In-situ investigation of β/α transformation in β-solidifying γ-TiAl alloys at different cooling rates

We report an in-situ study on β/α transformation of a β-solidifying γ-TiAl alloy using high-temperature laser scanning confocal microscopy. Combining the current theories of Ti alloys and a method to distinguish the crystal orientation, the special effect of cooling rate on the nucleation, growth of α phase from β matrix as well as the surface relief in Ti-40Al were researched. The results indicate that Ti-40Al exhibits both sympathetic and interface instability nucleation and grows in the form of Widmannstatten plate and equiaxed grain cooling at 2 K/min; just interface instability nucleation and Widmannstatten plate growth occur in the alloy at 108 K/min. Besides, high cooling rate can promote the growth rate of Widmannstatten α plates, whose growth direction is parallel to γ lamellar orientation nearly. These findings will provide insight into controlling the orientation and size of lamellar colonies of TiAl alloys to improve mechanical properties.