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Intermetallic Ti-Al-Cr based coatings with small additions of yttrium or zirconium were deposited on the γ-TiAl alloys Ti-48-2-2 and TNM-B1 using magnetron sputtering. The oxidation behavior of the coated substrates was investigated in the temperature range between 850 °C and 950 °C under cyclic thermal exposure conditions in laboratory air. Excellent oxidation protection to both TiAl alloys was provided by the Ti-Al-Cr-Y coating in the studied temperature range for up to 1000 1 h-cycles. Similar results were obtained for the Ti-Al-Cr-Zr coating on the Ti-48-2-2 substrate. When thermally cycled at 900 °C and 950 °C, the latter protective layer deposited on the TNM-B1 alloy locally degraded probably caused by cracks in the coating leading to substrate oxidation. During high temperature exposure the microstructure of the coatings changed due to depletion in Al and Cr. Because alloy Ti-48-2-2 contains 2 at.% Cr and has a higher Al content compared to the TNM-B1 alloy, interdiffusion between coating and the Cr bearing substrate is reduced, retarding phase transformation in the oxidation protective layers.

Tension specimens of a γ-TiAl based TNM-B1 alloy (nominal composition in at%: Ti-43.5Al-4Nb-1Mo-0.1B) were coated with oxidation protective intermetallic Ti-Al-Cr layers containing yttrium and zirconium. The coated specimens were tensile tested in air at room temperature (RT) and 850 °C in the as-deposited condition and after a pre-oxidation treatment at 850 °C for 300 h in air. For comparison, uncoated specimens with and without pre-oxidation were also tested. The elongation after fracture of the TNM-B1 alloy was 0.23% at RT; it increased to 8.3% at 850 °C. The as-deposited coatings spalled off during tensile testing at RT. No coating spallation was observed at elevated temperature and after pre-oxidation due to improved adhesion by interdiffusion bonding. Pre-oxidation deteriorated the RT tensile properties of the γ-TiAl based alloy. The RT ductility determined for pre-oxidized coated specimens was even more reduced probably caused by the brittleness of the coatings initiating premature cracks. When tensile tested at 850 °C, the coated specimens exhibited considerably enhanced elongation after fracture in both the as-deposited and pre-oxidized conditions in comparison to the bare substrate alloy. The increase in ductility was attributed to the high environmental protection of the substrate by the coatings.

Environmental/thermal barrier coatings (E/TBCs) of yttria partially stabilised zirconia (YSZ), gadolinium zirconate (GZO) and a combination of Y2SiO5 and GZO were applied on a Nb/Nb5Si3-based alloy with the nominal composition of Nb-25Ti-8Hf-2Cr-2Al-16Si (at%). A B-containing M7Si6-based (M=Ti, Nb, Fe, Cr) layer produced by pack cementation was used as bond coat. The ceramic topcoats were manufactured by electron-beam physical vapour deposition (YSZ and GZO) and magnetron sputtering (Y2SiO5).The different E/TBC systems were tested under cyclic oxidation conditions in air at 1100 and 1200°C. Lifetimes exceeding 1000 cycles of 1h dwell time at 1100°C were determined. The YSZ, GZO and combined Y2SiO5+GZO topcoats did not spall off during the maximum exposure time, exhibiting good adherence to the thermally grown oxide scale which consisted of mixed oxides of Fe,Ti, Cr and Nb embedded in silica. For the E/TBC systems with YSZ and GZO which were thermally cycled at 1200°C, lifetimes up to 850 cycles were measured; that of the system with a combined Y2SiO5+GZO topcoat exceeded again 1000 cycles.

A thermal barrier coating (TBC) system was manufactured on a Mo-containing γ-TiAl-based alloy utilizing the halogen effect to ensure the formation of thin protective oxide scales. Samples of the TNM-B1 alloy were treated with fluorine using hydrofluoric acid, an F-polymer spray, or plasma immersion ion implantation. They were subsequently exposed to air at 900 °C for 24 h to form a thermally grown oxide (TGO) layer consisting mainly of alumina. Using electron-beam physical vapor deposition, ceramic topcoats of 7 wt% yttria partially stabilized zirconia were deposited on the pre-oxidized fluorinated samples. The lifetime of this TBC system was determined by performing thermal cycling tests at 900 and 1000 °C in air. At 900 °C, lifetimes exceeding 1000 cycles of 1 h dwell time at high temperature were measured. The F-polymer treatment proved to be the most effective fluorination method. For TNM-B1 samples fluorinated by the latter treatment, lifetimes up to 1000 cycles were also determined at 1000 °C. The TBC exhibited good adherence to the TGO consisting mainly of alumina. An Al-depleted zone developed with small precipitates of the AlF3 phase below the oxide scale. Transmission electron microscopy examinations of the Al-depleted zone revealed the formation of the cubic Z-phase (Ti5Al3O2) which transformed into the α2-Ti3Al phase with prolonged high temperature exposure.

International audience; Environmental/thermal barrier coatings (E/TBCs) were applied on an Nb/Nb5Si3-based alloy. The substrate material with the nominal composition Nb-23Ti-4Hf-3Cr-6Al-20Si (at.%) was manufactured by plasma melting. Rectangular platelets of this alloy were coated with chromia-silica forming M7Si6-based (M = Ti, Nb, Cr, X with X = Ni, Co, Fe) bond coats produced by pack cementation. The Co- and Fe-modified M7Si6-based layers also contained boron. On these grit blasted bond coats, ceramic topcoats of 7 wt.% yttria partially stabilised zirconia (YSZ) and gadolinium zirconate (GZO) were deposited using electron-beam physical vapour deposition. The E/TBC systems were thermally cycled at 1100 °C and 1200 °C in laboratory air (1 h at high temperature, 10 min at ambient temperature). Cross-sectional examinations of the samples after cyclic testing were carried out using scanning electron microscopy to study the protection capability of the E/TBC systems and failure mechanisms.The lifetimes of the E/TBC systems exceeded the maximum exposure period of 1000 cycles at 1100 °C. When cyclically tested at 1200 °C, the samples with Ni-modified and boron containing Co-modified M7Si6-based bond coats failed within 700 cycles. Failure was caused by spallation of thick oxides scales thermally grown on the substrate after degradation of the bond coats. The E/TBC systems with boron containing Fe-modified M7Si6-based bond coats exhibited lifetimes approaching 1000 cycles at 1200 °C. For both YSZ and GZO topcoats, chemical reactions with the TGOs were observed.

To improve the oxidation resistance of a third generation Ti–Al alloy (TNM-B1) at high temperatures, aluminium rich coatings were deposited using pack cementation and magnetron sputtering. The coatings formed were analysed and compared with respect to phases formed and the containing elements. Oxidation tests were carried out on the additionally fluorine treated and preoxidised samples at 900 C in air. Compared to uncoated samples the beneficial effect of coating deposition was confirmed. Additionally, the concentration of incorporated oxygen in the subsurface zone and its influence on the mechanical properties were investigated using EPMA and microhardness measurements. Oxygen concentrations were determined for the three phases of the substrate material -TiAl, -TiAl, and -Ti(Al), and depended on the previous coating layer deposition.

Thermal barrier coatings (TBC) of yttria stabilized zirconia were deposited on the γ-TiAl based alloy Ti–45Al–8Nb (at.%) using electron-beam physical vapor deposition. The bond coats used were 10 μm thick intermetallic Ti–Al–Cr layers with additions of the reactive elements Y and Zr produced by magnetron sputtering. Cyclic oxidation tests at 900 and 1,000 °C in air revealed excellent oxidation resistance of the Ti–Al–Cr–Y bond coat associated with the precipitation of Y-rich particles in the thermally grown alumina scale as well as in the intermetallic layer. A less protective behavior was found with the zirconium containing bond coat. Lifetimes exceeding 1,000 1-h cycles were determined for both TBC systems at 900 °C. Edge chipping of the zirconia topcoat occurred at 1,000 °C. As observed by cross-sectional examination, a continuous alumina scale was still present on the samples with Ti–Al–Cr–Y bond coat, whereas the Ti–Al–Cr–Zr layer was severely degraded and a thick mixed oxide scale formed after 1,000 cycles at 1,000 °C.

Intermetallic titanium aluminide alloys are attractive light-weight materials for high temperature applications in automotive and aero engines. The development of γ-TiAl alloys over the past decade...

Establishing the formation of a thin alumina scale, fluorine-treatment of the surface of γ-TiAl alloys can function as a bond coat in thermal barrier coating (TBC) systems, eliminating problems associated with thickness and chemical compatibility of conventional bond coat types. In the present work, yttria partially stabilized zirconia (YSZ) coatings were deposited on two recently developed γ-TiAl alloys, two-phase TNB-V5 and β-stabilized TNM-B1, using electron-beam physical vapor deposition (EB-PVD). The deposition temperature was 900 °C and 1000 °C. Prior to TBC deposition, the substrate materials were treated with fluorine by dipping in dilute hydrofluoric acid or spraying of an organic fluorine-containing compound, followed, in general, by a pre-oxidation step at 900 °C for 24 h in air. With some of the samples, the latter pre-oxidation step was omitted. As revealed by scanning electron microscopy, the as-deposited zirconia topcoats were tightly adherent to both fluorine treated alloys, regardless of the fluorination technique, the TBC deposition temperature and the pre-oxidation treatment. The oxidation behavior of the coated specimens was determined under cyclic oxidation conditions at 900 °C in air. Cyclic short-term oxidation tests revealed that the pre-oxidation step was crucial to maintain the oxidation protection by the halogen effect after EB-PVD TBC application. Whereas a continuous alumina layer was found beneath the zirconia topcoat on pre-oxidized samples, a mixed layered oxide scale of TiO 2 and Al 2 O 3 formed on specimens without pre-oxidation. When thermally cycled at 900 °C, the TBC system on substrates fluorine-treated by hydrofluoric acid exhibited lifetimes exceeding 500 cycles of 1 h dwell time at high temperature. No spallation of the zirconia topcoat was observed for both TNB-V5 and TNM-B1 alloys. Therefore, YSZ TBCs offer a promising concept to thermally protect γ-TiAl based alloys in combination with the fluorine effect.

The oxidation behavior of γ-TiAl specimens coated with an intermetallic Ti-49Al-34Cr-4Zr layer was investigated at 1000°C under cyclic conditions in laboratory air. The 11 μm thick coating was produced using a combined technique of high power impulse magnetron sputtering and unbalanced magnetron sputtering. The as-deposited coating exhibited a dense layered structure and excellent adhesion to the substrate. The Ti-Al-Cr-Zr coating possessed high oxidation resistance associated with the formation of a thin continuous alumina scale for exposure time periods exceeding 1000 cycles of 1 h dwell time at 1000°C. During the high temperature exposure, the coating being amorphous in the as-deposited condition became crystalline exhibiting different polytypes of Ti(Cr,Al)2Laves phases with Ti probably partially substituted by Zr and Nb. Due to alumina formation and interdiffusion the coating was depleted in aluminum and chromium as well as enriched in titanium. After 1000 cycles at 1000°C, the coating consisted of an outer layer of the hexagonal C14 Laves phase and an inner layer of a probably orthorhombic phase whose structure was not yet determined. In both layers, pores and fine precipitates rich in Zr and Y were found.

CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl substrate material using high power impulse magnetron sputtering technology. The nitride coating provided effective oxidation protection to γ-TiAl at 850 °C for exposure time periods exceeding 2 000 cycles of 1 h dwell time at high temperature in air. High oxidation resistance was also observed at 900 °C. After exposure to air at 850 °C for 300 h, coated tension specimens exhibited a reduction in fatigue strength of about 70 MPa compared to the bare γ-TiAl alloy. Zirconia topcoats produced by electron-beam physical vapour deposition were well adherent to the CrAlYN/CrN coating with an oxy-nitride overcoat. When thermally cycled at 900 °C, the lifetime of this thermal barrier coating system on γ-TiAl exceeded 1 000 1 h cycles. Below cracks in the nitride coating, the substrate was oxidised and protrusions of an outer oxide scale with columnar structure formed.

TiAl-based intermetallic layers with additions of Ag, Cr, Y and Hf were deposited on γ-TiAl specimens using magnetron sputtering. The oxidation behaviour of the coated Ti-45Al-8Nb (at.%) alloy was investigated at 850 °C and 900 °C under cyclic oxidation conditions in air. The ternary Ti-50Al-2Ag (at.%) coating did not provide oxidation protection to the substrate material at 850 °C. Two-phase γ-TiAl + Ti(Cr, Al)2 Laves coatings exhibited high oxidation resistance at 900 °C for up to 1000 cycles of 1 h dwell time at high temperature. During thermal exposure phase dissolution and transformation occurred. The slow oxide growth rates observed for the Ti – Al – Cr intermetallic layers were associated with the formation of a thin continuous alumina top scale established by the Laves, B2 and Z-phases. Small additions of hafnium and yttrium improved the oxidation resistance. In contrast, Ti – Al – Cr – Ag coatings exhibited poor oxidation behaviour at 900 °C.

In this paper, we present the sulphidation/oxidation behaviour of a Ti45Al8Nb (at%) alloy coated with different protective surface films. Two intermetallic coatings are considered; TiAlCr and Al 2 Au deposited by physical vapour deposition. The coated alloy was subjected to a H 2 /H 2 S/H 2 O yielding pS 2 – 10 −1 Pa and pO 2 – 10 −18 Pa potentials at 750 °C for up to 1000 h. The corrosion kinetics were determined by means of discontinuous gravimetry and the as-received and exposed samples were characterised using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). The materials showed the development of a multilayered structure. In the case of the TiAlCr coated Ti45Al8Nb – base alloy, Al 2 O 3 , TiO 2 and Cr 2 S 3 developed. For the Al 2 Au coated Ti45Al8Nb samples an Al 2 O 3 scale containing TiO 2 nodules was observed at the surface.

The lifetime of thermal barrier coating (TBC) systems on gamma titanium aluminides was determined in the temperature range between 850 °C and 950 °C under cyclic oxidation conditions in air. Coupons of the alloy Ti–45Al–8Nb (at.%) were coated by pack aluminizing. A subset of samples was subsequently annealed at 910 °C for 312 h in argon. During this heat treatment, the two-phase (Nb,Ti)Al3 plus TiAl2 microstructure of the coating transformed into single phase γ-TiAl. On pre-oxidised aluminized, annealed and bare samples, TBCs of yttria partially stabilized zirconia were deposited using electron-beam physical vapour deposition (EB-PVD). No spallation of the TBCs was observed in cyclic oxidation tests at 850 °C for up to 3,000 cycles of 1 h dwell time at high temperature. The two-phase aluminide coating provided effective oxidation protection due to the formation of a continuous alumina scale. The lifetime of this TBC system exceeded 1,400 cycles at 950 °C, whereas an aluminized and annealed sample failed after approximately 500 cycles. The TBC on bare substrate failed when thermally cycled at 900 °C. In contrast, no spallation occurred with an aluminized and annealed specimen at this temperature during the maximum exposure length of 1,000 cycles, probably related to an increased aluminium concentration in the subsurface region. EB-PVD zirconia top coats were well adherent to the alumina scale and the thermally grown mixed oxides. Failure of the TBC systems observed with bare and annealed samples was associated with spalled oxide scales formed on γ-TiAl.

In the present study, the applicability of thermal barrier coatings (TBCs) on γ-TiAl alloys was investigated. Two alloys with the chemical compositions of Ti-45A1-8Nb-0.2Nb-0.15C and Ti-45Al-lCr-6Nb-0.4W-0.2B-0.5C-0.2Si were used. Before TBC deposition, the specimens were pre-oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron-beam physical vapour deposition (EB-PVD). The oxidation behaviour of the γ-TiA1 specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900 °C. Post-oxidation analysis of the coating systems was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). No spallation of the TBC was observed for pre-oxidised specimens of both alloys when exposed to air at 850 °C for 1100 cycles of 1 h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO 2 /Al 2 O 3 protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900 °C, the oxide scales on the alloy Ti-45Al-8Nb-0.2B-0.15C pre-oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900 °C.

Aluminium alloy 6013 sheet was butt welded using a 3 kW Nd:YAG laser and different filler powders. Two kinds of filler metals were used: gas atomised powders of the aluminium alloys AlMg5, AlSi12, AlSi12Mg5 and AlSi10Mg, as well as mixtures of powders of the elements Al and Si and the binary alloys Al–5Mg, Al–5Zr, Al–5Cr and Al–10Mn. Microstructure, hardness, tensile properties and corrosion behaviour of the welds were investigated in the as-welded T4 and T6 conditions and after a post-weld heat treatment to the T6 temper. The weld region exhibited a dendritic cellular structure in the fusion zone and a partially melted zone adjacent to the fusion boundaries. The hardness of the fusion zone depended upon the filler metals used. Post-weld artificial aging to the T6 temper improved the hardness, being associated with precipitation of strengthening phases, as confirmed by transmission electron microscopy. Joint efficiencies achieved for post-weld heat treated joints ranged from 80 to 90%. Strengths of welds in the as-welded T6 condition were lower due to the softened fusion zone and heat affected zone, being between 70 and 75% of the ultimate tensile strength of the base alloy 6013–T6. Optimum tensile properties were obtained with joints made with the filler powder AlSi12. Reasonable tensile properties were achieved for welds made with mixtures of elemental and binary alloy powders, but they did not reach those of joints made with aluminium alloy powders. The dispersoid forming elements Zr, Cr and Mn added to a mixed Al–7Si powder did not prove beneficial effects on weld quality. When exposed to an intermittent acidified salt spray fog, joints in the as-welded T4 and post-weld heat treated T6 conditions exhibited corrosion behaviour similar to that of the base sheet in the tempers T4 and T6. As-welded 6013–T4 joints were susceptible to stress corrosion cracking when immersed in an aqueous solution of 0.6 M NaCl + 0.06 M NaHCO 3 . Sensitivity to environmentally assisted cracking was associated with grain boundary precipitates in the heat affected zone.

Engine designers show continued interest in titanium aluminides based on the intermetallic γ-TiAl phase as lightweight structural materials to be used at moderately elevated temperatures. Although alloy development has made significant progress in terms of mechanical properties and environmental resistance, protective coatings have been developed that help to extend the lifetime of these alloys significantly. The major challenge of coating development is long-termstability of aprotective oxide scale that forms during service for which purpose alumina formation is essential. Furthermore, changes of coating chemistries at high temperatures must be controlled to avoid rapid degradation of the coatings due to diffusional losses into the substrate material and vice versa.

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of resisting temperatures in excess of 850°C. Under these conditions, environmental and thermal protection becomes a concern since oxidation might eventually limit the maximum service temperatures achievable. Therefore protective coatings are necessary to exploit the full potential of gamma titanium aluminides at moderately elevated temperatures; however, as yet no coating system tested has proven sufficient performance for long-term use in automotive, aerospace as well as power generation applications. Recently, promising coating systems have been developed that appear to have a fairly high potential for the targeted applications. A novel approach is the use of thermal barrier coatings (TBCs) on γ-TiAl, typically applied to nickel-based superalloys. This allows to reduce the metal surface temperatures in combination with component cooling. The paper is focussed on both development of oxidation resistant coatings as well as thermal barrier coatings for gamma titanium aluminides. TiAlCrY nitride and metallic overlay coatings based on TiAlCr were produced by magnetron sputtering techniques. Both coatings provide an excellent oxidation protection for γ-TiAl alloys in air at 750°C. Using the pack-cementation process, the surface region of γ-TiAl specimens was aluminized to TiAl3. Adopting state-of-the-art technology for nickel-base gas turbine components, EB-PVD yttria-stabilized zirconia TBCs were deposited on Ti-45Al-8Nb, either pre-oxidized or coated with various “bond coats” mentioned above. The oxidation behaviour of the specimens with environmental and thermal protective coatings was determined under cyclic oxidation conditions at 850 and 900°C in air up to 2000 h and compared to that of the γ-TiAl substrate material. Post-oxidation SEM/EDS analysis of the coating systems will be presented, highlighting the particular microstructure features which will be discussed with regard to oxidation behavior and lifetime of the coating systems

The stress corrosion cracking (SCC) behaviour of 8090-T8171 plate material was investigated in short transverse direction performing constant load tests and constant extension rate tests under permanent immersion conditions. At an applied stress of 100 MPa, smooth round tensile specimens were exposed to synthetic environments containing chlorides and various nonhalide anions. Environment-induced cracking was not observed in aqueous solutions of 0.6 M NaCI, LiCl, NH 4 Cl, or MgCl 2 . In 0.6 M NaCI solutions containing 0.06 M Na 2 SO 4 or Na 3 PO 4 , the SCC behaviour of 8090-T8171 plate was similar to that observed in pure 0.6 M NaCI solution. Sodium chloride solutions with additions of nitrate, hydrogen carbonate, or carbonate promoted stress corrosion cracking. Threshold stresses below 100 MPa were obtained from constant load tests using the latter environments. When sodium sulfite or sodium hydrogen phosphate was added, values being 100 MPa or slightly higher were determined. Lithium and ammonium present as cations in mixed salt electrolytes accelerated SCC failure. Lithium chloride solutions containing nitrate, hydrogen carbonate, carbonate, or sulfite were highly conducive to stress corrosion cracking. Very low SCC resistance was found for alloy 8090-T8171 exposed to synthetic environments with additions of ammonium salts. Constant extension rate tests were carried out using notched tensile specimens. Displacement rates were in the range 2 × 10 -6 - 2 × 10 -5 mms -1 . Aqueous 0.6 M NaCI solutions with additions of 0.06 M NH 4 HCO 3 , (NH 4 ) 2 SO 4 , or Li 2 CO 3 promoted environment-induced cracking with 8090-T8171 plate, as indicated by severe degradation of notch strength. The constant extension rate testing technique did not indicate SCC susceptibility using sodium chloride solutions containing sodium sulfate or lithium sulfate. For specimens exposed to substitute ocean water a slight degradation of notch strength was found at the lowest displacement rate applied.

Double layer coatings consisting of a 2 µm thick diffusion barrier and a 5 µm thick oxidation protective topcoat were deposited on Mo–Si–B alloys using magnetron sputtering. During vacuum annealing of the amorphous as-deposited coatings, the tetragonal D8l–Mo5SiB2 phase evolved in the Mo–12Si–21B (at.%) interlayer; in the Mo–48Si–24Al and Mo–71Si–8Al (at.%) topcoats, the C40–Mo(Si,Al)2 and the C11b–MoSi2 phases formed, respectively. The oxidation behavior of the coated samples was investigated at 800 and 1000 °C under cyclic conditions in air. Compared to the bare substrate, the coated samples exhibited significantly reduced mass loss at both temperatures. A scale with a mixture of silica and mullite-like oxides formed on the coatings at 800 °C; in addition, outer aluminum borate needles grew on the Mo–48Si–24Al topcoat. At 1000 °C both coatings formed dense scales of SiO2 and a mullite-like phase, being protective for more than 100 h.

International audience; The oxidation behaviour of Nb silicide based alloys, considered as potential ultra-high temperature material for gas turbine engine applications, can be improved by chromia-silica forming coatings with M7Si6 structure. To stabilise the protective oxide scales against water vapour corrosion in combustion atmospheres, environmental barrier coatings (EBCs) of yttrium silicate were deposited on an FeB modified M7Si6 based bond coat using magnetron sputtering. The 15-20 mm thick ceramic top coats with an approximate chemical composition of 25Y-13Si-62O (at.-%) were dense and amorphous after deposition. They were annealed in vacuum to get a crystalline structure with the predominant phase Y2SiO5. Samples with this EBC system were tested in rapidly flowing water vapour at temperatures between 1100 and 1300 degrees C for up to 16 h. At 1100 and 1200 degrees C, significantly reduced mass gains were determined for samples with yttrium silicate top coat in comparison to those coated only with FeB containing layers. The EBC partially transformed into the Y2Si2O7 phase and exhibited microporosity. At 1300 degrees C, the yttrium silicate layer decomposed forming yttrium oxides.

International audience; Coupons of a Nb-silicide-based alloy with the nominal composition of 47Nb-25Ti-8Hf-2Cr-2Al-16Si (at-%) were coated with Co-, Ni- and FeB-modified M7Si6-based layers using packcementation. Subsequently, on one side of the coated samples, environmental/thermal barrier coatings (E/TBCs) of yttria partially stabilised zirconia (YSZ) and gadolinium zirconate were deposited by electron beam physical vapour deposition. The lifetimes of the different E/TBC systems were determined performing thermal cycling tests at 1100 and 1200 degrees C in air. At 1100 degrees C, lifetimes approaching 1000 cycles (corresponding to 1000 h of high temperature exposure) were measured. However, severe oxidation occurred at the edges and rear side of the samples (areas without ceramic topcoat) due to local degradation of the oxidation protective coatings, resulting in substantial material recession with prolonged testing. When thermally cycled at 1200 degrees C, the E/TBC systems exhibited lifetimes exceeding 200 cycles. The sample with FeB containing bond coat and Gd2Zr2O7 topcoat survived even 400 cycles at this exposure temperature.

Intermetallic Ti–60Al–13Cr based coatings with additions of 1 at.-% yttrium or 4 at.-% zirconium were deposited on a γ-TiAl based Ti–45Al–8Nb (at.-%) TNB alloy using magnetron sputtering. The Zr containing layer was also used as bond coat for a thermal barrier coating (TBC) of yttria partially stabilised zirconia produced by electron beam physical vapour deposition. Cyclic oxidation tests at 850°C in laboratory air revealed excellent oxidation behaviour of the Y containing coating up to more than 2000 one hour cycles. Similarly high oxidation resistance was also observed for the Zr containing bond coat beneath the TBC. However, without ceramic topcoat, the Ti–60Al–13Cr–4Zr coating degraded after 400 one hour cycles due to the occurrence of cracks in the brittle intermetallic coating. Furthermore, fatigue tests of coated tension specimens were performed after an isothermal exposure at 850°C for 300 h. The deposition of the intermetallic Ti–Al–Cr based coatings reduced the fatigue strength by more t...

The corrosion behaviour of a scandium containing Al-Mg alloy sheet was investigated performing accelerated corrosion tests. The material was studied in an annealed temper (2h at 350°C) and after additional heat treatments at temperatures ranging from 85 to 150°C for time periods up to 3000h. In the annealed condition, the sheet was resistant against intergranular corrosion and stress corrosion cracking and exhibited a moderate susceptibility to exfoliation corrosion. The corrosion resistance was decreased by thermal exposure at temperatures between 85 and 150°C, as indicated by high mass loss and severe exfoliation corrosion attack observed in the NAMLT test and ASSET test, respectively. When alternately immersed in 3.5% NaCI solution under static loading conditions, the scandium bearing Al-Mg alloy was found to be immune against stress corrosion cracking. For sheet material heat treated in the temperature range from 85 to 150°C, environment-induced failure was observed under the severe loading conditions of the slow strain rate testing technique using an aqueous solution of 3% NaCl + 0.3% H 2 O 2 . The deterioration in corrosion resistance of the Al-Mg-Sc alloy sensitised by thermal exposure was associated with grain boundary precipitation of a magnesium rich phase.

Protective coating systems were applied to Mo–9Si–8B (at.%) alloys to prevent oxidation at elevated temperatures. The coatings produced by magnetron sputtering and subsequent annealing consisted of an outer oxidation protection layer and an interlayer between this and the substrate. Three amorphous outer layers with different compositions were deposited: Mo–45Si–25B, Mo–55Si–10B and Mo–29Si–15B (all in at.%). The interlayer was selected to give a diffusion barrier with the composition of the Mo 5 SiB 2 (T2) phase. All coatings were dense and well-adherent. During vacuum annealing the amorphous as-deposited coatings became crystalline exhibiting mainly the intermetallic Mo 5 SiB 2 compound as interlayer and the MoSi 2 , Mo 5 Si 3 and MoB phases in the top layers. The samples were exposed to dry laboratory air in the pesting regime at 800 °C and above, i.e. at 1000 and 1300 °C for up to 100 h under cyclic conditions. All coatings were protective at 800 and 1000 °C for at least 100 h and showed a marked improvement in mass change compared to the uncoated substrate. For protection at 800 °C higher boron content is preferential, while at higher oxidation temperatures a lower boron content provides improved oxidation protection. At 1300 °C stress induced failures like cracking, spallation and buckling occurred due to the relatively high CTE mismatch between PVD coating and substrate. Even though, the mass change was still markedly reduced as compared to the bare substrate.

Samples of a high niobium bearing γ-TiAl based alloy were coated with intermetallic Ti–52Al–15Cr–0.4Y (in at.%) layers produced by magnetron sputtering. On some of the specimens thermal barrier coatings of 7 wt.% yttria partially stabilised zirconia were deposited using electron-beam physical vapour deposition. The oxidation behaviour of the coated Ti–45Al–8Nb (at.%) alloy with and without thermal protection was investigated at 950 °C under cyclic oxidation conditions in air. The 20 μm thick Ti–Al–Cr–Y layer exhibited excellent oxidation resistance at this exposure temperature associated with the formation of a thin continuous alumina scale. Lifetimes exceeding 1000 cycles of 1 h dwell time at 950 °C were determined for the TBC system consisting of zirconia topcoat and Ti–Al–Cr–Y bond coat.

Intermetallic Ti-Al-Cr layers with small additions of Si, Zr, W, and Y were deposited on γ-TiAl specimens using magnetron sputtering. The oxidation behaviour of the coated γ-TiAl alloy was studied in the temperature range between 950 and 1000°C under cyclic oxidation conditions in air. Compared to the bare substrate material, the coatings exhibited higher oxidation resistance. During prolonged exposure the intermetallic layers degraded losing their capability to form a protective alumina scale. On coated γ-TiAl samples zirconia topcoats were deposited by electron beam physical vapour deposition. These thermal barrier coating systems exhibited lifetimes exceeding the maximum exposure length of 1000 1 h cycles at 950°C, but failed at 1000°C. Failure was caused by degradation of the bond coats resulting in spallation of the thermally grown oxides.

Titanium aluminides suffer from non-protective mixed-oxide scale formation during high-temperature exposure in oxidizing environments, so that they cannot be used at temperatures above approximately 800° C for longer times without additional treatment. A fluorine treatment on γ-TiAl alloys leads to the formation of a pure protective alumina scale and allows their use at service temperatures above 800°C. This thermally grown aluminum oxide layer can be used for bonding ceramic thermal barrier coatings to the TiAl substrate. Zirconia topcoats deposited by electron-beam physical vapor deposition were very adherent to F-treated TiA samples during cyclic oxidation tests at 900 to 1,000°C. A separate bond coat is not needed in this case.

The oxidation behaviour of CrAlN + 2 mol.% YN thin films deposited on γ-TiAl based alloy Ti–45Al–8Nb (at.%) was investigated at 850 and 900 °C under cyclic oxidation conditions in air. At both temperatures the coated samples exhibited significantly reduced mass gains compared to the bare substrate material. Slow oxide growth rates were observed for up to 2000 cycles of 1-h dwell time at high temperature, indicating effective oxidation protection provided by the CrAlYN coating to the γ-TiAl alloy. Microstructural examinations of coated specimens after high temperature exposure revealed an outer mixed scale of chromia and alumina grown on the degraded nitride film. Beneath the coating, TiN and Ti 2 AlN layers formed. When thermally cycled at 900 °C in air, thermal barrier coatings of yttria partially stabilized zirconia deposited on γ-TiAl samples coated with CrAlN + 2 mol.% YN thin films exhibited lifetimes exceeding the maximum exposure time period of 1000 1-h cycles. No spallation of the thermal barrier coating system was observed. The zirconia topcoats produced by electron-beam physical vapour deposition were well adherent to the mixture of chromium and aluminium oxides formed on the nitride coating.

Thermal barrier coatings (TBCs) of yttria partially stabilized zirconia were deposited on gamma TiAl samples using electron-beam physical vapour deposition. The specimens were coated with intermetallic Ti-Al-Cr layers and CrAlYN/CrN nanoscale multilayer coatings. The lifetime of the TBC systems was determined performing cyclic oxidation tests in air at temperatures between 850 and 950 degrees C. The TBC systems with Ti-Al-Cr and CrAlYN/CrN layers did not fail at 850 and 900 degrees C during the maximum exposure time period of 1000 cycles of 1 h dwell time at high temperature. No spallation of the thermal barrier coatings was observed. As revealed by post-oxidation microstructural analysis, the protective coatings were severely degraded when exposed at 900 degrees C, resulting in growth of mixed oxides on the substrate. Underneath the thermal barrier coating an outer oxide scale with a columnar structure was observed, consisting of rutile and alpha-Al2O3. Energy-dispersive X-ray spectroscopy analysis revealed zirconia and chromia being dissolved in the outer oxide scale. The columnar structure and the presence of zirconia indicated an effect of the TBC on the morphology of the outer oxide scale. The zirconia top coat exhibited an excellent adherence to this oxide scale formed on the protective layers when degraded, and at defects like cracks. When thermally cycled at 950 degrees C, the TBC system on specimens coated with Ti-Al-Cr failed by spallation of the thermally grown mixed oxides, whereas the thermal barrier coating was well adherent to the outer oxide scale at this temperature, too.

The stress corrosion cracking (SCC) behaviour of flat extrusions of the Al-Li-Cu-Mg-Zr alloys 8090-T851 and 2091-T851 was investigated performing accelerated tests under constant deformation, constant load, and slow strain rate conditions. The used corrosive environments were an aqueous 3.5% NaCl solution and an aqueous solution of 2% NaCl+0.5% Na 2 CrO 4 at pH=3. Round tensile specimens and tuning fork type specimens were alternately immersed in 3.5% NaCl solution according to ASTM G44 under constant deformation, whereas they were continuously immersed in the other tests

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of operating at temperatures in excess of 850 °C. Under these conditions, environmental and thermal protection becomes a concern since oxidation might eventually limit the maximum service temperatures achievable. Therefore protective coatings are necessary to exploit the full potential of gamma titanium aluminides at moderately elevated temperatures; however, as yet no coating system tested has proven sufficient performance for long-term use in automotive and aerospace applications. Thermal barrier coatings (TBCs), typically applied to nickel-based alloys, offer the potential to increase the service temperature of components by lowering the metal surface temperature in combination with cooling systems. The paper is focussed on development of thermal barrier coatings for gamma titanium aluminides. Different coatings were used for oxidation protection and bond coat application. Substrate specimens were either pre-oxidized or coated with PVD-Al 2 O 3 , TiAl-CrYN, or diffusion aluminides. Yttria-stabilized zirconia TBCs were deposited applying electron-beam physical vapour deposition. Cyclic and quasi-isothermal oxidation tests were carried out at 900 °C in air. Post-oxidation analysis of the coating systems was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy. Zirconia top coats offer a promising thermal protection concept to be applied on γ-TiAl components. However, high oxidation resistance has to be supplied by protective coatings. Diffusion layers of the TiAl 3 aluminide provided excellent environmental protection because of the formation of a continuous alumina scale. No spallation of the thermal barrier coatings was observed on aluminized specimens during 1000 1-h cycles and 3000 h of cyclic and isothermal oxidation testing, respectively.

The aerospace as well as the automotive industry permanently intend to reduce the weight of applied structural materials. Therefore, γ-TiAl is an attractive material fulfilling that requirement due to its low mass density, generally, and good mechanical properties in the temperature range between 700°C and 900°C. However, application of titanium aluminides above 750°C service temperature is limited by poor oxidation behaviour caused by formation of fast growing and porous titania. To decrease the oxidation rate of TiAl it is essential to prevent the formation of non-protective TiO 2 scales. In this study the affinity of titanium to oxygen was lowered by forming titanium silicides. Si-based coatings with a thickness of 10µm were deposited on the TiAl substrate by means of magnetron sputter technique. The coating system had to be pre-treated to form thermodynamically stable titanium silicides. A detailed study of phase formation in bare γ-TiAl under various exposure conditions, such as oxygen and nitrogen partial pressure, temperature and time, led to an optimized pre-treatment process of 100h at 1000°C at 10 -6 mbar. Thus no oxides and nitrides were formed during this procedure but phase alterations occurred caused by interdiffusion processes. Therefore this pre-treatment was successfully used to form titanium silicides in the coatings. Annealing at 1000°C under high-vacuum conditions resulted in the formation of a Ti 5 Si 3 layer, whereas less thermodynamically stable titanium silicides (Ti 5 Si 4 , TiSi and TiSi 2 ) formed when exposed to air at 750°C for 100h. With the latter pre-treatment, a mixed oxide scale was also found on the top of the coating. The oxidation behaviour of the pre-treated coatings on γ-TiAl was studied by cyclic testing at 900°C and 950°C in air. Phase formation and microstructure evolution of the samples were investigated after 10, 100 and 1000 cycles by SEM and EDX as well as XRD. Notably, the oxidation rate of the high-vacuum annealed samples is lower than that of the pre-oxidized ones. All specimens exceeded a lifetime of 1000 cycles at both test-temperatures at minimum.

A short review of the stress corrosion cracking (SCC) behaviour of aluminium alloys is given. Mechanisms of environmentally assisted cracking are outlined. For aluminium alloys, in which stress corrosion cracks propagate predominantly along grain boundaries, anodic dissolution and hydrogen embrittlement have been proposed. Transgranular stress corrosion cracking occurring at severe loading conditions has found particular interest concerning localised corrosion-deformation interactions. Accelerated test methods for assessing the SCC behaviour are described, including the slow strain rate testing technique and the breaking load method. Results of recent studies on environmentally assisted cracking of aluminium alloys are summarised. Most of the work published in the last two decades has been on aluminium-lithium based alloys and improved high strength Al-Zn-Mg-Cu alloys in corrosion resistant retrogressed and re-aged tempers.

Microstructure and corrosion behaviour of 6061 and 6013 sheet material were investigated in the naturally aged and peak-aged heat treatment conditions. Transmission electron microscopy did not reveal strengthening phases in the naturally aged sheet. In the peak-aged temper, β’’ precipitates were observed in alloy 6061, whereas both β’’ and Q’ phases were present in 6013- T6 sheet. Marked grain boundary precipitation was not found. Corrosion potentials of the alloys 6061 and 6013 shifted to more active values with increasing aging. For the copper containing 6013 sheet, the potential difference between the tempers T4 and T6 was more pronounced. When immersed in an aqueous chloride-peroxide solution, alloy 6061 suffered predominantly intergranular corrosion and pitting in the tempers T4 and T6, respectively. On the contrary, 6013 sheet was sensitive to pitting in the naturally aged condition, and intergranular corrosion was the prevailing attack in the peak-aged material. Both alloys 6061 and 6013 were resistant to stress corrosion cracking in the tempers T4 and T6.

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti–Al–Cr–Y–N and Ti–Al–Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti–45Al–8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD). Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti–Al–Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti–Al–Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti–Al–Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.

Al-based intermetallic coatings are widely used as bond coats and for oxidation protection of turbine blades and engine components. Here we present and discuss the structural and thermal properties of novel unbalanced-magnetron sputtered Al-based coatings (single-phase intermetallic Al2Au, binary Al-Zr, and nano-structured Al-Zr-Y) developed to protect gamma-TiAl from environmental attack at elevated temperatures. Al-Zr films exhibit a coarse-grained dual-phase Al3Zr2-Al2Zr structure and are nano-structured by alloying with ~5, 10, and 14 at% Y.Combined dynamic differential-scanning calorimetry and thermogravimetric analyses up to a temperature of 1150 °C reveal that the Al2Au film is very stable with only marginal mass gain from oxidation found between 800 and 1000 °C. High temperature X-ray diffraction shows that this coating retains its (311) texture up to 900 °C where Al2O3 formation leads to the depletion of Al in Al2Au and subsequently the precipitation of intermetallic AlAu. When gamma-TiAl is coated with Al2Au and exposed to cyclic oxidation tests at 750 and 850 °C good oxidation resistance is obtained as a protective oxide layer is formed.Dual-phase Al3Zr2-Al2Zr coatings form ZrO2 and Al2O3 in oxidizing atmosphere. However, the phase transition from monoclinic (m-)ZrO2 to tetragonal (t-)ZrO2 with the accompanying volume change causes flaking of the oxide. Yttrium addition to the Al-Zr films stabilizes the cubic (c-) and t-ZrO2 and hence avoids the fatal tetragonal-monocline transformation. The thermally grown c-ZrO2 based oxides allow good adhesion to thermal barrier coatings which are themselves based on c-ZrO2.

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)2 Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhib...

The stress corrosion cracking behaviour of plate material of the aluminium alloys 2024-T351, 8090-T8171, 7475-T651, and 7075-T7351 was investigated performing constant load tests. Short transverse tensile specimens were permanently immersed in aerated aqueous 0.6 M Na2Cl solutions with additions of Na2SO4, NaNO3, NaHCO3, NH4HCO3, Na2HPO4, Na2SO3 or Na2CO3. The concentration of the added salts was 0.06 M. The applied stress was 100 MPa, except with 7075-T7351 specimens, which were loaded at 300 MPa. Environment induced failure was not observed in neutral 0.6 M NaCl solution. The various salts added promoted intergranular stress corrosion cracking with the alloys 2024-T351, 8090-T8171, and 7475-T651. Threshold stresses were generally below 100 MPa. For 8090-T8171 exposed to chloride containing electrolytes with additions of sulfate, hydrogen phosphate, or sulfite, threshold stresses were approximately 100 MPa or higher. Similar results were obtained for 7475-T651 plate when immersed in chloride-hydrogen phosphate and chloride-carbonate solutions. Alloy 7075-T7351 was resistant against intergranular stress corrosion cracking. Specimens suffered pitting corrosion during immersion in the corrosive environments. Failure observed with 7075-T7351, in particular when exposed to the chloride-nitrate solution, was associated with reduction of cross-sectional area due to pitting and transgranular stress corrosion cracking. Anioneneinflusse auf das Spannungsrisskorrosionsverhalten von Aluminiumlegierungen Das Spannungsrisskorrosionsverhalten von Plattenmaterial der Aluminiumlegierungen 2024-T351, 8090-T8171, 7475-T651 und 7075-T7351 wurde in kurzer Querrichtung untersucht. Dauertauchversuche wurden unter konstanter Belastung in wassrigen 0,6 M NaCl Losungen mit 0,06 M Zusatzen von Na2SO4, NaNO3, NaHCO3, NH4HCO3, Na2HPO4, Na2SO3 oder Na2CO3 durchgefuhrt. Die aufgebrachte Spannung betrug 100 MPa, auser bei Zugproben der Legierung 7075-T7351, die mit 300 MPa belastet wurden. Wahrend bei Auslagerung in neutraler Natriumchlorid-Losung kein Versagen beobachtet wurde, losten die zugegebenen Salze bei den Legierungen 2024-T351, 8090-T8171 und 7475-T651 interkristalline Spannungsrisskorrosion aus. Die kritischen Grenzspannungen lagen uberwiegend unter 100 MPa. Bei Auslagerung in chloridhaltigen Elektrolyten mit beigegebenen Sulfat-, Hydrogenphosphat- oder Sulfitionen waren die kritischen Grenzspannungen fur die Legierung 8090-T8171 etwas hoher; sie hatten einen um 100 MPa oder daruber liegenden Wert. Gleiches galt fur die Legierung 7475-T651 in chloridhaltigen Losungen mit Hydrogenphosphat- oder Carbonatzusatzen. Plattenmaterial der Legierung 7075-T7351 war in den untersuchten Elektrolyten bestandig gegen interkristalline Spannungsrisskorrosion. Die Proben wurden jedoch durch Lochkorrosion geschadigt. Querschnittsverminderung infolge Lochkorrosion und transkristalliner Spannungsrisskorrosion fuhrten zum Versagen von Zugproben der Legierung 7075-T7351 innerhalb der maximalen Prufdauer von 30 Tagen, insbesondere bei Auslagerung in eine chlorid-nitrathaltige Losung.

The stress corrosion cracking behaviour of 7475 plate material in the tempers T651 and T7351 was investigated performing constant load tests. Short transverse specimens were permanently immersed in aerated aqueous 0.6 M sodium chloride solutions with additions of 0.03 M sodium sulphate, 0.03 M sodium nitrate and 0.1 M sodium bicarbonate. Chloride solutions containing sulphate, nitrate or/and bicarbonate promoted environment-induced cracking. A short transverse threshold stress below 50 MPa was found for 7475-T651 plate material. Neutral 0.6 M NaCl solution was less conducive to stress corrosion cracking. Specimens of alloy 7475 in the overaged temper T7351 failed at applied stresses above 300 MPa. This failure resulted from overload fracture caused by a reduction of cross-sectional area due to pitting corrosion, as confirmed by fractography showing severe pitting attack. Fractographic examinations of 7475-T7351 specimens failed during immersion in chloride-nitrate-bicarbonate containing solutions with or without addition of sulphate after long exposure time periods revealed slight transgranular stress corrosion cracking, too. Pronounced transgranular environment-induced cracking was observed on the fracture surfaces of specimens which were exposed to an aqueous solution containing chloride, sulphate, nitrate, and bicarbonate for 30 days at applied stress and were subsequently tensile tested in an inert environment.

The oxidation behaviour of two γ-TiAl-based alloys coated with thermal barrier coatings (TBCs) was investigated in air at 900 °C for up to 1000 h under quasi-isothermal conditions. Samples bare or with a 4 μm thick Ti–51Al–12Cr (at.%) protective layer were coated with yttria-stabilized zirconia using electron-beam physical vapour deposition. Prior to TBC deposition, the specimens were pre-oxidised in air and low-pressure oxygen. Thermal barrier coatings on samples of the alloy Ti–45Al–8Nb–0.2B–0.15C (at.%) did not spall off during the maximum exposure length, whereas failure occurred with pre-oxidised specimens of the alloy Ti–45Al–6Nb–1Cr–0.4W–0.2B–0.5C–0.2Si (at.%). Scanning electron micrographs revealed an outer oxide scale with a columnar structure grown by outward diffusion of Ti and Al cations. Below this columnar oxide layer, a broad porous zone formed consisting predominantly of titania. A discontinuous nitride layer was observed in the transition region between oxide scale and substrate. Spallation of the TBC was associated with cracking in the porous titania-rich layer. The zirconia top coat exhibited an excellent adherence to the thermally grown oxide scales. Pre-exposure in low-pressure oxygen did not improve the oxidation resistance. Post-oxidation microstructural examinations of specimens coated with Ti–Al–Cr revealed that the protective layer was severely deteriorated. On the degraded Ti–Al–Cr coating, an outer scale with columnar structure formed beneath the TBC.