Microstructure characteristics of the directionally solidified TiAl-Nb alloys with different solidification modes

Microstructure characteristics are investigated for the TiAl-Nb alloys with four typical solidification modes, namely, single β, hypo-peritectic, hyper-peritectic and single α solidification. The fully lamellar structures is formed in both the dendrite and interdendritic regions in single β, hypo-peritectic, and hyper-peritectic solidification alloys, and the order of lamellar thickness is hyper-peritectic, hypo-peritectic and single β solidification alloys. Some γ islands originating from the prior interdendritic regions are formed by the chemical segregation in hyper-peritectic solidification alloy, but in single α solidification alloy, some fine lamellar structure only exist in the dendritic core and the granular γ grains are observed in the primary α interdendritic region. For hypo-peritectic solidification alloy, lamellar grain boundaries are almost located in the core of primary β phases. Lamellar orientation can be changed along either the boundaries of dendrites or the ridge-like Nb-segregations. However, for hyper-peritectic solidification alloy, the lamellar boundary locates in both the interdendritic region and the core of primary β dendrite. The lamellar orientation is changed mainly along the dendritic Nb-segregation, not along the dendrite boundary. Not every α2 in lamellar grain has Burgers orientation relationship with the prior β dendrite for both hypo- and hyper-peritectic solidification alloys. Several macrosegregation regions consisting of the curved coarsened lamellae are produced only in hypo-peritectic alloy. These regions may be induced by relatively weak thermo-gravitational convection (TGC) effect, which do not change the lamellar grain orientation but make a small orientation angle deviation of the α2 phase from each other in the grain.