Achieving fine fully lamellar microstructure of casting TiAl alloy by simple heat treatment

An improved step heat treatment of a cast TiAl alloy was developed to overcome the challenges associated with microstructure refinement of such materials without hot working. A KIMS TiAl alloy (Ti-46Al-6Nb-0.5 W-0.5Cr-0.3Si-0.1C) was heat-treated to identify the phase transformation behavior, and this microstructure was successfully refined only by heat treatments. Through a pseudo-phase diagram, the heat-treatment temperature and processes can be determined. To facilitate the subsequent heat treatment process, the as-cast structure was eliminated first through homogenization heat treatment. After the heat treatment of each phase with different fractions in the (α + γ) region, most nucleation occurred at the grain boundaries. As the nucleation proceeded, the lamellar structure thickened during the (α + γ) heat treatment. The thickening also occurred because the phase transformation kinetics changed according to the heat-treatment temperature. Even after the heat treatment for the same duration, the rates of phase transformation were different owing to the different growth rates of the phases. The microstructure can be controlled through the step heat treatment, which improves the mechanical properties of the cast TiAl alloy at high temperatures.