Effects of boron, Hf and Ta content on boride morphology and microstructure of β-solidified γ-TiAl alloys

Boron is the most commonly used grain refining element in ?-TiAl alloys and the morphology of the borides has a significant influence on the room temperature plasticity of ?-TiAl alloys. In this paper, the effect of boron, Hf and Ta content on boride morphology and microstructure of a ?-TiAl alloy (Ti-43.5Al-4Nb-1Mo-0.5B) were investigated. The boride morphology transitions from flat to curved as boron content increases from 0.5% to 0.8%. With a further increase in boron content, the curved borides change into short rod-like and granular. Boride morphology changes from flat to curved When Hf is added. Conversely, when Ta is added, the length and width of the borides become smaller and dendritic borides with complex morphology appear. The combination of solid solution strengthening (boron, Hf and Ta), precipitation strengthening (borides) and grain refining increases the alloy's hardness. Furthermore, the flat borides are rich in Nb and Ta, whereas the curved borides are rich in Al. The present study shows that the addition of Ta can refine the size of borides, which may have a positive effect on improving the room temperature properties of the ?-TiAl alloy.