On Phase Equilibria and Phase Transformations in β/γ-TiAl Alloys – A Short ReviewPhasengleichgewichte und Phasenumwandlungen in β/γ-Titanaluminiden – Ein kurzer Überblick

Due to the fact that Molybdenum is a strong β stabilizer it plays an important role among TiAl-based alloys, especially since a significant volume fraction of the disordered body-centered cubic β-phase improves the processing characteristics during hot-working at elevated temperatures. Unfortunately the effect of Mo on the individual phases and their transition temperatures is insufficiently known though it is essential for designing engineering components. In this paper sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. Furthermore the effect of Mo on the phase transformation behavior during slow heating and rapid cooling in the course of isothermal short and long-term heat treatments is elaborated. Below a certain amount of Mo the high-temperature cubic β-phase transforms into ordered hexagonal α2'-martensite during water quenching. At higher contents the high-temperature β-phase can be retained upon quenching; only the ordering reaction β → βo takes place. The subsequent tempering at 1000 °C leads to a strong grain refinement as well as to a reduction in hardness.