Long-term oxidation of an as-cast Ni<Subscript>3</Subscript>Al alloy at 900 °C and 1100 °C

The oxidation behavior of a cast nickel aluminide alloy, IC221M, was examined after long-term aging in air for up to 16,600 hours at 900 °C and 5000 hours at 1100 °C. The oxidation products were identified using X-ray diffraction and energy-dispersive X-ray (EDX) spectroscopy with multivariate statistical analysis. At 900 °C, NiO dominates the oxidation products initially, but at longer times, NiAl₂O₄ spinel and Al₂O₃ predominate and remain stable for times up to 16,600 hours. Cross-sectional observation confirmed that a continuous surface oxide that is mostly a mixture of Al₂O₃ and NiAl₂O₄ protects the base metal. In its initial stages, the oxidation process at 1100 °C is qualitatively similar to that at 900 °C but with faster kinetics. However, as aging proceeds, NiO spalls freely from the surface, and a protective continuous oxide scale does not form. The oxidation mechanism can be qualitatively understood by the selective oxidation mechanism maps developed by Giggins and Pettit. An erratum to this article is available at .     

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     

Rare earth application for heat-resisting alloys

High-temperature oxidation resistance of Al₂O₃- and Cr₂O₃-forming heat-resisting alloys with rare earths (yttrium-implanted FeCrAl, -added FeCrAl, -added FeCrAlPt alloys, Y₂O₃- or CeO₂-coated NiCrSi, yttrium- or lutetium-added NiCr and NiCrSi) was studied in oxygen at high temperatures, by mass gain measurements, mass change measurements, amount of spalled oxide, observation of surface appearance, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA) and transmission electron microscopy (TEM). After oxidation at 1573 K for 18 ks in oxygen, oxide scale on FeCrAl alloy spalled from the entire surface, however, yttrium-implanted FeCrAl alloys showed good oxide adherence. After oxidation at 1473 K for 18 ks in oxygen, mass gain of FeCrAlY alloys decreased with increasing yttrium of up to 0.1 wt.% follwed by an increase with the yttrium content, and the mass gain of FeCrAl0.005Pt0.05Y alloy with appropriate additions of platinum and yttrium was lower than that of FeCrAl0.1Y alloy. Yttrium-added FeCrAl alloys showed good oxide adherence. TEM analysis revealed that the alumina/alloy interface of FeCrAl0.005Pt0.05Y alloy showed good coherency. The scale surface of FeCrAl alloy was rough, however, those of FeCrAlY and FeCrAlPtY alloys were smooth. Cyclic oxidation of NiCrSi, Y₂O₃- or CeO₂-coated NiCrSi alloys was studied up to 10 cycles (1 cycle: 300 s) at 1523 K in oxygen. Mass change of NiCrSi alloy increased up to 3 cycles and then decreased up to 10 cycles because of oxide spallation during cooling. On the other hand, mass change of Y₂O₃- or CeO₂-coated NiCrSi alloy increased up to 10 cycles, and these alloys showed good oxide adherence. Granular Cr₂O₃ particles on Y₂O₃-coated NiCrSi alloy were in size smaller than these on CeO₂-coated NiCrSi alloy. This result suggested that oxidation rate of Y₂O₃-coated NiCrSi alloy was lower than that of CeO₂-coated NiCrSi alloy. After oxidation at 1473 and 1573 K for 18 ks in oxygen, mass gain of yttrium- or lutetium-added NiCr and NiCrSi alloys decreased. Oxide scales on NiCrSi alloy markedly spalled along with alloy grain boundaries during cooling. On the other hand, yttrium- or lutetium-added NiCrSi alloys showed good oxide adherence. Granular Cr₂O₃ particles on yttrium- or lutetium-added NiCr and NiCrSi alloys decreased in size with increasing yttrium or lutetium, and increased with increasing oxidation temperature.     

Effect of Superficially Applied Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating on High-Temperature Corrosion Behavior of Ni-Base Superalloys

Inhibitors and oxide additives have been investigated with varying success to control high-temperature corrosion. Effect of Y₂O₃ on high-temperature corrosion of Superni 718 and Superni 601 superalloys was investigated in the Na₂SO₄-60 pct V₂O₅ environment at 1173 K (900 °C) for 50 cycles. Y₂O₃ was applied as a coating on the surfaces of the specimens. Superni 601 was found to have better corrosion resistance in comparison with Superni 718 in the Na₂SO₄-60 pct V₂O₅ environment. The Y₂O₃ superficial coating was successful in decreasing the reaction rate for both the superalloys. In the oxide scale of the alloy Superni 601, Y and V were observed to coexist, thereby indicating the formation of a protective YVO₄ phase. There was a distinct presence of a protective Cr₂O₃-rich layer just above the substrate/scale interface in the alloy. Whereas Cr₂O₃ was present with Fe and Ni in the scale of Superni 718. Y₂O₃ seemed to be contributing to better adhesion of the scale, as comparatively lesser spalling was noticed in the presence of Y₂O₃.     

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     

Understanding the Magnesiothermic Reduction Mechanism of TiO<Subscript>2</Subscript> to Produce Ti

Titanium dioxide (TiO₂) powders in the mineral form of rutile were reduced to metallic and an intermediate phase via a magnesiothermic reaction in molten Mg at temperatures between 973 K and 1173 K (700 °C and 900 °C) under high-purity Ar atmosphere. The reaction behavior and pathway indicated intermediate phase formation during the magnesiothermic reduction of TiO₂ using XRD (X-ray diffraction), SEM (scanning electron microscope), and TEM (transmission electron microscope). Mg/TiO₂ = 2 resulted in various intermediate phases of oxygen containing titanium, including Ti₆O, Ti₃O, and Ti₂O, with metallic Ti present. MgTi₂O₄ ternary intermediate phases could also be observed, but they were dependent on the excess Mg present in the sample. Nevertheless, even with excessive amounts of Mg at Mg/TiO₂ = 10, complete reduction to metallic Ti could not be obtained and some Ti₆O intermediate phases were present. Although thermodynamics do not predict the formation of the MgTi₂O₄ spinel phase, detailed phase identification through XRD, SEM, and TEM showed significant amounts of this intermediate ternary phase even at excess Mg additions. Considering the stepwise reduction of TiO₂ by Mg and the pronounced amounts of MgTi₂O₄ phase observed, the rate-limiting reaction is likely the reduction of MgTi₂O₄ to the Ti_tO phase. Thus, an additional reduction step beyond thermodynamic predictions was developed.     

Oxidation of Fe-V Melts Under CO<Subscript>2</Subscript>-O<Subscript>2</Subscript> Gas Mixtures

The oxidation mechanism of liquid Fe-V alloys with V content from 5 to 20 mass pct under different oxygen partial pressures using CO₂-O₂ mixtures with CO₂ varying from 80 pct to 100 pct was investigated by thermogravimetric analysis between 1823 K and 1923 K (1550 °C and 1650 °C). The products after oxidation were identified by scanning electron microscopy energy-dispersive spectrograph and X-ray diffraction. The results indicate that the oxidation process can be divided into the following steps: an apparent incubation period, followed by a chemical reaction step with a transition step before the reaction, and diffusion as the last stage. At the initial stage, a period of slow mass increase was observed that could be attributed to possible oxygen dissolution in the liquid iron-vanadium coupled with the vaporization of V₂O. The length of this period increased with increasing temperature as well as vanadium content in the melt and decreased with increasing oxygen partial pressure of the oxidant gas. This analysis was followed by a region of chemical oxidation. The oxidation rate increased with the increase of the O₂ ratio in the CO₂-O₂ gas mixtures. During the final stage, the oxidation seemed to proceed with the diffusion of oxygen through the product layer to the reaction front. The Arrhenius activation energies for chemical reaction and diffusion were calculated, and kinetic equations for various steps were setup to describe the experimental results. The transition from one reaction mechanism to the next was described mathematically as mixed-control equations. Thus, uniform kinetic equations have been setup that could simulate the experimental results with good precision.     

Electrochemical Behavior of an Amorphous and Nanocrystalline Fe<Subscript>79</Subscript>P<Subscript>13</Subscript>Si<Subscript>5</Subscript>V<Subscript>3</Subscript> Alloy in a Damp SO<Subscript>2</Subscript>-Contaminated Atmosphere

The electrochemical behavior of amorphous and nanocrystalline soft magnetic Fe₇₉P₁₃Si₅V₃ alloy in a 0.1 M Na₂SO₄ solution has been studied. Mössbauer studies show that the electrochemical characteristics of the alloy are comparable with those of an Finemet Fe₇₇Si₁₃B₇Nb_2.1Cu_0.9 alloy, whereas the studied alloy is inexpensive and can be prepared using natural alloy ferrophosphorus containing vanadium and silicon.     

Experimental Investigation and Thermodynamic Modeling of the B<Subscript>2</Subscript>O<Subscript>3</Subscript>-FeO-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> System for Recycling of NdFeB Magnet Scrap

NdFeB magnet scrap is an alternative source of neodymium that could have a significantly lower impact on the environment than current mining and extraction processes. Neodymium can be readily oxidized in the presence of oxygen, which makes it easy to recover neodymium in oxide form. Thermochemical data and phase diagrams for neodymium oxide containing systems is, however, very limited. Thermodynamic modeling of the B₂O₃-FeO-Fe₂O₃-Nd₂O₃ system was hence performed to obtain accurate phase diagrams and thermochemical properties of the system. Key phase diagram experiments were also carried out for the FeO-Nd₂O₃ system in saturation with iron to improve the accuracy of the present modeling. The modified quasichemical model was used to describe the Gibbs energy of the liquid oxide phase. The Gibbs energy functions of the liquid phase and the solids were optimized to reproduce all available and reliable phase diagram data, and thermochemical properties of the system. Finally the optimized database was applied to calculate conditions for selective oxidation of neodymium from NdFeB magnet waste.     

The Thermodynamic Behavior of Copper in the CaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> and BaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag System

The Cu solubility was measured in the CaO-B₂O₃ and BaO-B₂O₃ slag systems to understand the dissolution mechanism of Cu in the slags. The Cu solubility had a linear relationship with oxygen partial pressure in the CaO-B₂O₃ slag system, which corresponds with previous studies. Also, the Cu solubilities in slag decreased with increasing the slag basicity, which value of slope was close to –0.5 in logarithmic form. From the results of experiment, the Cu dissolution mechanism established as follows:

Effect of small concentrations of zirconium on high temperature oxidation behaviour of Fe-15 Cr-4 Al

Effect of 1% Zr on oxidation behaviour of Fe-15 Cr-4 Al alloy under isothermal conditions in air, O₂ and O₂-10% H₂O environments in the temperature range 1000–1150°C was investigated. The effect of zirconium concentration was studied at 1 200°C in air. Oxidation rate increases with increase in zirconium concentration. Parabolic rate of oxidation was observed. Limited study on cyclic oxidation was carried out at 1150°C in air. The cycle consisted of one hour holding at isothermal temperature followed by half an hour air cooling. The oxidised samples were examined by X-Ray diffractometry, SEM, EDAX. Extensive spalling was observed in the base alloy, Fe-15 Cr-4 Al in all environments. Zirconium additions eliminated the spalling of the scale. The EDAX analysis of a spalled region shows the presence of iron and chromium while the unspalled region is aluminium rich. A common structural feature, localised formation of granules/nodules was observed in the scale of zirconium containing alloys in all the environments. The number of granules increased with increase in zirconium concentration and was observed to be a maximum in 1% Zr and also increases with increasing temperature. The observations reveal that 1% Zr alloy shows lower oxidation rate in O₂-H₂O environment under isothermal conditions. X-Ray diffraction analysis shows the additional presence of Fe₂O₃ and Cr₂O₃ in α-Al₂O₃ scale which have not been detected in the α-Al₂O₃ scale formed in other environments, air and O₂. 0.2% Zr is most effective in increasing oxidation resistance of Fe-15 Cr-4 Al alloy both under isothermal and cyclic oxidation conditions.     

High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO<Subscript>2</Subscript> as Impurities

The objective of this study was to determine the mechanisms of carburization and decarburization of alloy 617 in impure helium. To avoid the coupling of multiple gas/metal reactions that occurs in impure helium, oxidation studies were conducted in binary He + CO + CO₂ gas mixtures with CO/CO₂ ratios of 9 and 1272 in the temperature range 1123 K to 1273 K (850 °C to 1000 °C). The mechanisms were corroborated through measurements of oxidation kinetics, gas-phase analysis, and surface/bulk microstructure examination. A critical temperature corresponding to the equilibrium of the reaction 27Cr + 6CO ↔ 2Cr₂O₃ + Cr₂₃C₆ was identified to lie between 1173 K and 1223 K (900 °C and 950 °C) at CO/CO₂ ratio 9, above which decarburization of the alloy occurred via a kinetic competition between two simultaneous surface reactions: chromia formation and chromia reduction. The reduction rate exceeded the formation rate, preventing the growth of a stable chromia film until carbon in the sample was depleted. Surface and bulk carburization of the samples occurred for a CO/CO₂ ratio of 1272 at all temperatures. The surface carbide, Cr₇C₃, was metastable and nucleated due to preferential adsorption of carbon on the chromia surface. The Cr₇C₃ precipitates grew at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. The decrease in activity of chromium triggered a reaction between chromia and carbide: Cr₂O₃ + Cr₇C₃ → 9Cr+3CO, which resulted in a porous surface scale. The results show that the industrial application of the alloy 617 at T > 1173 K (900 °C) in impure helium will be limited by oxidation.     

Effect of Scandium and Chromium on the Structure and Heat Resistance of Alloys Based on γ-TiAl

It is established that microalloying of γ-titanium aluminides with scandium provides an increase in heat resistance, structure refinement and modification, and formation of a dispersion-strengthened structure with a coherent bond between the strengthening and matrix phases. Proceeding from this an improvement might be expected in strength characteristics over a wide temperature range. The effect in scandium consists in changing the ratio of Al:Ti thermodynamic activities in the direction of forming aluminum oxide at the alloy surface during oxidation as a result of the deoxidizing effect of scandium and the formation of fine oxide inclusions. As a result of this aluminum does not form oxides within the alloy. The distribution of elements within the microstructure of γ-Ti ― Al with 5%Cr after oxidation at 900°C for 300 h is studied. It is established that the surface scale layer that forms sometimes contains Cr in addition to Al and O. A diffusion mechanism is suggested for realizing the Cr-effect according to which chromium and aluminum ions participate in place of titanium ions in forming Al₂O₃ ― Cr₂O₃ scale at the metal ― air atmosphere interface.     

Oxidation mechanism of three Fe-Al alloys with and without addition of 0.1 at% yttrium at 800 °C

The isothermal oxidation behavior of Fe-10Al, Fe-15Al, Fe-20Al alloys with and without the addition of 0.1 at% Y was studied at 800 °C under 1 × 10⁵ Pa of flowing pure O₂ for 24 h. The oxidation of three Fe-Al alloys can be divided into transient state and steady state oxidation stages. The oxidation of each stage is approximately in accordance with the parabolic law. The addition of 0.1 at% Y changes the oxidation behavior obviously and leads to a significant increase of the weight gain of Fe-10Al and Fe-15Al. The scale grown on Fe-10Al is much thicker and more complicated than that grown on Fe-20Al, which is composed of an exclusive thin layer of Al₂O₃ protective film. Due to the formation of a large number of nodules, the scales grown on Fe-15Al cannot provide full protection for the alloy. Scale microstructure of the three Fe-Al-0.1Y alloys is similar to their corresponding Fe-Al alloys. However, nodules with very small size still appear on the surface of Fe-20Al-0.1Y alloy. The critical Al concentration to form an exclusive Al₂O₃ protective layer for Fe-Al binary alloy is on the borderline between 15 at%–20 at%. For Fe-Al-0.1Y alloy, the presence of 20 at% Al is not enough to inhibit the growth of nodules.     

Effect of Oxide Scale Formation on the Behaviour of Cu in Steel during High Temperature Oxidation in O2‐N2 and H2O‐N2 atmospheres

Cu enrichment at the steel‐scale interface and its migration from there was investigated during the heating of steel cast at 1200°C under various oxidizing conditions. The behaviour of Cu enrichment was found to be largely dependent on the morphology of oxide scale formed during oxidation. At the early stage of oxidation, Cu‐rich phase formed and accumulated at the steel‐scale interface in both O₂‐N₂ and H₂O‐N₂ atmosphere. However, as the oxidation proceeded, the enrichment was vastly different for each oxidizing atmosphere. In the case of O₂‐N₂ oxidation, an oxide layer was formed initially at the steel surface, but a gap was developed soon after at the steel‐scale interface and grew in size, which practically separated the scale from the steel substrate. The scale layer formed under this condition was porous. The Cu‐rich phase initially formed at the interface seemed to migrate to the scale layer, leaving no Cu‐rich phase at the interface. In the case of H₂O‐N₂ oxidation, however, the scale layer formed was dense and tightly attached to the steel surface, and the Cu rich‐phase continued to accumulate at the interface. Regarding the behaviour of the Cu‐rich phase formed at the interface, it is proposed with experimental evidences that, when a gap forms at the steel‐scale interface, it is the vaporization of Cu in the Cu‐rich phase through the gap that brings Cu to the scale.     

Oxidation Behavior of NiAl-30.75Cr-3Mo-0.25Ho Alloy at High Temperatures

The oxidation behavior of NiAl-30.75Cr-3Mo-0.25Ho alloy from 1300 to 1500 K in air atmosphere was investigated. The results reveal that oxidation resistance of the alloy is improved by the addition of Ho. At 1500 K, the oxidation kinetic curve obeys the parabolic law (n≈0.5), whereas the oxidation kinetic curve of the tested alloy follows the cubic relations (n≈0.3～0.4) from 1300 to 1450 K. An activation energy of about 261 kJ·mol-1 was determined for the tested alloy. It is found that a continuous and compact Al2O3 layer has formed on the surface of NiAl-30.75Cr-3Mo-0.25Ho alloy after oxidation 100 h at various tested temperatures. A rich-Ho solution formed on the boundaries of Cr(Mo) phase. Doped little amount of Ho in NiAl-31Cr-3Mo alloy promotes the transformation from θ-Al2O3 phase to α-Al2O3 phase, and decreases the size of Al2O3 and the crack forming in the oxidation scale prolongs the spalling time of the film at high temperature. The volatilizing oxides of Cr, Mo and the reactive element effects (REEs) make the mass gain lower than that of pure NiAl.     

Effect of water vapour partial pressure on the chromia (Cr2O3)-based scale stability

The kinetics of the Cr₂O₃-based scale oxidation and volatilisation were studied in the presence of water vapour (H₂O). A commercial Cr₂O₃-based scale forming Type 310S stainless steel was examined at the ambient pressure (0.1?MPa) and 550°C in relatively low and high H₂O-containing environments of air-10% H₂O and air-70% H₂O, respectively. The increase in the partial pressure of H₂O (p_H2O) from 10 to 70% resulted in the transition of the oxidation and volatilisation kinetics from the parabolic rate law in air-10% H₂O to the paralinear rate law in air-70% H₂O. The kinetics transition was attributed to the increase in the Cr loss rate from the base scale after coupons exposure in air-70% H₂O. The significant role of Mn alloying element in the base scale protectiveness was also discussed in the context of the Cr₂O₃-based scale stability.     

The influence of sulfides on the oxidation behavior of nickel-base alloys

The oxidation of presulfidized chromium, Ni?Cr, and Ni?Al alloys, and complex nickel base alloys was studied at 1000°C in 1.0 atm of oxygen. Sulfur-rich surface layers were produced in the pretreatment by using H₂S?H₂ mixtures. Presulfidized chromium oxidized at a rate similar to that of sulfur-free chromium. The oxidation rate of presulfidized Ni?Cr alloys was affected by sulfur only when liquid nickel sulfide was present which accelerated the oxidation rate by creating rapid diffusion paths through the Cr₂O₃ scale. The oxidation behavior of presulfidized Ni?Al alloys, with aluminum contents sufficient for the formation of a protective Al₂O₃ layer in the sulfur-free condition, was influenced by sulfur only when aluminum sulfide was formed in the presulfidation treatment which caused the Al₂O₃ scale to be porous. The oxidation behavior of nickel-base alloys containing both chromium and aluminum was insensitive to the presence of sulfides when the concentration of aluminum in the alloy was such that a protective Al₂O₃ scale was formed during oxidation of the sulfur-free alloy and aluminum sulfide was not formed in the presulfidizing treatment.