Oxidation & fatigue behaviour of ɣ-Titanium aluminides coated with yttrium or zirconium containing intermetallic Ti-Al-Cr layers and TBC

Kurzfassung

Intermetallic titanium aluminide based alloys are light-weight high temperature materials partially replacing Ni-based superalloys, which have recently been introduced in the low pressure turbine of jet engines. Due to their inadequate oxidation behaviour above 800°C, protective coatings are expected to be applied for higher service temperatures. In the present work, intermetallic Ti-60Al-13Cr coatings with additions of 1 at.% yttrium or 4 at.% zirconium were deposited on a γ-TiAl based Ti-45Al-8Nb (at.%) alloy using magnetron sputtering. Samples with Ti-Al-Cr-Zr layers were also coated with a thermal barrier coating of yttria partially stabilized zirconia produced by electron-beam physical vapour deposition. The coated γ-TiAl specimens were thermally cycled at 850°C in laboratory air. The maximum exposure time period was 2000 cycles corresponding to 2000 h of exposure at high temperature. Post-oxidation analyses of the cross sections were performed using scanning and transmission electron microscopy with energy-dispersive X-ray spectroscopy and selected area diffraction. Furthermore, the fatigue behaviour of coated tension specimens was studied after isothermal exposure at 850°C for 300 h. The Ti-60Al-13Cr-1Y bond coat exhibits an excellent oxidation resistance at 850°C for up to 2000 cycles. Yttrium-aluminium oxides formed in the thermally grown alumina layer as well as in the intermetallic coating, which primarily precipitated at the grain boundaries, improving the oxidation resistance due to the so-called reactive element effect. A similar oxidation resistance was observed for the Ti-Al-Cr-Zr layer used as bond coat beneath the thermal barrier coating. However, without ceramic topcoat, the Ti-Al-Cr-Zr coating degraded after about 400 cycles, probably because of the substitution of zirconium in the intermetallic phases of the coating. The fatigue behaviour of the γ-TiAl alloy was deteriorated by the coatings. Compared to the thermally exposed bare substrate material, the fatigue strength of the tension samples with intermetallic coatings was about 400 MPa lower; it was further reduced by the thermal barrier coating.