Study on the microstructure, phase transition and hardness for the TiAl–Nb alloy design during directional solidification

Bridgman-type directional solidification experiments were conducted for the TiAl–Nb alloys. The microstructure, phase transition and the hardness were investigated for the composition design. The S- and β-segregation are observed corresponding to solute Nb enrichment in primary β/α phase during the solidification and in the retained β phase during the β → α transition respectively. The improved Nb addition can promote the L + β + α peritectic solidification to the fully β solidification in Ti–46Al alloy, and even the fully α solidification to the β solidification in Ti–50Al alloy. During the peritectic solidification the S-segregation and the further β-segregation lead to the highly solute enrichment and the inhomogeneous multi-phase structure, in which B2 phase and massive γ lamellar from the Nb enrichment in the core of dendrites and the coarse α2/γ lamellar from the Al enrichment in the interdendritic region greatly decrease the hardness in TiAl–Nb alloy, leading to the scatter mechanical properties. Finally the low-(Al, Nb) TiAl–Nb alloy with the fully β solidification is proper for the favorable mechanical property due to its homogeneous microstructure with the weak solute segregation. The solute segregation is greatly connected with the microstructure evolution during the directional solidified TiAl–Nb alloy. S-segregation leads to solute Al rich in the interdendritic liquid and Nb rich in the dendrite, which can promote the L + β + α peritectic reaction at the high growth rates. Peritectic α phase envelopes the primary β phase firstly by nucleation, and then the dissolution of primary β phase and the growth of peritectic α phase happen through the β → α transition, during which the further β segregation lead to the multi-phase structure (α2/γ lamellar and B2 phase) and the inhomogeneous solute distribution. [Display omitted] •Two types of S- and β-segregation of solute Nb are investigated in highly-Nb TiAl alloys.•The Nb addition can change the solidification process greatly in TiAl–Nb alloy.•The S- and β-segregation lead to the inhomogeneous microstructure and highly solute segregation.•The low-(Al, Nb) TiAl–Nb alloy with the fully β phase solidification is proper for the good property.