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The Role of Fundamental Material Parameters for the Fluorine Effect in the Oxidation Protection of Titanium Aluminides

Published online by Cambridge University Press:  01 February 2011

Hans-Eberhard Zschau  and
Michael Schütze
Hans-Eberhard Zschau
Affiliation:
zschau@dechema.de, DECHEMA e. V., Karl-Winnacker-Institut, Frankfurt am Main, Germany
Michael Schütze
Affiliation:
schuetze@dechema.de, DECHEMA e. V., Karl-Winnacker-Institut, Frankfurt am Main, Germany
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Abstract

To improve the insufficient oxidation resistance of Titanium Aluminides at temperatures above 750°C the fluorine effect offers an innovative way. The focus of this paper is to define the fundamental material variables for the fluorine effect related to the macroscopic behaviour (oxidation resistance) and its long time stability. The thermodynamic model predicted the fluorine effect for the TiAl within a corridor of total fluorine amount in terms of partial pressures. To realize the fluorine effect the required F-concentration [in at.-%] within the near surface region had to be found. Using fluorine ion implantation several fluences within 5e15 and 5e17 F cm-2 were implanted with an energy of 20 keV. The implantation depth profiles were calculated by using the Monte Carlo simulation code T-DYN and verified experimentally by using the non-destructive PIGE - technique (Proton Induced Gamma-ray Emission). After oxidation tests at 800°C – 1000°C a value of 2e17 F cm-2 / 20 keV was determined as an optimal implantation parameter set. Following these results the maximal fluorine concentration was identified to be a fundamental material variable for starting the alumina formation with a required fluorine amount of about 40-45 at.-%. However this maximum fluorine concentration showed a rapid decrease to values less than 5 at.-% only after a few hours of oxidation (900°C and 1000°C) followed by a slow decrease. Therefore the maximum fluorine concentration Cmax – now located at the metal/oxide – interface – was identified to be a fundamental parameter for the long time stability. An exponential decay function containing a constant term of about 1 at.-% was found to describe the time behaviour of Cmax for isothermal and cyclic oxidation (900°C, 1000°C). Because the alumina scale on the surface acts as a diffusion barrier for fluorine, the stability of Cmax is strongly influenced by the F-diffusion into the metal. From the F-depth profiles the diffusion coefficient of fluorine into the TiAl at 900°C was determined as a fundamental parameter for the long-term stability of Cmax showing a value of 1.56e-15cm2/s.

Keywords

oxidationion beam analysision-implantation
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1128: Symposium U – Advanced Intermetallic-Based Alloys for Extreme Environment and Energy Applications , 2008 , 1128-U04-10
Copyright
Copyright © Materials Research Society 2009

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References

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