Microstructure Evolution in the Mushy Zone of a β-Solidifying TiAl Alloy under Different Cooling Processes

A novel method termed mushy zone cooling control is applied to tailor the microstructure of a β-solidifying titanium aluminide alloy in this study. Ti-44.5Al–4Nb–2Cr–0.1B (at%) alloy is subjected to temperature-controlled cooling through the mushy zone by different processes. The solidified microstructure consists of α₂/γ lamellar colony, B2 phase, and massive γ. It is found that both the fast cooling and the thermal cycling can play a role in refining α₂/γ lamellar colonies and in reducing B2 phase, which is brittle at room temperature. α₂/γ colonies with an average size of 28 µm are obtained after 10 thermal cycles. The results also show that solidification microsegregation can be largely eliminated by fast cooling. However, α₂/γ colonies will continuously coarsen during such an isothermal holding process.