Effect of drawing rate on microstructure evolution of Ti-47.5Al-2Cr-2Nb-0.2B alloys under directional solidification

The directionally solidified samples of Ti-47.5Al-2Cr-2Nb-0.2B alloys were successfully prepared by a near-rapid directional solidification apparatus. The interface morphologies were observed as cellular interface at 2 μm/s drawing rate and fully-developed dendritic interface at 100 μm/s. The angle between lamellar (γ+α₂) orientation and the growth direction lay exhibited 0° or 45° at 2 μm/s which showed the leading phase growth in microstructures was β phase. However, at 100 μm/s, the lamellar orientation was perpendicular to the growth direction and the angle reached 90°, the leading phase changed to a phase. This phase selection process occurred at 100 μm/s matched with the prediction of the theoretical calculation result of 180 μm/s, based on the comparison of interface growth temperatures of a and β phase at different drawing rates in TiAl-based alloys under directional solidification.