Al-Ti-Zn System: thermochemistry of the Ti(Zn,Al)3 phases

Cubic cP4-AuCu3 trialuminides have long been of interest because of their possible use as low density, high-temperature structural materials. The inherent brittleness of trialuminides, however, has limited their use in such applications so, many of the efforts to increase the ductility of these alloys have focused on identifying alloying additions that can lead to the stability of the cubic cP4-AuCu3 phase, rather than the tetragonal (tI8-Al3Ti or tI16-Al11Ti5) structures that form in most binary trialumides. These phases are also of interest for their possible use as precipitates for high-temperature, creep-resistant Al-based alloys (i.e. Al3Sc precipitates have been successfully used to increase the creep resistance of Al alloys). The design of new alloys requires accurate knowledge of the thermodynamic stability of all relevant phases which may be integrated within CALPHAD formalism to calculate binary and ternary phase diagrams and phase transformation driving forces. A high temperature drop calorimeter has been employed to determine the standard entalpy of formation at 300K of the samples which consist in an appropriate amounts of Al, Ti and Zn powders weighed and mixed in a glove box and then pressed to obtain pellets. The compositions of the alloys were Ti(Zn,Al)3 with an Al content from 10 to 65 at% Al. After the measurements, composition and state of the samples were examined by means of microscopic analysis (optical and electron probe microanalysis) and powder X-ray diffraction methods. We faced some experimental problems due to intrinsic factors that may be unfavorable in finding an optimum condition for calorimetric runs (i.e. large difference in melting point between Ti, Al and Zn, high vapor pressure of Zn). The results obtained by calorimetry, X-ray diffraction analysis and electron probe microanalysis, are here presented and discussed.