As-cast microstructure characteristics dependent on solidification mode in TiAl-Nb alloys

Abstract In this work, as-cast microstructure characteristics are identified for the TiAl–Nb alloys with four typical solidification modes, namely, single β (B mode), hypo-peritectic (BA mode), hyper-peritectic (BAA mode) and single α (A mode) solidification. B mode alloys are chemically more homogeneous, and they show netty white ridge β-segregation and fine equiaxed lamellar grains compared to the other three mode alloys due to the β→α solid-phase transformation. Increasing Nb addition decreases the size of primary β dendrite and thus decreases the lamellar colony size of the B mode alloy. In BA and BAA mode alloys, much higher Al-segregation and worm-like β-segregation are formed in the interdendritic and dendritic regions, respectively. The low fraction and rapid growth of primary β dendrite in liquid and the subsequent peritectic α growth are responsible for the sharp casting texture and coarse lamellar colonies. A mode alloy is characterized by the columnar colony and weak casting texture due to the rapid growth of primary α dendrite accompanied by the lack of solid-state transformation refinement. As-cast microstructure evolution during cooling that depends on the phase transition sequence in the TiAl–Nb alloys with different solidification modes will also be proposed.