Microstructure and High Temperature Oxidation Property of Fe–Cr–B Based Metal/Ceramic Composite Manufactured by Powder Injection Molding Process

This study investigated the microstructure and high temperature oxidation property of Fe–Cr–B metal/ceramic composite manufactured using powder injection molding process. Observations of initial microstructure showed a unique structure where α-Fe and (Cr, Fe)₂B form a continuous three-dimensional network. High temperature oxidation tests were performed at 900, 1000 and 1100 °C, for 24 h, and the oxidation weight gain according to each temperature condition was 0.13, 0.84 and 6.4 mg/cm², respectively. The oxidation results according to time at 900 and 1000 °C conditions represented parabolic curves, and at 1100 °C condition formed a rectilinear curve. Observation and phase analysis results of the oxides identified Cr₂O₃ and SiO₂ at 900 and 1000 °C. In addition to Cr₂O₃ and SiO₂, CrBO₃ and FeCr₂O₄ formed due to phase decomposition of boride were identified at 1100 °C. Based on the findings above, this study suggested the high temperature oxidation mechanism of Fe–Cr–B metal/ceramic composite manufactured using powder injection molding, and the possibility of its application as a high temperature component material was also discussed.     

Oxidation Behavior of Graded NiCrAlYRe Coatings at 900, 1000 and 1100 °C

A graded NiCrAlYRe coating was prepared by combining arc ion plating (AIP) with chemical vapor deposition (CVD) aluminizing. Quasi-isothermal oxidation tests of the graded NiCrAlYRe coating and the conventional NiCrAlYRe coating were performed in air at 900, 1000 and 1100 °C for up to 1000, 1000 and 200 h, respectively. The results showed that the graded NiCrAlYRe coating exhibited better long time oxidation resistance than the conventional NiCrAlRe coating. This favorable oxidation behavior was attributed to the rapid formation of a protective α-Al₂O₃ scale and a sufficient Al reservoir. The structures and morphologies of oxide scales varied under different oxidation conditions. θ-Al₂O₃ was observed on both coatings during oxidation at 900 °C, however, the graded coating showed more favorable conditions for θ-Al₂O₃ to grow than the conventional coating. For the graded coating, phase transformation from θ-Al₂O₃ to α-Al₂O₃ resulted in a sharp decrease in the parabolic rate constant k_p between 900 and 1,000 °C.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Metastable-Stable Phase Transformation Behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scale Formed on Fe–Ni–Al Alloys

The oxidation behavior of Ni–Fe–41.5at.%Al alloys with different Fe/Ni ratios was investigated in air at 1000 °C in order to clarify the effect of Fe on the phase transformation of Al₂O₃ scale, using in-situ high-temperature X-ray diffraction by means of synchrotron radiation. The oxidation mass gain of alloys after 25 h of oxidation generally decreased with increasing Fe content; however, the initial oxidation mass gain was significantly decreased by increasing alloy Fe content. In-situ X-ray diffraction analysis indicated that higher alloy Fe contents promoted rapid formation of the stable α-Al₂O₃, while lower Fe in the alloy maintained the metastable Al₂O₃ for longer time oxidation. The effect of Fe on promoting α-Al₂O₃ formation can be explained by the initial formation of α-Fe₂O₃, whose structure is isomorphous with α-Al₂O₃. The additional effect of Fe on the growth rate of α-Al₂O₃ is also discussed.     

Effects of Nano Metal Coatings on Growth Kinetics of α-Al2O3 Formed on Ni-50Al Alloy

The effects of pure metal coatings, including Ni, Fe and Cr, on long-term oxidation kinetics, surface morphology and structure were studied. Ni-50Al alloy and Ni-coated, Fe-coated and Cr-coated samples were pre-oxidized at 900 °C in air. They were then oxidized isothermally at 1,000 °C in air. The bare Ni-50Al alloy oxidized rapidly during the initial stage of oxidation due to the formation of θ-Al₂O₃, but the oxidation rate decreased after α-Al₂O₃ had developed. Oxidation of the Ni-coated sample was slow from the beginning of oxidation even though the θ-Al₂O₃ was predominated for a longer oxidation time. No θ-Al₂O₃ developed on the Cr and Fe-coated samples, but the oxidation rates of these samples were much faster than those of bare and Ni-coated samples. Cross-sectional images revealed that the grain size of α-Al₂O₃, which formed on Cr and Fe-coated samples, was smaller than those of bare and Ni-coated samples. These metal coatings affected the microstructure of α-Al₂O₃ and they showed a strong effect on the growth rate of α-Al₂O₃ in the steady-state oxidation stage.     

Cyclic oxidation behavior of Ni3Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025% B between 900 and 1100°C

A Ni₃Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al₂O₃, NiAl₂O₄ and ZrO₂ were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al₂O₃, ZrO₂)-containing oxides that keyed the oxide scale to the matrix alloy.     

Rapid Assessment of Oxidation Behavior in Co-Based γ/γ′ Alloys

A high-throughput, non-destructive photostimulated luminescence spectroscopy (PSLS) technique was used to analyze fifty oxidized Co-based γ/γ′ alloy samples for the presence of α-Al₂O₃. Alloys were produced by combinatorial ion-plasma deposition, and oxidation was performed at 1100 °C for 1 h in air. PSLS measurements are compared with microscopy of oxides in cross-section to relate the presence of the luminescence signal of α-Al₂O₃ with the thickness of the oxide scale. Analysis of the current dataset validates the use of PSLS as a rapid screening technique of oxidation behavior for the present materials system.     

Preparation of Self-Healing α-Al2O3 Films by Low Temperature Thermal Oxidation

This paper reports a new approach to lowering the temperature necessary for the preparation of α-Al₂O₃. Oxidation of Al–Cr alloys, with Cr contents of 18, 23 and 27 %, was performed at temperatures ranging from 620 to 720 °C in air for 100 h. The resulting oxide films were analyzed by SEM, EDS, XRD and XPS. The results showed that α-Al₂O₃ films were obtained following oxidation of the 18 and 23 wt% Cr alloy samples at 720 °C and that rough surfaces were conducive to the formation of α-Al₂O₃ such that peened surface samples showed significant α-Al₂O₃ growth while polished samples showed no oxide by XRD. A 23 wt% Cr sample with a roughened surface exhibited the formation of α-Al₂O₃ at a temperature of 670 °C. Conversely, only a very thin oxide film was observed on a 27 wt% Cr sample after oxidation at 720 °C.     

The oxidation behaviour of an aligned Ni-Al-Cr3-C2 eutectic alloy under isothermal and thermal cycling conditions

The isothermal and thermal cycling oxidation behaviour of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy at temperatures from 800° to 1200 °C in flowing air have been investigated using several physical techniques. At all temperatures an initial, protective, external layer of α-Al₂O₃ develops on the alloy surface. However, this breaks down mechanically during thermal cycling, enabling a less protective Cr₂O₃-rich scale to form. The time of retention of the α-Al₂O₃ layer at temperature decreases with increasing temperature, failing after between 30 min and 2 h at 1100° and 1200 °C. However, if platinum metal is introduced into the hot zone at these temperatures, this period is increased to about 48 h. Following formation of the external Cr₂O₃ scale, internal oxide penetration into the alloy can be considerable, involving preferential oxide penetration down the alloy/carbide fibre interfaces. Thermal cycling does not influence markedly the oxidation behaviour, although it does result in formation of a greater quantity of nickel-rich oxide nodules on the scale surface following crack development in the Cr₂O₃-rich scale. This crack development is assisted by differential thermal contraction stresses.     

Effect of pore size on the high temperature oxidation of Ni-Fe-Cr-Al porous metal

This study investigated the effect of the pore size of Ni-22.4%Fe-22%Cr-6%Al porous metal on its hightemperature oxidation. Two types of open porous metals with pore sizes of 800 μm and 580 μm were used. A 24-hour isothermal oxidation test was conducted at three different temperatures of 900 °C, 1000 °C, and 1100 °C under a 79% N₂ + 21% O₂ atmosphere. The results of the BET analysis revealed that the specific surface area increased as the pore size decreased from 800 μm to 580 μm. The high-temperature oxidation results showed that porous metals exhibited far lower levels of oxidation resistance compared with bulk metals, and that the oxidation resistance of porous metals decreased with a decreasing pore size. According to the microstructural observations of the oxide layers, the 900 °C and 1000 °C oxidation layer contained Ni, Cr, and Al oxides mainly on the strut. The 800 μm porous metal strut exhibited similar oxidation behavior at 1100 °C to that found at lower temperatures. In contrast, the 580 μm porous metal strut was found to consist of Ni and Fe oxides in the upper layer and Ni, Cr, and Al oxides in the lower layer, representing a low oxidation resistance. For powders affixed to the strut inside the porous metal, a different oxide-forming behavior from that of the strut was observed. In addition, the Ni-Fe-Cr-Al porous metal high-temperature oxidation microscopic mechanism is also discussed.     

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.     

Oxidation behaviour of the alloy IC-6 and protective coatings

In this work, NiCoCrAlY coatings were deposited on a new Ni-base alloy, IC-6. The oxidation kinetic curves of alloy IC-6, K17 and NiCoCrAlY coatings on alloy IC-6 at 900-1100 °C were obtained. The results indicated that the oxide scales consisted of α-Al₂O₃, NiAl₂O₄, NiO, as well as a small amount of NiMoO₄ and MoO₂. These scales occurred after alloy IC-6 exposure at 900 °C for 100 h. The weight loss occurred when alloy IC-6 were exposed at 1050 and 1100 °C due to the formation of volatile MoO₃. After the NiCoCrAlY coating was deposited, the scales mainly contained α-Al₂O₃, when the specimens were oxidized at 900 °C, and α-Al₂O₃and Cr₂O₃ at 1050 °C. The formation of α-Al₂O₃ and Cr₂O₃ scales on NiCoCrAlY coating was directly responsible for improving oxidation resistance of the alloy IC-6.     

Improvement of the Oxidation Resistance of the Single-Crystal Ni-Based TMS-82+ Superalloy by Ni–Al Coatings with/without the Diffusion Barrier

Oxidation behavior of the uncoated base, Ni–Al coated and Re–Cr-Ni plus Ni–Al coated single-crystal (SC) Ni-based TMS-82+ superalloy is studied under cyclic air at 900 °C for 200 h to assess the oxidation resistance. Regardless of the coating processing, Ni–Al coating is effective in improving the oxidation resistance due to the formation of a continuous α-Al₂O₃ layer in the scale. For the uncoated base superalloy, the mass-gain curves are fitted by a subparabolic relationship, and complex oxide products including predominately NiO, some CrTaO₄, α-Al₂O₃, Cr₂O₃, a minor of spinels of (Ni, Co)Al₂O₄, AlTaO₄ and θ-Al₂O₃ are detected. Time-dependence of the oxide growth rate for both coated superalloy with/without the diffusion barrier is explained by the parabolic relationship. The oxide scales consist predominately of α-Al₂O₃ and a minor of θ-Al₂O₃. The diffusion barrier of σ-phase plays a negligible effect on the oxidation resistance during the cyclic exposure environment. The amount of detrimental γ′-phase and topologically close-packed (TCP) phases in the interdiffusion zone in the coated superalloy with the diffusion barrier is greatly reduced compared with that without the diffusion barrier due to the distinct barrier effect limiting diffusion of elements between the bond-coat and the substrate.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

Microstructure and Residual Stress of Alumina Scale Formed on Ti2AlC at High Temperature in Air

Ti₂AlC ternary carbide is being explored for various high temperature applications due to its strength at high temperatures, excellent thermal-shock resistance, and high electrical conductivity. A potential advantage of Ti₂AlC over conventional Al₂O₃-forming materials is the near-identical coefficient of thermal expansion (CTE) of Ti₂AlC and α-Al₂O₃, which could result in superior spallation resistance and make Ti₂AlC a promising option for applications ranging from bondcoats for thermal barrier coatings to furnace heating elements. In this study, isothermal and cyclic oxidation were performed in air to examine the oxidation behavior of Ti₂AlC. Isothermal oxidation was performed at 1000, 1200 and 1400 °C for up to 25 h and cyclic oxidation consisted of 1,000 1-hour cycles at 1200 °C. Characteristics of the oxide scale developed in air, including mass change, residual stress in the α-Al₂O₃ scale, phase constituents and microstructure, were examined as functions of time and temperature by thermogravimetry, photostimulated luminescence, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy via focused ion beam in situ lift-out. Above a continuous and adherent α-Al₂O₃ layer, a discontinuous-transient rutile-TiO₂ scale was identified in the oxide scale developed at 1000 and 1200 °C, while a discontinuous-transient Al₂TiO₅ scale was identified at 1400 °C. The continuous α-Al₂O₃scale thickened to more than 15 μm after 25 h of isothermal oxidation at 1400 °C, and after 1,000 1-hour cycles at 1200 °C, yet remained adherent and protective. The compressive residual stress determined by photoluminescence for the α-Al₂O₃ scale remained under 0.65 GPa for the specimens oxidized up to 1400°C for 25 hours. The small magnitude of the compressive residual stress may be responsible the high spallation-resistance of the protective α-Al₂O₃ scale developed on Ti₂AlC, despite the absence of reactive element additions.     

Stability of External α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Scales on Alloy 602 CA at 1100–1200 °C

An external ultrathin α-Al₂O₃ scale grown on the Ni-base alloy 602 CA during air oxidation at 800 °C was characterized by means of high-resolution TEM/EDX and electron diffraction. Alloy samples pre-oxidized at 800 °C were subsequently exposed at 1100, 1150 and 1200 °C for up to 100 h. Whereas the external alumina remained stable at 1100 °C, with the increasing exposure temperature, the pre-grown alumina scale tended to break down resulting in an external chromia scale accompanied by internal alumina precipitation. The transition from external to internal Al oxidation was investigated using SEM/EDX/EBSD. The critical Al depletion at the scale-alloy interface during the post-exposure at 1100–1200 °C was modeled using the CALPHAD-based thermodynamic-kinetic approach.     

Transient oxidation behavior of nanocrystalline CoCrAlY coating at 1050℃

Nanocrystalline CoCrA1Y overlay coating was prepared on M38G superalloy by magnetron sputtering deposition. To investigate the oxidation behavior and phase transformation of alumina during oxidation, the oxidation experiments were conducted at 1 050 ℃ for various time in the range of 5-180 min. The phase compositions of the oxide scales were investigated by using glancing angle X-ray diffraction（XRD）. The microstructure analysis of oxide scales was carried out by means of scanning electron microscopy（SEM）. The growth process of metastable alumina at the grain boundaries and transformation to stable alumina were discussed. The results show that at the initial oxidation stage the mixture of δ-Al2O3, γ-Al2O3 and α-Al2O3 is formed on the sample surface rapidly. Especially, δ-Al2O3 and γ-Al2O3 prefer growing at the grain boundaries of CoCrA1Y coating. With increasing oxidation time, δ-Al2O3 and γ-Al2O3 transform to θ-Al2O3, afterwards θ-Al2O3 transforms to α-Al2O3 gradually. After 180 min oxidation, θ-Al2O3 transforms into α-Al2O3 completely.     

The kinetics of the oxidation of aluminum—copper alloys in oxygen at high temperature

The oxidation behavior of high purity aluminum—copper alloys has been studied at temperatures from 475 to 575°C in dry oxygen at a pressure of 76 torr. The oxidation product was found to be duplex in nature consisting of both amorphous and crystalline γ-Al₂O₃. Roughly cylindrical crystals of γ-Al₂O₃ of constant thickness, at any given temperature and alloy content, grow into the metal from the amorphous oxide—metal interface by inward diffusion of oxygen through the overlying amorphous film. The amorphous oxide film, growing by outward diffusion of metal ions, forms between the crystals of γ-Al₂O₃ with accurately parabolic kinetics throughout the temperature and alloy composition ranges. Above the crystalline phase the amorphous oxide forms at a lower rate because of the additional resistance conferred to cation egress afforded by the crystalline oxide. In general, it was found that increasing copper content of the alloy in the range 0·1–4 per cent Cu caused a reduction in the crystal nucleation density and in the depth of intrusion of the crystals into the underlying alloy. Conversely, increasing copper content was also found to increase the rate of formation of amorphous γ-Al₂O₃ between the crystals of γ-Al₂O₃ and to increase the rate of radial growth of the crystals. These results are explained in terms of the defect nature of the oxidation products.     

Oxidation of β-NiAl,undoped and doped with Ce,Y, Hf

The high temperature oxidation of β-NiAl, undoped and doped with Ce, Y and Hf was studied in situ by thermogravimetry in He with p(O₂) = 5·10⁻⁶ bar at 1000°C and by high temperature X-ray diffraction at 950 and 1000°C in air. After the in situ experiments the samples were analysed by optical microscopy and SEM with EDX. It was observed by thermogravimetry that the weight gain for β-NiAl+Hf is lower than for undoped β-NiAl, whereas the weight gain for β-NiAl+Y was similar to that for β-NiAl. For β-NiAl+Ce an enhanced increase of mass gain was observed. The in situ X-ray-experiments show that in the first hours of the oxidation process the metastable Θ-Al₂O₃ is formed, which transforms to the protective α-Al₂O₃ after different periods of time, depending on the dopant element and the temperature. The analysis of the samples shows that the ternary phases along the grain boundaries were oxidized, whereas the ternary phase precipitates within the grains remained unoxidized. The extend of the grain boundary oxidation increases from β-NiAl+Hf and β-NiAl+Y to β-NiAl+Ce.     

Effects of a Steam Pre-treatment on the Formation and Transformation of Alumina Phases on Fe Aluminide Coatings

Several researchers have studied the transformation of metastable aluminas (γ- and θ-) to α-Al₂O₃ but very little is known regarding alumina scales formed under water vapour and their transformation to α-Al₂O₃. Some results have indicated that water vapour increases the oxidation rate of alumina-scale forming coatings but others have found the opposite, that is, that under water vapour the oxidation rates decrease as either transition aluminas do not form or the transformation to α-Al₂O₃ is accelerated. In addition, it was found that χ-Al₂O₃ is the only oxide that forms at the initial stages of oxidation under 100 % steam on Fe–Al coatings at 650 °C. Under these conditions, this oxide is very protective, and slowly transforms onto α-Al₂O₃. A preliminary study of the transformation of χ- to α-Al₂O₃ at 900 °C under laboratory air was carried out. χ-Al₂O₃ was generated by a steam pre-treatment on slurry Fe aluminide coatings deposited on P92.