High Temperature Oxidation Protection of Multi-Phase Mo-Containing TiAl-Alloys by the Fluorine Effect
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High Temperature Oxidation Protection of Multi-Phase MoͲContaining TiAlͲAlloys by the Fluorine Effect Alexander Donchev1, Raluca Pflumm1, Svea Mayer2, Helmut Clemens2, Michael Schütze1 1
DECHEMAͲForschungsinstitut, D-60486 Frankfurt am Main, Germany Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben, A-8700 Leoben, Austria 2
ABSTRACT Intermetallic titanium aluminides are potential materials for application in high temperature components. In particular, alloys solidifying via the β-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic β-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, their practical use at temperatures as high as 800°C requires improvements of the oxidation resistance. This paper reports on the fluorine effect on a multi-phase TiAl-alloy in the cast and hotisostatically pressed condition at 800°C in air. The behavior of the so-called TNM material (Ti43.5Al-4Nb-1Mo-0.1B, in at %) was compared with that of two other TiAl-alloys which are Nbfree and contain different amounts of Mo (3 and 7 at%, respectively). The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. After fluorine treatment all alloys exhibit slow alumina kinetics indicating a positive fluorine effect. Results of isothermal and thermocyclic oxidation tests at 800°C in air are presented and discussed in the view of composition and microstructure of the TiAl-alloys investigated, along with the impact of the fluorine effect on the oxidation resistance of these materials. INTRODUCTION The increasing demand for more efficient automotive or jet engines triggers the replacement of heavy Ni- or Co-based superalloys currently in use by lighter TiAl-alloys. This group of intermetallic materials offers promising properties for several high temperature applications. Hence, they are currently being used for turbocharger rotors in automotive engines [1] and have now been introduced for turbine blades in General Electric’s GENx engine which powers the Boeing 787 [2]. Recent developments have led to multi-phase alloys with a significant amount of the β/β0-phase next to the γ- and α2-phases which are common in technical TiAl-alloys [3, 4]. Their machinability and mechanical properties would allow their use at elevated temperatures. Even though, in the envisaged temperature range above 750°C their oxidation resistance must be improved. At such high temperatures, TiAl-alloys do not form a protective alumina scale like other aluminides (e.g. NiAl) because of the similar thermodynamic stabilities of Ti- and Al-oxide [5]. A fast growing and non-protective mixed TiO2/Al2O3/TiN scale develops on TiAl-alloys after high temperature exposure in air [6]. It has been proven that the fluorine effect changes the oxidation behavior of TiAl-alloys at temperatures higher than 750°C. Very small amounts of halogen, micro-alloyed at the metal/oxide interface, l
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