Phase equilibria in the Al-rich part of the Al-Ti system

While phase equilibria in the Ti-rich part of the Ti-Al system have been the subject of numerous investigations, the Al-rich part has attracted comparably much less attention. Within the current search for new weight-saving and oxidation-resistant alloys for structural applications, Al-based titanium aluminides have come more into focus. The increasing interest supported the identification of phases and determination of transformation temperatures in the Al-rich part of the Al-Ti system. These activities resulted in a qualitative understanding of phase equilibria in this part of the binary system. However, missing information about homogeneity ranges of the Al-rich titanium aluminides hindered a purposeful materials development of specific alloys. What impeded the determination of phase equilibria in this specific part of the Al-Ti system are a variety of experimental difficulties. By careful preparation, handling, and examination of equilibrated bulk alloys and diffusion couples, the homogeneity ranges of the A-rich titanium aluminides have now been established by electron-probe microanalysis (EPMA). In the temperature range 800 degrees C to 1300 degrees C the maximum solubility of aluminum in gamma-TiAl and the composition ranges of the phases TiAl(2), Ti(2)Al(5), and TiAl(3) were established. The present results show that the maximum solubility of aluminum in gamma-TiAl is not almost independent from temperature as assumed in recent reports but increases from 56 at% Al at 800 degrees C to more than 63 at% Al at 1300 degrees C. Thus it is possible to generate A-rich TiAl-based alloys with specific microstructures by decomposition of the supersaturated gamma-TiAl solid solution and alloys comprising lamellar microstructures have been produced.