Comparison of different fluorine-treatments for improved high temperature oxidation resistance of TiAl-alloys

Intermetallic TiAl-alloys can replace the heavier Ni-based superalloys in several high temperature applications with regard to their mechanical properties but they can not be used at temperatures above 800°C in oxidizing environments for longer times because of an insufficient oxidation resistance. Despite an Al-content of about 45 at.% in technical alloys no protective alumina layer is formed because the thermodynamic stabilities of titanium oxide and aluminum oxide are in the same order of magnitude. Therefore a mixed TiO2/Al2O3-scale is formed which is fast growing so that the metal consumption rate is quite high. On the other hand the formation of a slowly growing alumina layer is promoted by a fluorine treatment. This so-called fluorine effect leads to the preferential intermediate formation of gaseous aluminum fluorides at elevated temperatures if the fluorine content at the surface is being kept within a certain concentration range. These fluorides are then converted into solid Al2O3 due to the high oxygen partial pressure of the high temperature service environment forming a protective pure Al2O3 surface scale. In this paper results of high temperature oxidations tests of several technical TiAl-alloys will be presented. Different F-treatments such as dipping or spaying which are easy to apply or, alternatively, more sophisticated ion beam and plasma-based techniques have been used for surface modification, and their results will be compared. The weight change data of the F-treated specimens are always lower than those of the untreated ones. Analytical methods such as light microscopy, scanning electron microscopy and energy dispersive X-ray analysis reveal the formation of a thin alumina layer on the F-treated samples after optimization of the process while a thick mixed scale is found on the untreated samples. The results will be discussed in view of both the development of an optimized process, and the future use of TiAl-components in high temperature environments.