The Role of fundamental Material Parameters for the Fluorine Effect in the Oxidation Protection of Titanium Aluminides
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1128-U04-10
The Role of fundamental Material Parameters for the Fluorine Effect in the Oxidation Protection of Titanium Aluminides Hans-Eberhard Zschau and Michael Schütze DECHEMA e.V., Karl-Winnacker-Institut, Theodor-Heuss-Allee 25, D-60486 Frankfurt am Main, Germany
ABSTRACT The increasing interest in Gamma-TiAl based alloys is motivated by their excellent specific strength at high temperatures which offers a high application potential in aerospace and automotive industries. To improve the insufficient oxidation resistance at temperatures above 750 °C the fluorine effect leading to the formation of a protective alumina scale offers an innovative way. After F implantation of TiAl and oxidation at 900 °C/1000 °C the fluorine maximum determined by using PIGE is found to be at the metal/oxide interface. The time dependent behaviour of the fluorine content is characterized by a fast decrease of the fluorine concentration during heating followed by a moderate decrease. As parameter for the stability cFmax was defined. The time behaviour can be described by an exponential decay function with a constant part of 0.3 – 1.2 at.-% offering a stable oxidation protection by the fluorine effect and its possible technical application. A theoretical modelling performed by using the diffusion coefficient of F in TiAl at 900 °C fits the experimental data with a good agreement. INTRODUCTION Due to their high specific strength at high temperatures the γ-TiAl based alloys are believed to have a high application potential in high temperature technologies, especially in aerospace and automotive industries. In contrast to the presently used Ni-based superalloys their specific weight is about 50 % lower leading to a lower moment of inertia for rotating parts like turbocharger rotors, exhaust valves or turbine blades. Thus lower mechanical stresses and a reduced fuel consumption can be expected. However the oxidation resistance of γ-TiAl inhibits an industrial use at temperatures above 750 °C [1]. To reach higher service temperatures the surface modification is expected to avoid any detrimental influence on the excellent mechanical properties of the material. The oxidation resistance can be improved by using the so-called “halogen-effect”. In [2-7] a dense alumina scale protecting the material against corrosion was formed on the surface after the application of small amounts of chlorine. However the alumina scale failes during thermocyclic loading. After fluorine ion implantation [8] or liquid phase treatment with diluted HF [9] in combination with oxidation at 900 °C/air a dense alumina scale was achieved showing pronounced adherence even during cyclic oxidation. The halogen effect can be explained by a thermodynamic model assuming the preferred formation and transport of volatile Al-halides through pores and microcracks whithin the metal/oxide interface and their conversion into alumina forming a protective oxide scale on the surface [4]. This model was proven by the F-depth profiles obtained by PIGE (Proton Induced Gamma-ray Emission)
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