Microstructure and microsegregation in directionally solidified Ti-46Al-8Nb alloy

Directional solidification experiments were conducted for Ti-46Al-8Nb alloy at the growth rates ranging from 3 to 70 μm/s. The microstructure evolution and microsegregation pattern were investigated. In the range of growth rate, a regular dendritic structure appears and the primary dendrite spacing decreases with increasing growth rate. The peritectic reaction is observed during the solidification and the final microstructure is composed of α ₂/ γ lamellar structure and retained β(B2) after directional solidification. The lamellar orientation is found to be parallel and 45° to the primary growth direction of β dendrite. Peritectic reaction leads to significant chemical inhomogeneity, in which aluminum is rich in interdendritic liquid and niobium is rich in the core of β dendrite during the solidification. With the nucleation and growth of α phase, the segregation amplitude of niobium increases, which promotes the formation of B2 phase, while aluminum rich in the interdendritic becomes homogeneous gradually.