Oxidation Behaviour of TBC Systems on γ-TiAl Based Alloy Ti–45Al–8Nb

Abstract

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)Al 3 plus TiAl 2 microstructure of the coating transformed into single phase c-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 c-TiAl.

Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C

“…Similar Nb-rich precipitates formed upon cyclic oxidation of a Tie45Ale8Nb alloy at 850 C have been identified as (Nb,Ti) 2 Al [59]. In the ternary NbeTieAl system, other phases are also possible, such as Ti 4 Nb 3 Al 9 or Nb 2 TiAl [60,61].…”

Environmental protection of γ-TiAl based alloy Ti-45Al-8Nb by CrAlYN thin films and thermal barrier coatings

“…In contrast to the sample coated with CrAlN, a continuous Al 3 Nb zone did not form in the samples with yttrium containing thin films. For some regions, precipitation of a (Nb,Al)-rich phase was observed, being probably the s-(Nb,Ti) 2 Al phase, as found in the subscale region of oxidised g-TiAl alloys with high niobium content [29,32,34]. Obviously, the alumina/chromia scale was an effective barrier to oxygen diffusion, preventing oxidation of the substrate material.…”

“…EB-PVD zirconia topcoats exhibited excellent adherence to the mixed alumina/chromia scale grown on the coating during thermal exposure. Spallation of the TBC system was not observed up to 1000 1-h cycles at 900 C. Recent investigations on the performance of TBCs on g-TiAl alloys revealed good adherence of the EB-PVD zirconia topcoats to the alumina scales formed on Ti-Al-Cr and aluminide coatings as well as to mixed alumina and titania scales grown on bare substrate materials or degraded oxidation protective layers [9,[29][30][31][32][33][34]. Failure observed with TBC systems on g-TiAl alloys was associated with spallation of the thermally grown oxide scale, predominantly in the porous titania layer.…”

“…Due to the thermal insulation provided by ceramic topcoats with low thermal conductivities, internally cooled components can operate at increased temperatures. TBC systems on g-TiAl alloys exhibited promising performance when thermally cycled in air at temperatures between 850 and 1000 C [9,[28][29][30][31][32][33][34]. Failure observed was related to spallation of oxide scales formed on the substrate after degradation of the protective coatings.…”

Influence of thermodynamic activities of different masteralloys in pack powder mixtures to produce low activity aluminide coatings on TiAl alloys

“…Because of the brittleness of the TiAl 3 and TiAl 2 phases the coating layers easily spall off during cyclic oxidation under thermal stresses. Zhou et al [19] have determined that a TiAl 3 layer formed on TiAl alloyed with Nb or Cr have much better cyclic oxidation resistance than the TiAl 3 layer formed on nonalloyed TiAl and related it to the change in crystal structure of TiAl 3 from the brittle tetragonal D0 22 to the more ductile cubic L1 2 structure. For the alloy used in this investigation, TNM ® -B1, the XRD phase analysis shows that the TiAl 3 phase has the unfavored D0 22 tetragonal structure (space group I4/mmm).…”