Oxidation behaviour of TiAl-based intermetallic coatings on γ-TiAl alloys

Abstract

TiAl-based intermetallic layers with additions of Ag, Cr, Y and Hf were deposited on c-TiAl specimens using magnetron sputtering. The oxidation behaviour of the coated Ti-45Al-8Nb (at.%) alloy was investigated at 850 8C and 900 8C under cyclic oxidation conditions in air. The ternary Ti-50Al-2Ag (at.%) coating did not provide oxidation protection to the substrate material at 850 8C. Two-phase c-TiAl + Ti(Cr, Al) 2 Laves coatings exhibited high oxidation resistance at 900 8C 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 8C. R. Braun et al.: Oxidation behaviour of TiAl-based intermetallic coatings on c-TiAl alloys Int.

Thermodynamic modeling of the Ti–Al–Cr ternary system

“…Cr:Cr,Ti + y Ti y " Cr y " Ti L 0 Ti:Cr,Ti + y Cr y Ti y " Cr L 0 Cr,Ti:Cr + y Cr y Ti y " Ti L 0 Cr,Ti:Ti +y Al y Cr y " Ti L 0 Al,Cr:Ti (14) E G Ø C15 = y Cr y " Cr y " Ti L 0 Cr:Cr,Ti + y Ti y " Cr y " Ti L 0 Ti:Cr,Ti + y Cr y Ti y " Cr L 0 Cr,Ti:Cr + y Cr y Ti y " Ti L 0 Cr,Ti:Ti (15) The Ti 3 Al is modeled as (Al,Cr,Ti) 3 (Al,Ti) in the ternary system, which is based on the substitution of Cr for Ti in Ti 3 Al confirmed by Hao et al [56]. Its ref G Ø , id G Ø , and E G Ø can be described by the following expressions:…”

“…The studies of structure and some properties of Ti-Cr alloys and the formation of nanocomposites containing Al and TiAl 3 in Ti-Al alloys are also reported in recent years [10][11][12]. In additon, the Ti-Al based coatings including Cr for high temperature oxidation protection of ␥-TiAl are also investigated recently [13][14][15]. Furthermore, Cr is the common ␤-stabilizer for the titanium alloys [16], and much cheaper than the elements V, Mo and Nb [17][18].…”

Lifetime of 7YSZ thermal barrier coatings deposited on fluorine-treated γ-TiAl-based TNM-B1 alloy

“…The halogen effect was no longer operative. Instead of a continuous thin alumina layer, a nitride layer interrupted with alumina particles was observed at the scale/substrate interface, as usually found for oxidized Nb‐ and Mo‐containing γ‐TiAl‐based alloys . Below the TBC, protrusions of an outer oxide scale with a columnar structure developed.…”

Oxidation Protection of γ-TiAl Alloys by Intermetallic Ti-Al-Cr-Zr Coatings

“…The high oxidation resistance of Ti-Al-Cr based alloys was associated with the Ti(Cr,Al) 2 Laves phase exhibiting low oxygen permeability and being capable of forming a protective alumina scale [7,8]. During long-term exposure at elevated temperatures, Ti-Al-Cr based coatings were found to degrade by depletion in chromium [9,10]. This Cr loss was caused by rapid diffusion of chromium into the substrate and precipitation of a quaternary phase, probably a (Ti,Nb)(Cr,Al) 2 Laves phase, at the coating/substrate interface, resulting in dissolution of the Laves phase in the protective layer.…”