High temperature oxidation protection of Ti 3 Al-based alloys with different Nb-content by a combined Al-/F-treatment
- PDF / 7,805,338 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 17 Downloads / 220 Views
High temperature oxidation protection of Ti3Al-based alloys with different Nb-content by a combined Al-/F-treatment A. Donchev, M. Galetz, M. Schütze DECHEMA-Forschungsinstitut, D-60486 Frankfurt am Main/Germany ABSTRACT Orthorhombic titanium-based Ti2AlNb alloys cannot be used above a temperature limit of about 800°C due to accelerated oxidation and environmental embrittlement. This embrittlement is caused by the high oxygen solubility which deteriorates the mechanical properties. Even if these materials possess an Al content up to ca. 25at.% no protective alumina layer is formed. Instead a non-protective fast growing mixed scale is found. Several attempts have been made to increase their operation temperature e.g. by coatings but none has proven sufficiently protective so far. One new way presented in this paper is to enrich Al in a narrow surface zone by using a powder pack process (aluminization) followed by a fluorination step. Exposure tests at elevated temperature have shown that the aluminized specimens form an alumina layer during exposure in oxidizing environments. Due to the gradient in the Al-concentration interdiffusion with the substrate and the Al-rich diffusion zone occurs which lowers the Al concentration in the diffusion zone. If the Al content drops below a critical value, Ti oxides will also form, which deteriorates the protection provided by the alumina scale. The subsequent fluorination triggers the fluorine effect which stabilizes the protective alumina layer. Untreated specimens are covered with a thick non protective scale and exhibit oxygen ingress in the subsurface zone while treated specimens reveal a thin protective alumina layer and no inward diffusion of oxygen. In this paper results of exposure tests of untreated and treated orthorhombic Ti2AlNb alloys will be presented and compared with the Nb-free α2-phase Ti3Al and Nb-containing Ti3Al-based alloys. INTRODUCTION The need for reducing the human impact on the climate is pushing materials engineers to use lighter materials to enhance the efficiency of e.g. jet or automotive engines. Thus CO2 emission could be reduced. Intermetallic alloys based on titanium and aluminum offer great potential for several high temperature applications. Especially the γ-TiAl phase is a major topic in research due to its low specific weight and good mechanical properties. Orthorhombic Ti2AlNb-based alloys also have interesting mechanical properties which make them candidates for several high temperature applications [1]. Due to oxidation and environmental embrittlement their use at temperatures above 800°C has not been possible so far [2]. Surface treatments such as coatings are a common solution for several metallic materials with all its benefits and problems [3]. So far a sole aluminide coating has not been proven sufficient for the protection of ο–Ti2AlNb alloys. The enforcement of the protective effect of aluminide coating by the fluorine effect is presented in this paper for the first time. The fluorine effect has been known from γ-TiAl alloys with an
Data Loading...
