Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

Sulfidation Behavior of Inconel 738 Superalloy at 500–900°C

The high-temperature sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500–900°C in pure sulfur vapor over the range 10²–10⁴ Pa. The sulfidation kinetics followed the parabolic rate law in all cases. The sulfidation rates increased with increasing temperature and sulfur pressure. The scales formed were bilayered and temperature-dependent. At T700°C, the outer scale consisted of mostly NiS (with dissolved Co) and minor (CoS₂ and NiCo₂S₄, while the inner layer was a heterophasic mixture of NiS, NiCo₂S₄, and minor amounts of Al₂S₃ and chromium sulfide (Cr₂S₃/Cr₃S₄). At T750°C, the outer scale consisted of mostly Ni₃S₂ (with dissolved Co) and minor amounts of Co₃S₄ and Cr₂S₃/Cr₃S₄, while the inner layer was a complex, heterophasic mixture of Ni₃S₂, Cr₂S₃/Cr₃S₄, CoCr₂S₄, and minor Al₂S₃. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations and the inner scale grew by the inward transport of sulfur. The formation of Al₂S₃ and Cr₂S₃/Cr₃S₄ partly blocked the transport of cations through the inner scale and consequently reduced the sulfidation rates as compared to pure nickel.     

Oxidation Behavior of Ti--50Al Intermetallics with Thin TiAl3 Film at 1000°C

Isothermal oxidation tests at 1000°C in air indicate that the Ti--50Al alloy with about 8 m TiAl₃ layer on the surface can resist the oxidation for 10 hr. From the FESEM and EPMA/EDS results, the rapid oxidation behavior is attributed to the formation of oxide nodules through the protective Al₂O₃ and TiAl₂ layers on the outer surface. Upon increasing the oxidation time at 1000°C, the size and the number of oxide nodules increase. After 3 hr of oxidation at 1000°C, a laminated layer is formed in between the oxide nodule and substrate, which consists of two nearly parallel phases. The EDS results suggest that these two phases are Ti--Al--O compounds. After 20 hr oxidation, the oxidation nodules and laminated layers disappear and a complex oxide scale is formed which is similar to the bare Ti--50Al oxidized at 1000°C.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

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.     

Microstructural Study of Fe-Cr-Al/Al Composite Coatings During Oxidation and Sulfidation at 900℃

The microstructure of a composite coating system,which was composed of an inner layer of Fe-Cr-Al and an outer layer of aluminum,was studied after it was respectively oxidized and sulfurdized at elevated temperatures. Apart from the Al2O3 scale formed on the surface,the microstructure of the composite coatings exposed at 900℃ in air for 4h was a three-layer structure. The first layer consisted of a solid solution of Cr and Fe in α aluminum and an intermetallic compound FeAl3,while the second layer was a single phase of the aluminide and the third layer still remained the same appearance as the original Fe-Cr-Al coating. The microstructural observation of the specimen tested at 850-900℃ at low oxygen pressure and high sulfur pressure for 576h revealed that the surface coatings of the specimen had transformed into a duplex structure containing an outer layer and a thicker aluminide layer beneath. X-ray diffraction results showed that the out layer was composed of Al2S3 and Al2O3 and that AlCrFe2 was the main phase composition of the aluminide layer,with a few of Al2S3 and Al2O3 accompanied.     

High-Temperature Oxidation Behavior and Mechanism of a New Type of Wrought Ni–Fe–Cr–Al Superalloy up to 1300°C

The oxidation behavior of a new type of wrought Ni–Fe–Cr–Alsuperalloy has been investigated systematically in the temperature range of1100 to 1300°C. Results are compared with those of alloy 214, Inconel600, and GH 3030. It is shown that the oxidation resistance of the newsuperalloy is excellent and much better than that of the comparisonalloys. Scanning electron microscopy (SEM), electron probe microanalysis(EPMA), and X-ray diffraction (XRD) experiments reveal that the excellentoxidation resistance of the new superalloy is due to the formation of adense, stable and continuous Al₂O₃ and Cr₂O₃ oxide layer at hightemperatures. Differential thermal analysis (DTA) shows that the formationof Cr₂O₃ and Al₂O₃ oxide layers on the new superalloy reaches a maximum at1060 and 1356°C, respectively. The Cr2O3 layer peels off easily, and thesingle dense Al₂O₃ layer remains, giving good oxidation resistance attemperatures higher than 1150°C. In addition, the new superalloypossesses high mechanical strength at high temperatures. On-site testsshowed that the new superalloy has ideal oxidation resistance and can beused at high temperatures up to 1300°C in various oxidizing andcorrosion atmospheres, such as those containing SO₂, CO₂ etc., for longperiods.     

Oxidation Behavior and Oxide Layers of Ti–50Al Intermetallics by Preoxidation in High-Pressure Oxygen

The oxidation resistance of Ti–50Al intermetallics is improved bypreoxidation for 1, 4, or 16 hours in high-pressure, pure oxygen(3.9 atm) at 900°C. Specimens preoxidized for 1 hr exhibit betteroxidation resistance than others. Prolonged preoxidation time candeteriorate the oxidation resistance and reduce the parabolic-lineartransition time during subsequent cyclic oxidation in 800°C air. Theoxide-mound occurrence is an important factor for evaluating theeffectiveness of the preoxidation treatment in oxygen. The formationmechanism of Z-phase (Ti₅Al₃O₂) in the Al-depleted layer beneath theflat oxide scale and that beneath the oxide mound are also proposed inthis study.     

Oxidation behavior and mechanism of porous nickel-based alloy between 850 and 1000 °C

The oxidation behavior and mechanism of a porous Ni–Cr–Al–Fe alloy in the temperature range from 850 to 1000 °C were investigated by optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) analyses and X-ray photoelectron spectroscopy (XPS). The results show that the oxidation kinetics at 950 and 1000 °C of this porous alloy is pseudo-parabolic type. Complex layers composed of external Cr₂O₃/NiCr₂O₄ and internal α-Al₂O₃ are formed on the surface of the oxidized porous alloys. γ' phases favor the formation of NiO/Cr₂O₃/NiCr₂O₄ during the initial oxidation. Many fast diffusion paths contribute to the development of the oxide layers. The decrease of the open porosity and the permeability with exposure time extending and temperature increasing can be controlled within a certain range.     

Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C

The oxidation behavior of FeCoNi, FeCoNiCr, and FeCoNiCrCu equi-molar alloys was studied over the temperature range 800–1000 °C in dry air. The ternary and quaternary alloys were single-phase, while the quinary alloy was two-phase. In general, the oxidation kinetics of the ternary and quinary alloys followed the two-stage parabolic rate law, with rate constants generally increasing with temperature. Conversely, three-stage parabolic kinetics were observed for the quaternary alloy at T 900°C. The additions of Cr and Cu enhanced the oxidation resistance to a certain extent. The scales formed on all the alloys were triplex and strongly dependent on the alloy composition. In particular, on the ternary alloy, they consist of an outer-layer of CoO, an intermediate layer of Fe₃O₄, and an inner-layer of CoNiO₂ and Fe₃O₄. Internal oxidation with formation of FeO precipitates was also observed for this alloy, which had a thickness increasing with temperature. The scales formed on the quaternary alloy consisted of an outer layer of Fe₃O₄ and CoCr₂O₄, an intermediate layer of FeCr₂O₄ and NiCr₂O₄, and an inner layer of Cr₂O₃. Finally, the scales formed on the quinary alloy are all heterophasic, consisting of an outer layer of CuO, an intermediate-layer of CuO and Fe₃O₄, and an inner-layer of Fe₃O₄, FeCr₂O₄, and CuCrO₂. The formation of Cr₂O₃ on the quaternary alloy and possibly that of CuCrO₂ on the quinary alloy was responsible for the reduction of the oxidation rates as compared to the ternary alloy.     

Softening produced by the microstructural instability of the intermetallic alloy Ti–46.5Al–2W–0.5Si

The influence of heat treatments on the softening process of a TiAl intermetallic alloy containing three phases (namely γ, α₂ and B2) has been examined. At the annealing temperatures studied, the lamellar microstructure exhibits faster softening kinetics than the globular one due to the transformation that takes place between the α₂ and B2 phases while the globular structure softens by a coarsening/dissolution process of the respective B2 and α₂ globules initially present.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. I: Oxidation Kinetics and Morphology at 800°C

The oxidation of nickel-matrix/aluminum-particle composite coatings was studied using thermogravimetric (TG) analysis in air at 800°C for up to 100 hr. Long-term oxidation behavior was investigated with furnace exposures up to 2000 hr. The coatings were applied to nickel substrates by the composite electrodeposition technique and vacuum heat treated for 3-hr at 825°C prior to oxidation testing. The heat-treated coatings contained a two-phase (Ni)+(Ni₃Al) microstructure and the overall coating composition was approximately 7 wt.% Al. Also examined were uncoated nickel substrates and bulk Ni–Al alloys containing 6.2, 9.0, and 14 wt.% Al. For all samples, mass-gain kinetics were obtained from thermogravimetric (TG) experiments and furnace exposures and the composition and morphology of the oxidation products were examined using optical microscopy, scanning-electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). An outer NiO layer and an inner -Al₂O₃ layer formed on the composite-coating surface. The addition of a small amount of Si (about 1–2 at.%) was found to have little effect on Ni–Al composite-coating oxidation behavior. The Ni–Al coatings behave similarly to bulk + (Ni₃Al) or single-phase (Ni₃Al). In addition, at lower temperatures, such as 800°C, the coatings benefit from a small grain size that enhances Al diffusion to the surface to form the protective alumina layer. Based on oxidation kinetics and morphology, a critical Al content of about 6 wt.% was found, below which internal oxidation and higher oxidation mass gains were observed.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. II: Oxidation Kinetics and Morphology at 1000°C

The oxidation behavior of nickel-matrix/aluminum-particle composite coat–ings was studied using thermogravimetric (TG) analysis and long-term furnace exposure in air at 1000°C. The coatings were applied by the composite-electrodeposition technique and vacuum heat treated for 3 hr at 825°C prior to oxidation testing. The heat-treated coatings consisted of a two-phase mixture of (Ni)+ (Ni₃Al). During short-term exposure at 1000°C, a thin -Al₂O₃ layer developed below a matrix of spinel NiAl₂O₄, with -Al₂O₃ needles at the outer oxide surface. After 100 hr of oxidation, remnants of -Al₂O₃ are present with spinel at the surface and an inner layer of -Al₂O₃. After 1000–2000 hr, a relatively thick layer of -Al₂O₃ is found below a thin, outer spinel layer. Oxidation kinetics are controlled by the slow growth of the inner Al₂O₃ layer at short-term and intermediate exposures. At long times, an increase in mass gain is found due to oxidation at the coating–substrate interface and enhanced scale formation possibly in areas of reduced Al content. Ternary Si additions to Ni–Al composite coatings were found to have little effect on oxidation performance. Comparison of coatings with bulk Ni–Al alloys showed that low Al -alloys exhibit a healing Al₂O₃ layer after transient Ni-rich oxide growth. Higher Al alloys display Al₂O₃-controlled kinetics with low mass gain during TG analysis.     

Effect of Ion Plating TiN on the Oxidation of Sputtered NiCrAlY-Coated Ti3Al-Nb in Air at 850-950°C

A single sputtered NiCrAlY coating and a complexcoating of inner ion-plated TiN and outer sputteredNiCrAlY were prepared on the intermetallic compoundTi₃Al-Nb. Their oxidation behavior wasexamined at 850, 900, and 950°C in air by thermalgravimetry combined with XRD, SEM, and EDAX. The resultsshowed that Ti₃Al-Nb followed approximatelyparabolic oxi dation, forming an outer thinAl₂O₃-rich scale and an inner TiO₂-rich layer doped withNb at the three temperatures. The TiO₂-richlayer doped with Nb dominated the oxidation reaction.The single NiCrAlY coating did not follow parabolicoxidation exactly at 850 and 950°C, but oxidizedapproximately in a parabolic manner, because theinstantaneous parabolic constants changed slightly withtime. Besides the Al₂O₃ scale,TiO₂ formed from the coating surface at the coating-substrate interface. Thedeterioration of the coating accelerated with increasingtemperature. The NiCrAlY-TiN coating showed two-stageparabolic oxidation at 850 and 900°C, and anapproximate parabolic oxidation at 950°C. The TiN layerwas effective as a barrier to inhibit coating-alloyinterdiffusion.     

Improvement of Oxidation Behavior of a Ti–48Al–2Cr Alloy by a Nitridation Treatment

The effect of a nitridation treatment for 10 hr at 800°C on the oxidation resistance of a Ti–48Al–2Cr (at.%) alloy in air at 800°C was evaluated. Results prove that nitridation decreases by about 40% the total mass gain of nonnitrided material, although the oxidation mechanism is the same for both materials. The oxidation can be divided into two stages. The formation of a nonprotective mixed alumina–rutile scale during the transient stage results in a high oxidation rate. A further decrease in the oxidation rate arises from the establishment of an external alumina-rich layer during the steady stage. The main difference between the scale developed on both materials is the continuous nature of the nitride layer present in the nitrided material during the entire exposure. The thin continuous nitride layer formed during the nitridation treatment acts beneficially as a diffusion barrier, preventing oxygen dissolution in the ₂-Ti₃Al phase during the transient stage. Furthermore, the oxygen gradient through the oxide scale is kept low, because no oxygen is removed at the scale–alloy interface.     

Precipitation evolution in Al–Zr and Al–Zr–Ti alloys during aging at 450–600 °C

The transformation of Al₃Zr (L1₂) and Al₃(Zr_1−xTi_x) (L1₂) precipitates to their respective equilibrium D0₂₃ structures is investigated in conventionally solidified Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys aged isothermally at 500 °C or aged isochronally in the range 300–600 °C. Titanium additions delay neither coarsening of the metastable L1₂ precipitates nor their transformation to the D0₂₃ structure. Both alloys overage at the same rate at or above 500 °C, during which spheroidal L1₂ precipitates transform to disk-shaped D0₂₃ precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the α-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D0₂₃ phase with α-Al. The transformation is very sluggish and even at 575 °C coherent L1₂ precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates.