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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.