Comparison of Different Fluorine-treatments for Improved High Temperature Oxidation Resistance of TiAl-alloys
- PDF / 227,329 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 105 Downloads / 216 Views
Comparison of Different Fluorine-treatments for Improved High Temperature Oxidation Resistance of TiAl-alloys A. Donchev1, M. Schütze1, A. Kolitsch2, R. Yankov2 DECHEMA e.V. Karl-Winnacker-Institut, Theodor-Heuss-Allee 25; D-60486 Frankfurt/Main, Germany 2 Forschungszentrum Dresden-Rossendorf e.V. Institute of Ion Beam Physics and Materials Research, Bautzener Landstraße 128, D-01328 Dresden, Germany
1
ABSTRACT Intermetallic TiAl-alloys can replace the heavier Ni-based superalloys in several high temperature applications with regards to their mechanical properties, however they can not be used at temperatures above 800°C in oxidizing environments for longer times because of insufficient oxidation resistance. Despite an Al-content of about 45 at.% in technical alloys, no protective alumina layer is formed because the thermodynamic stabilities of titanium oxide and aluminum oxide are of the same order of magnitude. Therefore a mixed TiO2/Al2O3-scale is formed which is fast growing so that the metal consumption rate is quite high. On the other hand the formation of a slow growing alumina layer is promoted by a fluorine treatment. This so called fluorine effect leads to the preferential intermediate formation of gaseous aluminum fluorides at elevated temperatures if the fluorine content at the surface stays within a defined concentration range. These fluorides are converted into solid Al2O3 due to the high oxygen partial pressure of the high temperature service environment forming a protective pure Al2O3 surface scale. In this paper results of high temperature oxidations tests of several technical TiAlalloys will be presented. Different F-treatments e.g. dipping or spaying which are easy to apply have been used and their results will be compared. The mass change data of the F-treated specimens are always lower than those of the untreated ones. Post experimental investigations such as light microscopy, scanning electron microscopy and energy dispersive X-ray analysis reveal the formation of a thin alumina layer on the F-treated samples after optimization of the process while a thick mixed scale is found on the untreated samples. The results will be discussed in view of an optimized procedure and the future use of TiAl-components in high temperature environments. INTRODUCTION Fluorine treatment of TiAl-alloys is a confirmed technique to improve the oxidation resistance at elevated temperatures in oxidizing environments [1-4]. The oxidation mechanism is changed from mixed oxide scale formation which is non protective on untreated alloys [5] to alumina formation [6]. Therefore TiAl-components could be used after a fluorine treatment in several high temperature applications such as aero or automotive engines. This -Al2O3 layer protects the alloy even under thermocyclic conditions in wet and SO2 containing air [7]. The fluorine treatment has to be precise. In this paper the results of different fluorine treatments and their effect on the oxidation behavior are presented. Isothermal and thermocyclic oxidation tests were perf
Data Loading...
