Effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound

The effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound was investigated in static air. The bare TiAl alloy exhibited poor isothermal and cyclic-oxidation resistance at 800–1000°C due to the formation of TiO₂-base scales which tend to spall during cooling. A sputtered Ti-50Al-10Cr coating remarkably improved the oxidation resistance of TiAl, due to the formation of an adherent Al₂O₃ scale at 800–1000°C. After long-term oxidation (at 900°C for 1000 hr), TiAlCr coating still provided excellent protection for the TiAl alloy. Minor interdiffusion occurred due to the inward diffusion of Cr, while no Kirkendall voids were found at the coating/ substrate interface. In contrast, NiCrAlY and CoCrAlY coatings reacted extensively with the TiAl alloys. Moreover, the TiAlCr coating alloy is based on -TiAl and TiAlCr Laves phases, which may offer improved mechanical properties. The TiAlCr coating exhibited a better combination of oxidation resistance and substrate compatibility than conventional aluminide and MCrAlY coatings.     

Oxidation behavior of TiAl coated with a fine-grain Co-30Cr-4Al film

The oxidation behavior of TiAl coupons coated with a fine-grain Co-30Cr-4Al (mass %) film of about 30-m thickness has been studied at 1100–1400 K in a flow of purified oxygen at atmospheric pressure for up to 500 ks. Three oxidation stages were recognized: initial transient, parabolic, and accelerated stages. However, at 1100 K a parabolic stage continues for more than 800 ks. The activation energy for parabolic oxidation agrees with reported values for the oxidation of alumina-former alloys, although the mass gains during the parabolic stages are relatively small at 1200 and 1300 K. Micropores developed mainly at the scale/coating and coating/substrate interfaces as oxidation proceeded. This is attributable to recrystallization of the coating during oxidation and a Kirkendall effect due to preferential diffusion of Co into the substrate. The accelerated oxidation can be explained in terms of the formation of rutile mounds on the scale.     

TEM investigations of the early stages of TiAl oxidation

The early stages of TiAl oxidation at 900°C and 1000°C in air have been investigated by transmission electron microscopy (TEM). The investigations revealed that at the beginning of oxidation, i.e., after 4 min, TiO₂ and Al₂O₃ grow in a preferential orientation on the -TiAl substrate. After 4 h of oxidation an oxide scale structure can already be found similar to that known from long-term oxidation. In addition, besides -Al₂O₃, the formation of a second aluminum oxide phase and of titanium nitrides is observed. The processes at the metal-oxide interface of oxidation in the early stages, consisting of a repeated cycle of Al₂O₃ formation, Al₂O₃ dissolution, outward migration of Al through the scale, and reprecipitation of Al₂O₃ in the outer scale, are described by a model. The four stages observed in the kinetics of TiAl oxidation are explained on the basis of the results obtained and the structure of the oxide scale.     

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO₂ (rutile), while the inward-growing part is composed of a mixture of TiO₂ and -Al₂O₃. A barrier layer of Al₂O₃ on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al₂O₃ barrier dissolved and re-precipitated in the outer TiO₂ layer. This shift leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al₂O₃ layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al₂O₃ barrier layer formed; large Al₂O₃ particles were part of the outward-growing layer. A brittle 2-Ti₃Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti₂AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al₂O₂₃ formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.     

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.     

Effect of tempering time on the oxidation behavior of 21/4Cr-1Mo steel

21/4Cr-1Mo ferritic steel is generally used in the normalized and tempered condition for steam-generator tubes in fast breeder reactors. The microstructure of the alloy changes on tempering, depending on the type of carbides formed, as a function of tempering time. At the very early stage of tempering (up to 30 min), most of the cementite (Fe₃C) dissolves, causing trapping of Cr and Mo to form their carbides. With an increase in the tempering time more Cr and Mo are used up for carbide formation. This leads to a decrease in the available Cr content, which is generally responsible for the formation of a protective oxide scale. It is thus expected that a decrease in the oxidation resistance will occur with an increase in tempering time. Oxidation tests were carried out on specimens prepared after five tempering treatments; viz. 10 min, 30 min, 60min, 120min, and 180min at 998 K. Oxidation was carried out in air both for long duration (1000 hr at 773 K) and for short duration (6hr at 873, 973, and 1073 K). Results indicate a decrease in the oxidation resistance with increasing tempering time. At 1073 K, however, no significant change in oxidation was noticed. This could be perhaps due to the vanishing of the tempering effect by the dissolution of carbides at this temperature.     

Observations on the effect of low sulfur activity on the oxidation of chromium-depleted zones in a stainless steel

Coupon specimens of 20% Cr-25% Ni-Nb-stabilized steel have been oxidized in an atmosphere of CO ₂-2%CO-670 vppb COS at 0.1 MPa pressure for periods up to 500 hr at 1123 K. Standard specimens, annealed at 1203 K prior to testing, showed an enhancement of iron in the surface scale and a much increased propensity to spall compared with control tests in a sulfur-free atmosphere. The main purpose of the work was to examine the effect of sulfur on the ease of formation of a healing layer under duplex attack produced by depleting specimens in chromium by prior vacuum annealing. It is shown that although a chromium-rich layer had formed, extensive breakdown occurred in the sulphidizing atmosphere leading to continued internal oxidation. Sulfur was found to partition at the base of this attack and to be associated with a large concentration of nickel. Spaliation was also enhanced in the depleted specimens, the favored site being the interface between the spinel and outer iron-rich oxide of the duplex structure. Partitioning of both sulfur and carbon was observed at this interface in those regions of the specimen showing healing layer breakdown.     

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.     

TEM Observations of the Initial Oxidation Stages of Nb-Ion-Implanted TiAl

Coupon specimens of TiAl were implanted with Nb ions at an acceleration voltage of 50 kV with a dose of 10²¹ ions m.^–2 They were then slightly oxidized during heating to 900 or 1200 K, or at 1200 K for 3.6 ksec (1 hr) in a flow of purified oxygen under atmospheric pressure. The implanted specimens and oxidized specimens were characterized and observed by AES, X-ray diffractometry, SEM, TEM, EDS, and EPMA. Implantation improves the oxidation resistance significantly by forming virtually -Al₂O₃ scales. The implanted layer is about 75 nm thick; the outer part of 30-nm thickness is -Ti phase and the rest of 45-nm thickness is amorphous. Heating to 900 K in O₂ results in partial crystallization of the amorphous layer to Ti₅Al₃O₂ (Z-phase) and to 1200 K results in oxide scales of 270 to 400 nm thickness consisting mainly of Al₂O₃. The fraction of Al₂O₃ in the scale increases toward the substrate. Oxidation at 1200 K for 3.6 ksec results in Al₂O₃-rich scales of about 400-nm thickness. The oxide grain size is very fine, about 80 nm in size, and becomes smaller toward the outer scale surface. This implies that implantation enhanced the nucleation of Al₂O₃ grains relative to the growth of TiO₂ grains. This finding and the formation of -Ti phase are thought to be responsible for the excellent oxidation resistance obtained.     

Effect of cold work on the oxidation resistance of2\tfrac{1}{4} Cr-1 Mo steel

The effect of cold work on the oxidation rate of 21/4 Cr-1 Mo steel in pure oxygen at 1 atm pressure at temperatures ranging from 400 to 950C has been studied for short exposure periods (max. 4 hr). The specimens had been cold worked up to 90% by cold rolling. The results indicate negligible effect of cold work on the oxidation kinetics up to 700C, beyond which there is general reduction in oxidation. The effect was pronounced at 900C. The increased resistance to oxidation has been attributed to the faster diffusion of chromium in the cold-worked material compared to the annealed one, leading to the formation of a chromium-rich spinel which helps in slowing down the oxidation of the alloy. The findings have been corroborated by the examination of all samples oxidized at 900C by optical, EPMA, SEM, and EDAX analyses.     

Long-time oxidation of iron at 1100°C in air as a function of time

Long-time oxidation experiments, 180 hr, 360 hr, and 660 hr, were carried out on AREMA iron in ambient air at 1100°C. Fe samples, 3 mm in diameter and 39 mm in length, were completely oxidized. Weight-gain measurements, electron microanalysis, and X-ray diffraction (XRD) analysis were used as experimental techniques. The completely oxidized samples remained cylindrical, compact, and solid. The content of hematite in the scale mass increased exponentially from 88.1% (180 hr) to 88.4% (660 hr), with the limit value 100% at t . No wustite and no pure Fe traces have been found in completely oxidized samples.     

Improvement in the oxidation resistance of TiAl by preoxidation in a SiO2-powder pack

The resistance of TiAl coupons to cyclic oxidation at 1300 K in a flow of purified oxygen under atmospheric pressure has been significantly improved by preoxidation. The preoxidation was performed by heating the specimens, buried in a silica powder and encapsulated in a silica tube under a vacuum of 1.3×10^–3 Pa at 1200 K for 100 ks. After 20 cycles (400 hr) of oxidation the mass gain was still very small, and correspondingly the scale thickened very slightly. This superior oxidation resistance is attributable to the formation of a scale very rich in alumina by the preoxidation under a very low-oxygen partial pressure. This low-oxygen partial pressure may be near the dissociation pressure of silica and much lower than that attained by modern vacuum pumps.     

The oxidation and resistance of tantalum-nitride thin films

The oxidation of reactively-sputtered, tantalum—nitride thin films has been studied between 473 and 773 K in air. Films with thicknesses that correspond to sheet resistances of 43, 75, and 150 / were evaluated in this study. X-ray diffraction revealed that the films and the oxidation products were amorphous. The oxidation products were characterized with AES and XPS. The oxidation process was monitored by measuring the change in sheet resistance with time. Sheet resistance measurements were performed with a four-point probe. The oxidation kinetics can be represented by the equation R/R ⁿ =kt. R/R is sheet resistance change in %, t is time, and k is the rate constant. The oxidation kinetics, for the thickest film, were parabolic above about 598 K, and approximately quartic below this temperature. The 75 / films deviated from parabolic kinetics below 603 K, and approached approximate cubic kinetics at 498 K. However, the data were forced to fit parabolic kinetics with good correlation. For the thinnest film, no deviation from parabolic kinetics was observed. The rate constants obeyed the Arrhenius relation. An activation energy of 147 kJ/mole (1.52eV) was calculated, for the thickest film, during parabolic kinetics. This value is approximately equal to the activation energy obtained during quartic kinetics. A model that explains the deviation from parabolic to quartic oxidation kinetics at lower temperatures is presented. In this model, the change in resistance as a function of time was due to simultaneous change in film resistivity, as a result of oxygen dissolution into the film, and oxide-scale growth. The temperature at which this deviation occurs should be a function of film thickness.     

The effect of aluminum ion implantation on the oxidation of an Fe-6 at.% Al alloy at 1173 K

Samples of an Fe-6 at.% Al alloy were implanted with doses of 5×10¹⁶–3×10¹⁷ Al⁺ ions/cm² at 100 kV, and subsequently oxidized at 1173 K in O₂ gas at 27 kPa. By comparison with the behavior of unimplanted specimens, no effect on oxidation was observed if the dose of implant was less than 1×10¹⁷ Al⁺ ions/cm². At doses of 1×10¹⁷ Al⁺ ions/cm² or higher, a change in the oxide scaling morphology was effected from a duplex -Fe₂O₃--Al₂O₃ scale to an -Al₂O₃-enriched film with a dispersion of -Fe₂O₃ nodules. Oxygen uptake by the high-dose samples was due principally to nodule formation and weight gains after 45 hr were lower by 50–70% as compared with the unimplanted material. Nucleation and growth of the nodules occurred only within the first 5 hr of reaction, but their presence acted as an upper limit on the benefits provided by implantation of aluminum. Nodule formation was shown to be characteristic of the oxidation properties of the alloy material, not an extraneous feature introduced by the implantation process itself. Surface preparation prior to implantation had little effect on oxidation: high-dose electropolished samples behaved similarly to mechanically polished samples when implanted to comparable doses.     

A Comparison of the High-Temperature Oxidation of 17Cr-2Ni and 18Cr-8Ni Austenitic Stainless Steels at 973 K

The oxidation behavior in air at 973 K of theaustenitic stainless steels 18Cr8Ni and17Cr-2.5Ni-10.5Mn-2Cu-0.17N (low-nickel content), wasstudied in a thermobalance. The steels were heated fromroom temperature up to 973K at 10 K min^-1, oxidizedfor 80 hr and then cooled to room temperature at 80 Kmin^-1. The two steels had the same weightgains, 0.18 mg cm^-2, which is equivalent tooxidation layers about 1.15 m theoretical thickness. In both cases, thegeneral shapes of the WS^-1 (mgcm^-2) curves vs. t (hours) were parabolic,but X-ray diffraction of the oxidized surfaces, surfaceand crosssection optical microscopy and SEM observations and EDSmicroanalysis show important differences betweenthem.     

Microalloying effects in the oxidation of TiAl materials

The influence of microalloying on the oxidation behaviour of γ-TiAl based alloys was studied. The microalloying elements were added by ion implantation. Oxidation tests at 1173 K in air showed that the addition of chlorine into TiAl improves the oxidation resistance resulting in a decrease of the oxidation rate by about 2 orders of magnitude compared to unalloyed TiAl. Microstructural investigations revealed that the formation of a protective alumina layer on top of Cl-implanted TiAl is the cause for the decrease in the oxidation rate. AES measurements in the initial stage of oxidation showed that chlorine is located under the alumina layer in the metal phase. Thermodynamic calculations, investigation of the temperature dependence of the chlorine effect and of the oxidation kinetics of preoxidized Cl-implanted samples support the model of a selective Al-transport via AlCl. Furthermore, the influence of small additions (in the ppm range) of P, B, C and Br on the oxidation kinetics of γ-TiAl-based alloys has been investigated. P, B and C implanted TiAl showed a different oxidation behaviour and oxide scale microstructure compared to Cl and Br microalloyed TiAl. Especially the P implanted sample revealed an extensive nitride formation connected with a breakaway oxidation after 100 h.     

Characterization of the initial oxide formed on annealed and unannealed 20Cr-25Ni-Nb-stabilized steel in 50 torr CO2 at 973 K

The oxidation of annealed and unannealed 20Cr-25Ni-Nb (wt.%) steel in 50 torr CO₂ at 973 K has been studied in situ using X-ray photoelectron spectroscopy with a view to the characterization of the chemical composition and nature of the oxides formed. The oxide first formed on the annealed steel is shown to be iron rich. Analysis of the bulk oxide using a variety of different spectroscopic techniques including X-ray diffraction, energy dispersive X-ray analysis, and scanning Auger microscopy showed that virtually all of the oxide scale formed after 100hr is a spinel of the type (Fe)Fe_2–x)Cr_xO₄ and is composed of an outer, iron-rich layer covering an iron/chromium-rich layer. By contrast, the oxide first formed on the unannealed steel is chromium rich, and is shown to be patchy consisting of a mixture of different oxides. This layer changes on further oxidation to develop into a manganese-iron-chromium spinel, which is present as the major oxide phase after 100 hr. The reasons for these differences are discussed, and it is argued that a major influence on oxidation behavior is the presence of cold work in the unannealed steel enhanc ing the diffusion of chromium to the surface.     

Formation of Al2O3 Scales on Single-Phase RuAl Produced by Reactive Sintering

The oxidation behavior of single-phase RuAl produced by powder metallurgy combined with arc melting was investigated. Oxidation was conducted at 1000°C; oxide scale growth and phase formation were studied using scanning-electron microscopy (SEM) and x-ray diffraction (XRD). A dense protective scale with an Al–depleted sublayer was formed during oxidation. The oxide scale is the stable -Al₂O₃. The oxide-scale morphology shows the presence of whiskers, with a needle-like form, which suggests that the growth of the oxide scale is produced by outward diffusion of Al. At the beginning, oxidation follows a parabolic law, but, after 100 hr of oxidation; the growth rate is slower than expected from a parabolic law.     

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.     

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.