Comparison of Different Fluorine Treatments for the Protection of TiAl-Alloys Against High Temperature Oxidation

Abstract

The oxidation resistance of TiAl-alloys can be improved by several orders of magnitude by treating the surface of the materials with small amounts of halogens especially Cl and F. The oxidation mechanism changes due to the so called halogen effect. The formation of a fast growing mixed oxide scale on untreated alloys is suppressed, instead a thin protective alumina scale is formed on samples after optimum treatment. The different methods only influence the surface region of the components so that the bulk properties are not affected. Recent results achieved with complex TiAl-samples showed the potential that the fluorine effect could be used for TiAl-components in several high temperature applications e.g. jet engines. TiAl-specimens were treated with fluorine and chlorine in several ways and their performance during high temperature oxidation tests in air was investigated. Results of isothermal and thermocyclic oxidation tests are presented. The long term stability of the fluorine effect lasted for at least one year under thermocyclic exposure at 900°C in laboratory air. The results are discussed in terms of later use of the fluorine effect for technical applications.