmite resulted by 5 h milling of gibbsite are 8 nm and 140 m²/g, respectively. The nanocrystalline boehmite has shown reduction in the α-Al₂O₃ formation temperature as well as in the activation energy of α-Al₂O₃ formation. The average crystallite size of nanocrystalline boehmite derived α-Al₂O₃ is measured to be 100 nm by TEM analysis and the BET surface area of resulted nanocrystalline α-Al₂O₃ is 12 m²/g.     

Healing of Double-Oxide Film Defects in Commercial Purity Aluminum Melt

The possibility of the formation of bonding between the two layers of a double-oxide film defect when held in a commercial purity liquid Al alloy was investigated. The defect was modeled experimentally by maintaining two aluminum oxide layers in contact with one another in a commercial purity Al melt at 1023 K (750 °C) for times ranging from 7 minutes to 48 hours. Any changes in the composition and morphology of these layers were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results showed that the oxide layers started to bond to one another after approximately 5 hours, and the extent of the bonding increased gradually by the holding time. The bonding is suggested to form because of the transformation of γ- to α-Al₂O₃. A complete bonding formed between the layers only when the oxygen and nitrogen trapped between the two layers were consumed, after approximately 13 hours. The results also confirmed that the nitrogen within the atmosphere of an oxide film defect reacts with the surrounding Al melt to form AlN at the interface of the defect and the melt.     

Interfaces in XD processed TiB2/NiAl composites

Interfaces of TiB₂−NiAl and α-Al₂O₃−NiAl in TiB₂/NiAl composites have been investigated by analytical electron microscopy. Although no consistent crystallographic orientation relationships have been found between NiAl and TiB₂ or Al₂O₃, semicoherent interfaces between α-Al₂O₃ and NiAl have been observed by high-resolution electron microscopy (HREM) in areas where the low indexed crystallographic planes of α-Al₂O₃ aligned with that of NiAl. No semicoherent interfaces between NiAl and TiB₂ have been observed. Silicon segregation was consistently detected by X-ray energy-dispersive spectroscopy (EDS) at the TiB₂/NiAl interface region. Segregation has not been detected in the α-Al₂O₃−NiAl interface region. The segregation layer observed at the TiB₂−NiAl interface is too thin to absorb any of the thermal residual stress.