Phase Equilibria and Phase Transformations in Molybdenum-Containing TiAl Alloys
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Phase Equilibria and Phase Transformations in Molybdenum-Containing TiAl Alloys Svea Mayer1, Christian Sailer1, Hirotoyo Nakashima2, Thomas Schmoelzer1, Thomas Lippmann3, Peter Staron4, Klaus-Dieter Liss5, Helmut Clemens1 and Masao Takeyama2 1 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben, Austria 2 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo, Japan 3 Institute of Materials Research, Helmholtz-Zentrum Geesthacht Outstation at DESY, Hamburg, Germany 4 Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Geesthacht, Germany 5 The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia ABSTRACT Molybdenum, being a strong ȕ stabilizer, is an important alloying element in TiAl alloys, since a significant volume fraction of the disordered bcc ȕ-phase at elevated temperatures improves the processing characteristics during hot-working. Unfortunately, the effect of Mo on the individual phases and their transition temperatures is not completely known but is necessary for designing engineering applications. In this paper, sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. Further, the phase transition temperatures given by the phase diagrams are compared with results from isothermal heat treatment studies, differential scanning calorimetry measurements and insitu high-temperature diffraction experiments. Combining all of these results, a revised phase diagram is proposed. INTRODUCTION Due to their attractive properties, intermetallic TiAl alloys are considered for hightemperature applications in aerospace and automotive industries. Their advantages are mainly seen in low density (3.9 - 4.2 g/cm³, depending on composition and constitution), high specific yield strength, high specific stiffness, good oxidation resistance, resistance against ”titanium fire”, and good creep properties up to high temperatures [1]. One recent approach is the design of so-called ȕ/Ȗ-alloys with microstructures in which homogeneously distributed ȕ- and Ȗ-phases are the main constituents [2]. Based on the fact that Mo is a strong ȕ stabilizer, the ternary system Ti-Al-Mo is well-suited for studying this type of alloys. In this paper, which is based on a recent study [3], representative sections of the ternary Ti-Al-Mo phase diagram, derived from thermodynamic calculations and experimental results, are presented. In principle, advanced multi-phase TiAl alloys contain three intermetallic phases, namely Ȗ, Į2 and ȕo, which are all ordered at ambient temperature. An increase of temperature induces that the ordered hexagonal Į2-phase transforms to the disordered hexagonal Į and the ordered ȕo-phase transforms to the disordered bcc ȕ-variant. For the prediction of the constituent phases and related transition temperatures, thermodynamic calculations based upon the CALPHAD method were conducted. For this purpose, the software package MatCalc was e
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