Evaluation of thermodynamic data for an advanced γ-TiAl CALPHAD database (ADVANCE)
Within the EU project „ADVANCE - Sophisticated experiments and optimisation to advance an existing CALPHAD database for next generation TiAl alloys” MPIE collaborated with Thermocalc-Software AB, Stockholm, Montanuniversität Leoben and Helmholtz-Zentrum Hereon, Geesthacht. At MPIE the focus lay on the production and heat treatments of model alloys and their analysis through metallography, X-ray diffraction, electron probe microanalysis and differential thermal analysis. Colleagues in Leoben performed transmission electron microscopy and at Helmholtz-Zentrum Hereon in situ synchrotron X-ray diffraction was carried out. All obtained data were optimised at the company Thermocalc and checked for consistency before they were implemented into the database.
Alloys based on the intermetallic phase γ-TiAl constitute a new class of materials. They have high specific strength and are therefore specifically suited for applications where the weight of moving parts has to be considered, e.g. compressor blades in aero engines or turbo chargers for vehicles. Compared to established Ni- and Co-base superalloys, γ-TiAl alloys are lighter and therefore more energy-efficient.
Today’s alloy developments crucially depend on data-based CALPHAD programs. CALPHAD is an acronym for calculation of phase diagrams. The databases contain specific thermodynamic values for phase equilibria and phase transformations, and data of phase compositions in dependence on temperature and composition of an alloy. The databases allow to calculate which phases are stable at certain temperatures, their volume fractions and compositions within regions, where no experimental results are available. This enables prediction of the microstructures and, from this, conclusions about expected mechanical properties. Thereby, employment of these databases enables a time-efficient development of novel materials.
Within the project thermodynamic data for the alloy systems Ti-Al-X (X = Nb, Mo, W, Zr, O, B, Si, Zr) and Ti-Al-Nb-Y (Y = W, Mo) were obtained. Work on individual systems started by critical evaluation of the existing literature [1-5]. From these assessments, temperature-composition ranges needing further study were identified.
One of the key systems for the development of novel TiAl-based alloys for application in aero engines is Ti-Al-Nb. Therefore phase equilibria [7-9], transformation kinetics [10, 11] and the influence of oxygen on phase equilibria at high temperature  in this system were investigated in detail. Silicon can improve the oxidation resistance of TiAl-based alloys and fine silicide precipitates enhance the creep resistance at high temperatures. Because strength crucially depends on the volume fraction of the silicide, phase equilibria between the Ti-Al phases and Ti5Si3 were determined [5, 13]. Zirconium is of interest in recent alloy developments because of substantial solid strengthening in TiAl. Re-determination of the phase equilibria [14, 15] showed that they are actually quite different from previously established ones because those included phases, which are stabilised by impurities.