On The Influence of Nb on the Transition Temperatures of Titanium Aluminides
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0980-II07-03
On The Influence of Nb on the Transition Temperatures of Titanium Aluminides Harald F. Chladil1, Helmut Clemens1, Masao Takeyama2, Ernst Kozeschnik3, Arno Bartels4, Rainer Gerling5, and Sascha Kremmer6 1 Department of Physical Metallurgy and Materials Testing, Montanuniversity Leoben, Leoben, 8700, Austria 2 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo, 1528552, Japan 3 Institute for Materials Science, Welding and Forming, Graz University of Technology, Graz, 8010, Austria 4 Materials Science and Technology, Technical University Hamburg-Harburg, Hamburg, 21073, Germany 5 Institute for Materials Research, GKSS Research Centre, Geesthacht, 21502, Germany 6 R&D, Bohler Schmiedetechnik GmbH&CoKG, Kapfenberg, 8605, Austria
ABSTRACT Phase transformations and phase transition temperatures in several Ti-45Al and Ti-45Al-(5-10)Nb (at%) alloys were investigated experimentally and compared to thermodynamic calculations. The present study combines scanning electron microscopy, highenergy and conventional X-ray diffraction as well as differential scanning calorimetry for the characterization of the prevailing phases and phase transformations. Thermodynamic calculation based on the CALPHAD method was used to predict phase stabilities. Modifications of a commercially available database, based on the thermo-physical measurements and long-term annealing treatments, were introduced in order to achieve better agreement between calculated and experimental results. INTRODUCTION Intermetallic γ-TiAl based alloys exhibit increasing technical importance for hightemperature applications in automotive and aerospace industries [1,2]. Their advantage is mainly seen in low density, combined with high specific yield strength and stiffness, good creep properties up to high temperatures as well as good oxidation resistance [3,4]. Current γ-TiAl based alloys are an example of complex multi-phase materials. Knowledge of the constituent phases and their transition temperatures is the basis for smart heat treatments [5], which, for example, are applied for optimization of mechanical properties. Thus, for effective alloy development through hot working and subsequent heat treatments, the influence of alloying elements on volume fraction and thermodynamic stability of the phases present at given temperatures has to be fully understood. For casting, the solidification path has to be known, whereas, for thermo-mechanical processing, the eutectoid temperature and α- and β-transus temperatures, which sensitively depend on alloy composition, are of particular importance. Practically, the transformation temperatures are determined by heat-treatments followed by a metallographic examination of the microstructure and/or by means of differential scanning calorimetry (DSC).
Alternatively or complementarily, appearing phases and transition temperatures can be predicted by thermodynamic calculations employing a commercially available TiAl database [6]. For multi-component γ-TiAl based alloys with alloying elem
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