Equiaxed to Columnar Transition of Ti46Al8Nb0.5B Intermetallics during Directional Solidification

In this paper, we report a phenomenon that equiaxed-to-columnar transition occurs during directionally solidified Ti46Al8Nb0.5B (at%) alloy. By analyzing microstructure and composition, it is found that this transition is induced by macro-segregation during directional solidification. Macro-segregation changes the solidified primary phase from β phase to α phase, which governs the microstructural evolution. At the early stage of solidification, β phase is the primary solidification phase, resulting in the formation of fine-equiaxed grains. As solidification proceeds, Al element enrichment and Nb weakened in liquid phase causes the primary phase transforms from β phase to α phase. The continuous growth of primary α phase forms columnar grains in the later stage of solidification. Flexural strength test shows the average value of bending strength with 652 MPa at equiaxed-grain region is much higher that at columnar-grain region with 395 MPa. These results reveal that even a small change in the composition of TiAl intermetallics will have a great effect on the solidification structure and mechanical properties.