Improvement of the γ-TiAl Oxidation Resistance by Aluminizing

In combination with good stiffness and strength, titanium aluminides offer the potential for component weight savings on the order of 50% over superalloys and steels. The target application range for Ti-Al intermetallics is 600-1000°C. However, the use of TiAl based components above 800°C is limited especially due to their poor oxidation resistance. The present project deals with aluminizing and Fluorine microalloying as two possible methods for improving g-TiAl oxidation resistance. This project includes also the investigation of the applicability of the thermal barrier coating concept for γ-TiAl. The long term application of TiAl alloys based on the gamma-phase at temperatures above 800°C requires suitable Al-diffusion coatings or additions of small amounts of fluorine to provide the needed oxidation resistance. Without these pre-treatments these alloys containing large amounts of titanium suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti-50at.-%Al alloy to TiAl₃, the most promising oxidation resistant phase in the Ti-Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800-1000°C in air for up to 300 h. Three different diluted HF solutions (0.01, 0.1, and 1 vol.-%) were applied to the TiAl specimens as a droplet. The isothermal oxidation behavior of the HF-treated specimens was investigated at 900°C in air for up to 100 h. Fluorine can have a beneficial effect on oxidation resistance in a certain F-range which has been quantified by thermodynamic calculations and by the experimental investigations. An alternative strategy for developing oxidation-resistant alloys is to design coating systems that lower the metal surface temperature. As the use on Ni-base alloys shows, the Thermal Barrier Coatings (TBC) concept offers the potential to drop metal-surface temperatures by up to 150°C in conjunction with component cooling. In this project, the TBC concept was applied to TiAl alloys for the first time. Yttria stabilized zirconia was used as a TBC onto modified γ-TiAl alloys, to lower the temperature of the metal substrate surface and to prolong the life of these alloys. As potential bond coats, the following surface treatments were applied: Al-diffusion coating, Al-diffusion coating combined with short term pre-oxidation in air, short term pre-oxidation in oxygen of a rough surface finish γ-TiAl sample without aluminide coating. Oxidation resistance of the modified γ-TiAl alloys/TBC systems was evaluated by isothermal oxidation tests at 900°C for 100 h in air. The pack-cementation and oxidation tests were carried out at Dechema (KWI). The Yttria stabilized zirconia TBC were applied by Atmospheric Plasma Spraying on the sand-blasted and modified γ-TiAl alloys by ATZ-EVUS. The morphology of the different specimens before and after oxidation was characterized by optical microscopy and scanning electron microscopy in plan view and cross sections. The composition of the modified γ-TiAl alloys and oxidation products was determined by energy dispersive X-ray analysis and electron-probe microanalysis. The proton induced gamma-ray emission and Rutherford backscattering spectrometry techniques were used at the Institute of Nuclear Physics at the University of Frankfurt/Main, to determine element concentration depth profiles. Alumina phases in oxide scales were identified using fluorescence spectroscopy at Forschungszentrum Jülich GmbH.