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.     

Phase equilibria in the subsolidus region of the NiO-α-Al2O3 system between 1000 and 1920°C

Nickel spinel, Ni_1–xAl_2(1+x/3)O₄, is the only intermediate compound in the quasibinary NiO--Al₂O₃ system at temperatures between 1000 and 1920°C. The spinel equilibrated with NiO occurs at its stoichiometric composition, NiAl₂O₄, independent of temperature. An alumina rich spinel, 0.17 x 0.62, equilibrated with -Al₂O₃ increases in alumina content with increasing temperature. Aluminum oxide solubility in NiO increases from 1 mole % at 1000°Cto 3 mol % at 1800°C. Nickel oxide solubility in -Al₂O₃ was found to increase from 2 mole % at 1000°C to 3 mole % at 1920°C.     

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.     

Experimental observations in support of the dynamic-segregation theory to explain the reactive-element effect

The addition of reactive elements can have a significant effect on the oxidation behavior of alumina- and chromia-forming alloys. A model has been developed to explain the effects associated with the addition of reactive elements that is based on the segregation of reactive-element ions to scale grain boundaries and the metal-oxide interface. Reactive-element ions use these interaces as pathways for diffusion from the metal substrate to the gas interface of the scale. The driving force for this outward diffusion is the oxygen potential gradient across the scale. Doping of the scale grain boundaries results in scale growth primarily by inward oxygen diffusion, while doping at the metal-oxide interface slows the growth of interfacial voids and thus improves scale adhesion.     

X-ray topographic study of β-NiAl upon growth of an α-Al2O3 film

The Berg-Barrett X-ray topographic method was employed as a microstructural technique to seek correlations of the metal substructure to the morphological features of -Al₂O₃ films grown on -NiAl. An analysis of diffraction micrographs using {112} and {002} reflections from individual grains in -NiAl revealed its subgrain structure to a depth of 30 . The dimensions of these subgrains were directly related to the density of oxide ridges in the -Al₂O₃ films and to the dimensions and shapes of cavities at the NiAl-Al₂O₃ interface.     

Sulfidation of manganese in H2S-H2 atmospheres at temperatures between 1073 and 1273 K

The sulfidation behavior of Mn in H₂S-H₂ atmospheres, atm, maintained at a total pressure of 1 atm was investigated at temperatures between 1073 and 1273 K. The reaction kinetics obeyed the parabolic rate law; the parabolic rate constant was proportional to where n=6.1±0.3. A value of 21,000±2000 cal/mole was estimated for the activation energy of the growth of the sulfide scale under constant sulfur pressure. Polycrystalline columnar -MnS scales were formed which exhibited texture with preferred (111) orientation. The parabolic rate constants at different sulfur pressures from this and previous investigations were used to evaluate the manganese self-diffusion coefficient in -MnS at .     

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.     

Oxidation Behavior of Cast Ni3Al Alloys and Microcrystalline Ni3Al+5% Cr Coatings with and without Y Doping

Ni₃Al+5% Cr and Ni₃Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al₂O₃ formed on the coatings as the main oxidation products, with the formation of - and -Al₂O₃ scales at 900 and 1200°C, respectively. The spallation resistance of the Al₂O₃ scales formed on the coatings was superior to the oxide scales formed on cast Ni₃Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.     

TEM Study of Scale Microstructures Formed on Ni–10Cr and Ni–10Cr–5Al Alloys with and Without Y Addition

A study was conducted to investigate the effect of Y addition on the isothermal-oxidation behaviors of Ni–10Cr, Ni–10Cr–0.5Y, Ni–10Cr–5Al, and Ni–10Cr–5Al–0.5Y alloys. The alloys were oxidized in air for 50 hr at 1000°C. The oxides formed on the alloys were characterized using primarily cross-sectional transmission-electron microscopy techniques along with light microscopy, scanning-electron microscopy, and X-ray diffraction. Although the Al-containing alloys showed comparatively better oxidation behavior, all alloys exhibited nonprotective scaling, as suggested by the thick oxides formed. The major component of the outer oxide was NiO. However, modified Y-containing alloys formed protective layers (i.e., -Cr₂O₃ for NiCrY and -Al₂O₃ for NiCrAlY) at the scale–alloy interface following the nonprotective scaling. The spalling resistance of the modified Y-containing alloys was better than their counterpart unmodified Y-free alloys, while their overall oxidation mechanism remained unchanged after Y addition.     

Characterization of the High-Temperature Oxidation Behavior in Fe-25Mn-1.5Al-0.5C Alloy

The high-temperature oxidation behavior of aFe-25Mn-1.5Al-0.5C alloy was investigated by using XRD(X-ray Diffraction) and electron microscopy. The oxidelayers could be macroscopically classified into two zones, the external and internal-oxidezones. The external-oxide zone was confirmed to be inthe order of Mn₂O₃,Mn₃O₄,MnFe₂O₄, (Mn,Fe)O andMnAl₂O₄ phases from the outsidesurface to the matrix, while the internal oxides, composed of interandintra-granular oxides, were identified asMnAl₂O₄ formed by the selectiveoxidation of manganese and aluminum. On the other hand,the Upsilon matrix was transformed to and phases by theselective oxidation of manganese.     

Application of an acoustic-emission technique to the high-temperature oxidation of Fe-Cr-Al alloys

Acoustic-emission analysis combined with thermogravimetry has been used to investigate the oxidation behavior of undoped and doped Fe-Cr-Al alloys. It was demonstrated that acoustic-emission signals which were detected upon isothermal oxidation of undoped Fe-20Cr-5Al arise from buckling of the finegrain -Al₂O₃ layer. The acoustic-emission signals which were detected upon isothermal oxidation of Fe-18Cr-12Al are attributed to repeated cracking of the coarse-grain -Al₂O₃ layer. The mass-gain curve results from superimposed diffusion-controlled oxide growth and accelerated oxide growth after cracking of the oxide layer. The present study shows that acoustic-emission analysis is very useful as it complements thermogravimetric results. The advantage of acoustic-emission analysis is that it reveals cracking and spalling of the oxide layer, which is not recorded by thermogravimetry.     

The Corrosion of Titanium Aluminides in H2/H2S/H2O Atmospheres at 800–1000°C

The corrosion behavior of three Ti–Al intermetallics containing 20, 30, and 40 wt.% Al was studied over the temperature range 800–1000°C in a H₂/H₂S/H₂O gas mixture. Ti–20Al and Ti–40Al alloys had the single-phase structure of Ti₃Al and TiAl, respectively, while Ti–30Al was a two-phase mixture of Ti₃Al+TiAl. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but decreased with increasing Al content. The Ti–40Al alloy exhibited the best corrosion resistance among all alloys studied. The scales formed on Ti–Al intermetallics were heterophasic and duplex, consisting of an outer-scale layer of pure -TiO₂ and an inner layer of -TiO₂ with minor amounts of -Al₂O₃ and Ti_l-xS. The amount of -Al₂O₃, which increased with increasing Al content, is responsible for the reduction of the corrosion rates as compared with those of pure Ti oxides.     

Analytical study of surface and bulk oxidation of Astroloy

Nickel-base alloys, such as Astroloy, used for aeronautical turbine disks, are sensitive to time-dependent cracking in environments containing oxygen. The mosaic structure of the alloy consisting islands (200 nm average size) surrounded by the -phase (100 nm thick) induces complex oxidation phenomena. Various analytical approaches allow the delineation of all the steps from segregation to oxidation occurring on the surface of such a duplex structure. The protection of Astroloy by its outer oxide layer against oxygen penetration was studied also, using alternative ¹⁶O₂ then ¹⁸O₂ oxidation. In association with STEM studies, it is shown that the outer oxide scale is not a real barrier against oxygen penetration and that inner precipitation of chronium (+ aluminium and titanium)-enriched oxides, takes place especially in the structure.     

Effect of Palladium Incorporation on Isothermal Oxidation Behavior of Aluminide Coatings

A (Ni, Pd)Al coating has been prepared by low-pressure pack cementation on the nickel-base superalloy IN738 with a preplating of Pd–20 wt.% Ni alloy. The coating consists mainly of a single phase of -Pd(Ni)Al in the outer part and -Ni(Pd)Al in the inner part. The oxidation behavior of the (Ni,Pd)Al coating at 900–1100°C was studied by TGA, XRD, and SEM/EDS. Results show that the transformation of - to -Al₂O₃ is more rapid on the (Ni,Pd)Al coating than that on a simple NiAl coating. The addition of Pd to the aluminide coating facilitates the transformation of - to -Al₂O₃e oxide scale formed and accelerates the diffusion of Ti from the substrate to the coating surface simultaneously.     

The formation of α-Al2O3 scales at 1500°C

The growth of Al₂O₃ scales on -NiAl was studied at 1500°C. Oxidation rates, diffusion mechanisms, and microstructures were examined in order to achieve a complete understanding of the scale development. Variation of the Al content within the phase field had little effect on the oxidation behavior. Ionimplanted yttrium (2×10¹⁶/cm²) was observed to provide a short-term improvement in scale adhesion but little long-term effect. When doped with Y or Zr, the first 1 m of -Al₂O₃ was observed to grow mainly by an inward oxygen growth mechanism. At longer times when the implant was ineffective, microstructural observations indicate a mixed-growth mode.     

The kinetics and mechanism of manganese sulfidation

The kinetics and mechanism of manganese sulfidation have been studied as a function of temperature (673–1373 K) and sulfur vapor pressure (10–10⁵ Pa). It has been found that the rate-determining step of -MnS scale growth is the outward diffusion of metal. In the high-temperature range (>1000 K) coarsegrain scale is formed, and manganese diffuses in the form of Mn²⁺ cations and electrons via doubly ionized cation vacancies and electron holes (volume diffusion). In the low-temperature range (<800K), on the other hand, a fine-grain scale is formed, and manganese diffuses mainly through grain boundaries, but also in the ionized form. Finally, in the intermediate-temperature range (800–1000 K) grain-boundary diffusion prevails in the initial period of the reaction, and volume diffusion predominates during the later stages, as a result of grain growth of the scale.     

Effect of the θ–α-Al2O3 Transformation in Scales on the Oxidation Behavior of a Nickel-Base Superalloy with an Aluminide Diffusion Coating

The high-temperature oxidation behavior of a nickel-base superalloy andaluminide-diffusion coating has been investigated over the temperaturerange from 800–1100°C and analyzed by TGA, XRD, EDAX, andSEM. The -NiAl coating was formed by low-pressure gas-phasecementation at 950°C for 3 hr. It was found that the formation of-Al₂O₃ from -Al₂O₃ on the -NiAl coating resulted in asharp decrease in the parabolic rate constant kp by one order ofmagnitude at 1050°C during transient oxidation. The transformationcaused the anomalous behavior of the oxidation kinetics curves of thisdiffusion coating in the temperature range 800–1100°C withinthe first 100 hr. A change in the morphology of scales also occurredwith the transformation. A growth stress was characterized by theformation of convoluted scales, which were observed on the surfaceafter oxidation. The oxidation mechanism of this -NiAl diffusioncoating is described.     

The Oxidation Behavior of Oxide-Dispersed β-NiAl: I. Short-Term Performance at 1200°C

Oxide dispersions were added to -NiAlalloys using a powder-metallurgy technique. During 20,2-hr cycles at 1200°C, scale adhesion of theexternal alpha Al₃O₂ was improvedby the addition of Y₂O₃ and ZrO₂. Relative to an undoped, castNiAl alloy, no improvement was observed forTiO₂ and HfO₂ additions andnegative effects were observed forAl₂O₃ andLa₂O₃ additions. The variouscation additions also had differing effects on the scale morphologyand isothermal growth rates at 1200°C. The effect ofthe dopants added as oxide dispersions was compared tosimilar alloy additions of the dopants to -NiAl.     

Tensile Deformation of Iron Oxides at 600–1250°C

Tensile tests of virtually pure FeO, -Fe₃O₄, and -Fe₂O₃ were performed at 600–1250°C at strain rates of 2.0×10^–3–6.7×10^–5 s^–1 under controlled gas atmospheres. Mechanical properties and deformation/fracture behavior were investigated. For -Fe₂O₃, brittle fracture resulted at 1150–1250°C and the fracture strain was below 4.0% at a strain rate of 2.0×10^–4 s^–1. -Fe₃O₄ deformed plastically above 800°C. Steady-state deformation was indicated at 1200°C; elongation of 110% was obtained. Plastic deformation observed at 800 to 1100°C was considered to result from dislocation glide. Using TEM, the Burgers vector of dislocations observed in deformed -Fe₃O₄ was determined to be <110>, its slip system was estimated to be {111}<110>. FeO deformed plastically above 700°C. Steady-state deformation became predominant above 1000°C. Elongation of 160% was obtained at 1200°C. Strain rates of FeO at 1000°C and 1200°C were proportional to the fourth power of the saturated stress, indicating that plastic deformation was affected by dislocation climb.