Microstructures of directionally solidified Ti-46Al-5Nb alloys

Intermetallic Ti-46Al-5Nb alloys were directionally solidified at constant growth rates (V) ranging from 10 to 200μm/s under the temperature gradient of G=3.8×10³K/m. Quenching was performed at the end of directional solidification (DS) experiments. Macrostructures and microstructures in the longitudinal section as well as the cross section have been investigated. Fully lamellar microstructures were observed in the above range of growth rates. At the growth rates of 10μm/s the primary phase was α while it changed to β when the growth rate was up to 20μm/s. Both the primary dendritic arm spacing (λ) and the interlamellar spacing (λ_s) decreased with increasing growth rate according to the relationships λ ∞ V^-0.37 and λ _s∞V^-0.39, respectively. Width of columnar grain decreased with increasing growth rate, and was uneven mainly because of the peritectic transformation in interdendritic and dendritic regions. Statistical result of the volume fraction of the lamellar microstructures in different orientations shows that, the one angled 0-30° to the growth direction has the largest volume fraction.