Fundamentals of the long time oxidation protection of gamma-titanium aluminides at high temperatures by using the fluorine effect

SCHU 729/15-1

Bild Forschungsprojekt

Hochtemperaturofen in Betrieb

Laufzeit: 01.11.2004 - 31.10.2006
Geldgeber: Deutsche Forschungsgemeinschaft (DFG)
Bearbeiter: Dr. Hans-Eberhard Zschau
Arbeitsgruppe: Hochtemperaturwerkstoffe

The increasing interest in Gamma-TiAl based alloys is motivated by their excellent specific strength at high temperatures which offers a high application potential in aerospace and automotive industries. Their benefit is a specific weight reduced of about 50 % compared to the presently used Ni-based superalloys. To improve the insufficient oxidation resistance at temperatures above 750°C the fluorine effect offers an innovative way. The doping of fluorine on the TiAl surface leads to the formation of a dense alumina scale which protects the TiAl against corrosion. As materials cast TiAl and technical Gamma-Met were used. Two methods of F-doping were applied: ion implantation and dipping into diluted HF, followed by long time isothermal (4000 hours) and thermocyclic (2600 hours) oxidation in air at 900°C and 1000°C. The fluorine concentration at the metal/oxide interface is essential for the long time stability of the effect. Therefore extended PIGE measurements using a particle accelerator were performed to obtain the F-depth profiles within the first 1.6 micrometer. Despite the high F loss during heating and within the first hours of oxidation the results show a nearly constant fluorine maximum between 0.3 and 1.2 at.-%. This stability results from two conditions: Firstly the formed alumina scale acts as a diffusion barrier against fluorine loss through the surface. Secondly the low diffusion coefficient of fluorine in TiAl at high temperatures allows only a very low fluorine transport into the metal. In order to install a higher F reservoir within the near surface region multiple implantations were simulated using the T-DYN software package, followed by implantations with several fluences and energies. The F amounts were enhanced by a factor of 2 compared to the single implantation case. The results obtained with ion beam methods show the excellent technical potential of the surface modified TiAl.

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