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

Intermetallic TiAI-alloys can replace the heavier Ni-based superalloys in several high temperature applications with regards to their mechanical properties, however they can not be used at temperatures above 800DC in oxidizing environments for longer times because of insufficient oxidation resistance. Despite an AI-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 of the same order of magnitude. Therefore a mixed Ti02/Ah03-scale is formed which is fast growing so that the metal consumption rate is quite high. On the other hand the formation of a slow growing alumina layer is promoted by a fluorine treatment. This so called fluorine effect leads to the preferential intermediate formation of gaseous aluminium fluorides at elevated temperatures if the fluorine content at the surface stays within a defined concentration range. These fluorides are converted into solid Ah03 due to the high oxygen partial pressure of the high temperature service environment forming a protective pure Ah03 surface scale. In this paper results of high temperature oxidations tests of several technical TiAIalloys will be presented. Different F -treatments e.g. dipping or spaying which are easy to apply have been used and their results will be compared. The mass change data of the F –treated specimens are always lower than those of the untreated ones. Post experimental investigations 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 an optimized procedure and the future use of TiAI-components in high temperature environments.