Microstructure evolution and solute segregation in directionally solidified TiAl alloys with high Nb content

TiAl-Nb alloys have been determined as the advanced direction for the development of the high temperature TiAl alloys, so being one of the advanced materials for turbines of aircraft engines and gas-burning power-generation plants. However, highly-Nb addition can lead to the complex solidification behavior of TiAl-Nb alloy and multi-phase microstructure, which is important for the mechanical properties during the alloy design. Bridgman type directional solidification experiments were conducted for Ti-46Al-(8,9,10)Nb alloy. The effect of the growth rate and Nb content on the microstructure, phase transition and microsegregation was investigated, and finally the selection diagram of the phase transition and the microstructure of the directional solidified TiAl-Nb alloy were obtained. The results show that the planar-cellular-dendritic evolution of solid-liquid interface can be observed with the increase of the growth rate. Meanwhile the fully β phase solidification changes to the peritectic solidification with the increase of the growth rate, and correspondingly the final microstructure is composed of the α₂/γ lamellar structure and a multiphase microstructure (B2 phase, α₂/γ lamellar structure) respectively. The increase of the β-stabilizer Nb content can promote the fully β phase solidification and the formation of the multiphase microstructure (B2 phase, α₂/γ lamellar structure). The contribution of the growth rate and the Nb content to the phase transition and the microstructure is connected with the solute segregation (S-segregation, β-segregation) closely. The increase of the S-segregation amplitude can easily promote the peritectic reaction, which always leads to the highly solute segregation and the concentrated distribution of plenty of B2 phase in the core of the dendrite. β-segregation is the mainly origin of the B2 phase formation, in which the Nb enrichment in the retained β phase directly determines the morphology and the dimension of the B2 phase. Finally according to the selection diagram of the solidification process and the microstructure of the Ti-46Al alloy with the growth rate and the Nb content, the high Nb content and the low growth rate during fully β solidification should be selected for the prefer microstructure with the homogeneous distribution and the low solute segregation.