Grain refining of Er added to Ti–22Al–25Nb alloy and morphology of erbium precipitates

In order to investigate the effect of erbium (Er) on the microstructure of orthorhombic Ti₂AlNb-based alloys, four testing alloys were prepared by adding different contents of the rare earth metal Er to Ti–22Al–25Nb alloy and optical microscopy, X-ray diffraction, scanning electron microscopy, electron probe microanalysis, energy-dispersive spectrometry, and transmission electron microscopy were used. The results show that the addition of Er is capable of grain refining and the refinement effect is more obvious with increasing content of Er. The Er₂O₃ dispersoids formed by internal oxidation and Al₃Er compound particles are observed in Er-doped alloys and the number of Er precipitates is increased with increasing Er addition. It is likely that the solubility of Er in the Ti₂AlNb alloy is very low and Er precipitates tend to segregate at grain boundaries, which together with the surface activity of rare earth elements is supposed to decrease the prior B2 grain size of Ti–22Al–25Nb alloy effectively.

Rare earth element Er with a high-segregation tendency segregated at boundaries during cooling process and consequently led to the formation of Er-rich precipitates. These precipitates, acting as heterogeneous nucleation site, played an important role in refining grain size and functioning as obstacles to the migration of boundaries as well.