Improvement of the High Temperature Oxidation Behaviour of TiAl Alloy by Cl Implantation using PIII

The intermetallic compound TiAl is very interesting for high temperature application due to its high temperature strength and low density. Above 700 °C, the use in e.g. aircraft engines is hindered because of a low oxidation resistance. Even though the compound contains 50at% Al, this is not sufficient to form a protective Al2O3 scale. Instead, a mixed oxide scale composed of TiO2 and Al2O3 is formed. Extended alloy development has shown, that additional elements which are beneficial for the oxidation often have a bad influence on the mechanical bulk properties. Cl doping has a strong effect even in very low concentrations of about 500 ppm ("Microalloy"). Ion Implantation of 1016 Cl/cm2 protects TiAl at 900 °C for up to 1000 h against oxidation in a wide range of implantation energies.

In order to apply this effect for complicated shaped parts, the high temperature oxidation behavior after plasma immersion ion implantation of chlorine into different TiAl alloys was investigated. A specialized chamber has been setup up for the strongly etching Cl plasma. Particularly with regard to contaminations, the chamber and the RF- antenna had to be made from aluminum. Since it is known that the Cl effect in TiAl requires a well defined Cl concentration, a variation of the implantation parameters has been performed. The resulting depth profiles of Cl are investigated using depth profiling with Auger electron spectroscopy (AES). After treatment (3*105 to 3*106 30 kV pulses of 5µs at a repetition rate from 150 Hz to 1500 Hz), the Cl is located close to the surface. The process is governed by the interplay between etching and implanting during the HV pulses. Therefore the retained dose depends mainly on the temperature during implantation, which is controlled by the repetition rate of the pulses. Oxidation tests at 900°C in air for 100 h showed a strong reduction of the oxidation, which are comparable to conventional beam line im-plantations of Cl.