An In Situ High-Energy Synchrotron X-Ray Diffraction Study of Directional Solidification in Binary TiAl Alloys

Phase formation and microstructure selection during solidification in gamma-TiAl alloys are highly relevant, both with respect to the microstructure and texture of cast alloys and to directional solidification, which allows to obtain a unique combination of properties. However, during cooling to room temperature, the solidifying phases transform to low-temperature phases, which mask the prior situation during solidification. A new inductive crucible-free zone melting furnace is used in this work, which is specially designed for in situ investigations of solidification using synchrotron radiation. Herein, alloys with 45 and 48 at% Al are studied, and it can be shown that with varying withdrawal rate, a change from alpha solidification to synchronous beta + alpha solidification occurs in Ti-48Al, as it is predicted in the literature. Furthermore, it is observed that by increasing the withdrawal rate, a preferred growth of the beta phase occurs in alloys with 45 at% Al, which is interpreted as a transition to dendritic solidification. The observations are compared with a microstructure selection map calculated according to the nucleation and constitutional undercooling (NCU) model proposed in the literature.