Improvement of the resistance of titanium aluminides to environmental embrittlement

Titanium aluminide alloys have recently been applied for first time as structural alloys in low-pressure turbine blades by an engine manufacturer. Indeed, they exhibit equivalent mechanical properties under service conditions as nickel-based superalloys but for half of their density. This enables an increase of the performances of the engine and/or a fuel consumption reduction. However, for engine applications requiring a higher thermal resistance, e.g. above 750°C, TiAl alloys possess an insufficient oxidation resistance and suffer from environmental embrittlement. In this interdisciplinary work, focus has been made from the surface preparation of alloys, development of coatings against environmental embrittlement up to the testing of mechanical properties after high temperature oxidation. The GE alloy (Ti-48Al-2Cr-2Nb) was used for the investigations as it exhibits a relative high ductility at room temperature (εR ~ 1.5 %) and has been already extensively investigated. Surface engineering showed that the surface temperature during the machining of the alloy increases significantly leading to embrittlement and ductility loss. Therefore, post polishing steps were achieved to obtain suitable surface roughness for the coating process without local embrittlement. Aluminum enriched coatings (between 50 and 60 at.%) combined with alloying elements, i.e. Cr, Nb, Si, Y, to improve the oxidation behavior and the corrosion resistance were produced by metal oxide chemical vapour deposition (MO-CVD), physical vapor deposition (PVD) and thermal spraying techniques (HVOF, APS). In addition, fluorine treatment (halogen effect [1]) either by plasma-immersion-ion-implantation (PI³) or by fluorine based polymer spraying was applied to the coating to enhance the growth of a dense and protective alumina layer on the top of the coating. The adhesion properties of thin coating (CVD, PVD) were investigated by applying repeated impacts of 2 to 15 mJ (Mercedes test [2]). All tests highlight the good impact resistance of the asdeposited coatings and no significant damage has been detected even after 105 impacts using a force of 800N.
After oxidation, the CVD deposited coatings exhibit very good behavior under repeated impacts. Thin (CVD, PVD) as well as thick (thermal sprayed) coatings were obtained and were tested for oxidation and corrosion resistance. CVD and PVD coatings exhibit thicknesses between 3 and 10 μm whereas the thickness of thermal sprayed coatings ranges between 150 and 300 μm (Fig. 1). Figure 1. SEM images of produced coatings. (a) MO-CVD, (b) PVD, (c) HVOF with ethene, (d) HVOF with kerosene, (e) HVOF with kerosene + Al addition in the powder, (f) APS. The produced coatings were tested for oxidation and corrosion. After pre-oxidation, the samples exhibit a better corrosion resistance in presence of salts (75-25 NaCl-NaSO4 mixture) at 850 °C after 350 h in air compared to literature data obtained at 650 °C [3]. The mechanical properties were investigated by means of 4-point bending and tensile tests on coated samples after 100 h oxidation at 900 °C in laboratory air. Results of mechanical testing are shown in Fig. 2. The CVD process combined with fluorine implantation (PI³) offers the best combination to remedy environmental embrittlement.
Using this process, after 100h of oxidation, it has been shown that 90% of the initial fracture strain and fracture stress can be maintained. For long term service (thousands of hours) coating thickness should be increased to offer an aluminium reservoir for the oxidation and to provide an efficient physical barrier to hot gas and corrosion agents. Figure 2. Fracture stress vs. fracture strain for the investigated coated samples.