Oxidation Behavior of Two-Phase γ + σ Nb-Ti-Al Alloys

The oxidation behavior of a series of single-and two-phase Nb-Ti-Al alloys, selected from the sameextended + tie-line, was investigated at1200°C in air. The single-phase alloy suffered from extensive internal oxidation andoxidized at a much higher rate than the single-phase alloy. In a two-phase + microstructure, the phase was preferentiallyattacked to form internal alumina andTi-rich nitride. This preferential attack of limited the extent to which the phase wasinternally oxidized, but also interrupted the formationof a continuous alumina scale. The single-phase alloyalso did not form a continuous alumina scale. Theinability of the phase to form continuousalumina was attributed to a combination of nitrideformation and internal oxidation. The oxidation behaviorof the two-phase + Nb-Ti-Al alloys isdiscussed in terms of mechanisms developed for theoxidation of binary, two-phase alloys.     

The Influence of the Alloy Microstructure on the Oxidation Behavior of Ti–46Al–1Cr–0.2Si Alloy

The influence of microstructure of the two-phase alloyTi–46Al–1Cr–0.2Si on the oxidation behavior in air between600 and 900°C was studied. The oxidation rate, type of scale, and scalespallation resistance were strongly affected by the type of microstructure,i.e., lamellar in as-cast material and duplex after extrusion at1300°C. The oxidation rate was affected by the size and distribution ofthe ₂-Ti₃Al phase, being faster for the extrudedmaterial with coarse ₂-Ti₃Al. The type of oxide scaledetermines the spalling resistance. Cast material developed a uniform scalethat spalled off after short exposure times at 800 and 900°C when a criticalthickness was reached. The extruded material presented a heterogeneous scalewith predominant thick regions formed on -TiAl-₂-Ti₃Algrains and thin scale regions formed on -TiAl grains. Thistype of scale could permit an easier relaxation in the matrix of stressesgenerated by both thermal-expansion mismatch between scale and alloy andoxide growth, resulting in a higher spallation resistance.     

Oxidation Resistance of Ion-Implanted γ-TiAl-Base Intermetallics

A series of alloy elements, including the detrimental, neutral, and beneficial elements for bulk alloying into -TiAl base intermetallics, i.e., V, Cr, Y, Er, Nb, and W, has been implanted into a -Ti–50Al intermetallic in order to explore the mechanism of high-temperature oxidation resistance for the ion-implanted intermetallic. The oxidation resistance was investigated under cyclic-oxidation conditions at oxidation temperatures from 800 to 1000°C for 200 hr in air. At a lower oxidation temperature of 800°C, the V-ion implantation has a detrimental effect on the oxidation resistance of Ti–50Al, while a neutral and beneficial effect was observed for Er-, Y- and Cr-, Nb-, W-ion implantation, respectively. At 900°C, V-, Er-, Y-, and Cr-ion implantation all showed a neutral effect, whereas Nb- and W-ion implantation apparently improved the oxidation resistance. With increasing oxidation temperature to 1000°C, Y- and Cr-ion implantation kept the neutral effect, and the beneficial effect of Nb-ion implantation disappeared gradually. The oxidation behavior of ion-implanted -TiAl base intermetallics is different from that of bulk-alloyed materials due to the two alloying methods, although the effect of the alloy elements on the oxidation resistance has not essentially changed in the -TiAl base intermetallics.     

Effects of Laser Surface Modification of Nimonic with Aluminum on Oxidation Behavior

Using a high-power CO₂ laser, aluminum-surface alloying was carried out on a nickel-base superalloy (Nimonic 80A) with the aim of improving its oxidation resistance. After the treatment, scanning-electron microscopy (SEM) studies show the alloy area to have a two-phase structure of Ni solid solution and Ni₃Al intermetallic. These layers were subsequently subjected to a laser-remelting treatment with different beam-scanning speeds in order to homogenize their structure. Metallographic studies indicate the formation of a single dendritic phase rather than the two-phase structure present in the unmelted alloyed tracks and a decrease in the aluminum content throughout the laser track. To establish their oxidation behavior at high temperatures, the alloyed layers and remelted alloyed layers were oxidized at 1273 K for varying times, between 24 and 300 hr, comparing their behavior with that of untreated specimens. The results indicate the formation of a protective alumina layer on the alloyed specimens. The oxidation behavior differs, depending on the scanning rate (of the laser beam over the specimen surface) during remelting. Oxidation of the remelted specimens at the maximum rates studied (500 mm/min) leads to the formation of protective oxides on the superalloy. However, when remelting takes place at lower rates (100 and 300 mm/min), the amount of aluminum present is insufficient to develop a continuous protective-oxide layer.     

On the oxidation mechanism of alumina formers

The oxidation mechanism of the intermetallic compound -NiAl and Fe-23Cr-5Al-0.2Zr alloys modified by reactive-element additions was studied in the temperature range 1273–1473 K by means of the two-stage oxidation method using the oxygen isotope¹⁸O as a tracer. It was found that outward cation diffusion predominates in the scale on -NiAl. The contribution of the inward oxygen transport increased with increased reaction temperature and oxidation time. At 1473 K, implanted yttrium suppressed inward oxygen diffusion for oxidation times less than 1 hr. In the case of Fe-Cr-Al-Zr alloys the counter-current transport of reactants was observed on the non-implanted materials. Implanted yttrium was found to alter the transport mechanism. This effect appeared to be directly related to that of yttrium on the scale morphology and microstructure. Yttrium promoted the formation of cracks which provided an additional surface for the reaction.On leave from Academy of Mining and Metallurgy, Krakow.     

Oxidation-Resistant Aluminide Coatings on γ-TiAl

The long-term application of TiAl alloys based on the -phase at temperatures above 750–800°C requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, these alloys, containing large amounts of titanium, suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti–50 at.% Al alloy to TiAl₃, the most promising, oxidation-resistant phase in the Ti–Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800–1000°C in air for up to 300 hr. The aluminide coating greatly improves the oxidation resistance of -TiAl, forming a protective alumina scale. The rapid aluminum interdiffusion between the TiAl₃ coating and the -TiAl substrate determined the effective life of the coating. In addition, the oxidation behavior of the TiAl₂ phase formed by interdiffusion of the coating system was studied by oxidation of cross sections.     

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 .     

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.     

The Effect of Niobium-Ion Implantation on the Oxidation Behavior of γ -TiAl Alloys in Static and Flowing Air

-TiAl (Ti–50Al at.%) alloys were implanted with Nb ions at an acceleration energy of 50 keV, at a dose of 1.2×10¹⁷ ions/cm². The cyclic-oxidation behavior of the unimplanted and Nb⁺-implanted TiAl specimens was investigated at 850°C in static air and in air with a flow velocity of 12.0 m/s (1000 ml/min). In static air, the unimplanted TiAl specimen showed rapid oxidation during a transition period of about 80 hr, after which partial scale spallation occurred and a net mass loss was observed. In flowing air, the whole scale spalled off after each cycle. On the other hand, Nb-ion implantation led to the formation of an adherent protective Al₂O₃ scale during oxidation in both static and flowing air, thereby significantly improving the cyclic-oxidation resistance of -TiAl alloys. A remarkable deference in the initial-oxidation behavior between unimplanted and Nb⁺-implanted specimens was also observed. A mixed TiO₂/A₂O₃ scale on the unimplanted specimen developed at a high growth rate during the very initial stage of oxidation. In contrast, the initial scale growth rate was significantly decreased by Nb-ion implantation and an Al₂O₃-rich layer was found present as the inner part of the initial scale on Nb⁺-implanted TiAl. Flowing air appeared to cause severe scale spallation during oxidation of unimplanted TiAl, but not to have any influence on the adhesion of the scale on Nb⁺-implanted TiAl.     

Beneficial Effects of Rhenium Additions on the Cyclic-Oxidation Resistance of β-NiAl + α-Cr Alloys

This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior of-NiAl + -Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in the phase and causing the interdendritic -Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the -Cr precipitates fromthe subsurface region of the alloy. The maintenance of -Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl₂O₃-rich scale.     

High-Temperature Oxidation Behavior of the Intermetallic Compound NbAl3: Influence of Two Processing Techniques on the Oxidation Mechanism

The NbAl₃ intermetallic compound was prepared two different ways:first, by the classical induction-melting technique; the end product is acoarse-grain massive compound, including cracks and pores. Second, bymechanically activated annealing process (M2AP); the end product is afine-grain, powder of submicron crystallites. The oxidation behavior in airunder atmospheric pressure over the temperature range 500–1350°Cwas studied for each material in order to determine the influence of theNbAl₃ microstructure on the oxidation mechanism. In all cases,the massive compound does not form the expected compact alumina, protectivescale. In the lower temperature range, the pest phenomenonoccurs. No grain disintegration was evidenced by oxidation of the M2APNbAl₃ powder despite the high number of crystallites forming onegrain. This is a good argument with expected behavior for a massive materialproduced from the M2AP precursor by powder metallurgy processing.     

The kinetics of the oxidation of aluminum-zinc alloys in oxygen at high temperature

The oxidation behavior of high-purity aluminum-zinc alloys has been studied at temperatures from 475 to 575°C in dry oxygen at a pressure of 76 Torr. The oxidation product is duplex in nature consisting of both amorphous and crystalline -Al₂O₃. The crystalline -Al₂O₃ grows as roughly cylindrical crystals of constant thickness at any given temperature and alloy content; the crystals grow into the metal from the amorphous oxide-metal interface by inward diffusion of oxygen through the overlying amorphous film. The amorphous oxide film existing between the crystals of -Al₂O₃ grows with accurately parabolic kinetics throughout the temperature and alloy composition range. The amorphous oxide existing above the crystalline phase grows at a lower rate because of the additional resistance to cation diffusion conferred by the underlying crystalline phase. Increasing the zinc content of the alloy from 0.1 to 1.0% causes a reduction in the crystal nucleation density and an increase in the radial growth rate. The presence of Zn has no effect on the growth kinetics of the amorphous oxide between the crystals of -Al₂O₃.     

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.     

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.     

Effect of NiAl Microcrystalline Coating on the High-Temperature Oxidation Behavior of NiAl–28Cr–5Mo–1Hf

The effect of an NiAl microcrystalline coating prepared by magnetron sputtering on the high-temperature oxidation behavior of NiAl–28Cr–5Mo–1Hf was investigated in static air at 1000–1150°C. The additions of Cr, Mo, and Hf changed the single -phase structure into a multiphase structure [-NiAl, -Cr(Mo), and Heusler phase]. The NiAl–28Cr–5Mo–1Hf alloy formed a nonprotective mixed scale of Al₂O₃+Cr₂O₃+HfO₂ 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 of NiAl–28Cr–5Mo–1Hf due to the formation of a compact and adherent Al₂O₃ scale at all test temperatures. It was found that the --Al₂O₃ transformation caused the anomalous behavior of the oxidation–kinetics curves of the NiAl microcrystalline coating in the temperature range 1000–1150°C. A change in the morphology of scales occurred with the transformation.     

Internal oxidation of Fe-Al alloys in the α-phase region

The internal oxidation behavior of Fe-0.069, 0.158, and 0.274 wt% Al alloys was investigated in the -phase region. The internal oxidation experiments have been made over the temperature range from 1023 to 1123 K using a mixture of iron and its oxide powders. A parabolic rate law holds in the present alloys, where the rate constant, K_p, depends upon the oxidation temperature as well as the aluminum content. The internal oxidation of Fe-Al alloys is, therefore, controlled by a diffusion process of oxygen in the alloy. The oxide formed in the oxidation layer is the stoichiometric FeAl₂O₄ (hercynite). The aluminum concentration, N _Al ^Io , in the oxidation layer was calculated by taking account of counterdiffusion of aluminum. Furthermore, the oxygen concentration, N _O ^S , at the specimen surface was evaluated on the basis of thermodynamics. Using these estimated values of K_p, N _Al ^IO , and N _O ^S , the diffusion coefficient of oxygen, D _O ^IO , in the oxidation layer, where the oxide particles were dispersed, was also calculated. D _O ^IO increases as the volume fraction of the oxide, f^IO, increases. The diffusion coefficient of oxygen, D_O, in -iron was determined by extrapolating D _O ^IO to f^IO=0.     

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.     

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.     

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.     

Analytical techniques in high temperature corrosion

This paper considers the application of analytical techniques (particularly AES, SIMS, RHEED, laser Raman, Mössbauer, TEM, EELS) to high-temperature oxidation studies. Specific systems reviewed include NiO on Ni, oxides on Fe, Cr and their alloys, and Al₂O₃ on Al and -NiAl. The often complementary information provided by the various techniques leads to a better understanding of oxide growth mechanisms on an atomic sale, interfacial segregation phenomena, and the role of reactive elements in modifying transport processes in oxides.