Mater. Res. Soc. Symp. Proc. Vol. 1 © 2015 Materials Research Society DOI: 10.1557/opl.2015.160 Oxidation Protection of Multiphase Mo-Containing γ-TiAl-Based Alloys under Cyclic Test Conditions A. Donchev 1 *, R. Pflumm 1 , M. Galetz 1 , S. Mayer 2 , H. Clemens 2 , M. Schütze 1 1 DECHEMA‐Forschungsinstitut, Theodor-Heuss-Allee 25, D-60486 Frankfurt am Main, Germany 2 Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben, Franz-Josef Str. 18, A-8700 Leoben, Austria * donchev@dechema.de, +49697564386 ABSTRACT Intermetallic titanium aluminides solidifying via the disordered β-phase are of great interest for several high-temperature applications in automotive and aircraft industries. In this paper the thermocyclic oxidation behavior of three β-solidifying γ-TiAl-based alloys at 800°C and 900°C in air, with and without fluorine treatment, is reported for the first time. The behavior of the well-known TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B, in at.%) is compared with that of two Nb-free model alloys which contain different amounts of Mo (Ti-44Al-3Mo and Ti-44Al-7Mo, in at.%). During thermocyclic high-temperature exposure in air a mixed oxide scale develops on all three untreated alloys. Small additions of fluorine in the subsurface region of the alloys change the oxidation mechanism from mixed oxide scale formation to alumina at both temperatures. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. INTRODUCTION The demand for increasing the efficiency of aero and automotive engines requires the development of new light-weight structural materials which can withstand higher temperatures. Intermetallic titanium aluminides are one group of high-temperature materials which possess a low specific weight next to good mechanical properties at elevated temperatures. In this context, γ-TiAl based alloys which solidify via the disordered β-phase are potential candidates for use as forged turbine blades in jet engines and turbine wheels in automotive turbochargers [1]. Besides the α 2 -Ti 3 Al and γ-TiAl phase they possess a significant volume fraction of the disordered body-centered cubic β-TiAl phase at elevated temperatures. This phase ensures good hot-workability as described in [2]. At room temperature and up to service temperature, however, the β-phase shows an ordered B2 lattice structure, therefore termed β o . Nevertheless, the practical use of such multiphase alloys at a temperature as high as 800°C or above requires reliable oxidation protection. In this study the cyclic oxidation behavior of the Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy at 800°C and 900°C in air is compared with that of Ti-44Al-3Mo (TiAl-3Mo) and Ti-44Al-7Mo (TiAl-7Mo). During thermocyclic high-temperature exposure in air a mixed oxide scale develops on all three alloys in untreated condition which partial spalls off. This is in contrast to the previously published results, where no spallation was observed during isothermal exposure [3]. The non- protective behavior can be changed via the so-called fluorine effect, as demonstrated for previously investigated TiAl alloys with a two-phase microstructure, consisting not only of α 2 and γ but also α 2 , γ and β [3, 4, 5]. Small additions of fluorine in the subsurface region of the alloys prior to thermocyclic high-temperature exposure change the oxidation mechanism from mixed oxide scale formation to alumina. The oxidation resistance of the fluorine treated samples was significantly improved when compared to the untreated samples due to alumina formation [3, 5]. The results of thermocyclic oxidation tests at 800°C and 900°C in air are 760