The Effect of Molybdenum on the Phase Transformation Behavior of beta/gamma-TiAl alloys

Molybdenum is a strong beta-stabilizer and an important alloying element in gamma-TiAl based alloys. It offers the opportunity to suppress the formation of alpha(2)-phase and is used to generate so-called beta/gamma-alloys with improved hot working behavior. Unfortunately, the effect of Mo on the individual phases and their transition temperatures during slow heating and rapid cooling in the course of isothermal short and long-term heat treatments is not completely understood. In this paper, the results of investigations of two model alloys with different Mo contents by scanning electron microscopic and calorimetric studies are reported. Below a certain Mo content the high-temperature p-phase transforms into alpha'-martensite during water quenching. At higher Mo contents the high-temperature p-phase can be retained upon quenching, only an ordering reaction is observed. Subsequent tempering at 1000 degrees C leads to a strong grain refinement and hardness reduction.