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 suc-cessfully 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 (alpha + gamma) region, most nucleation occurred at the grain boundaries. As the nucleation proceeded, the lamellar structure thickened during the (alpha + gamma) heat treatment. The thickening also occurred because the phase transformation kinetics changed according to the heat-treatment temperature. Even after the heat treat-ment 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 me-chanical properties of the cast TiAl alloy at high temperatures.

Automated summary

An improved step heat treatment method was developed to refine the microstructure of a cast TiAl alloy without hot working. The phase transformation behavior was identified and the heat treatment temperature and process were determined through a pseudo-phase diagram. The as-cast structure was eliminated through homogenization heat treatment before the heat treatment of each phase in the (alpha + gamma) region. Nucleation mainly occurred at the grain boundaries and the lamellar structure thickened during the (alpha + gamma) heat treatment. The step heat treatment effectively controlled the microstructure and improved the mechanical properties of the cast TiAl alloy at high temperatures.