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 2μm/s drawing rate and fully-developed dendritic interface at 100μm/s.The angle between lamellar(γ+α2) orientation and the growth direction lay exhibited 0° or 45° at 2μm/s which showed the leading phase growth in microstructures was β phase.However,at 100μm/s,the lamellar orientation was perpendicular to the growth direction and the angle reached 90°,the leading phase changed to α phase.This phase selection process occurred at 100μm/s matched with the prediction of the theoretical calculation result of 180μm/s,based on the comparison of interface growth temperatures of α and β phase at different drawing rates in TiAl-based alloys under directional solidification.