Effect of boron on microstructure and mechanical properties of multicomponent titanium alloys

• Two high strength Ti–Nb–Cu–Co–Al–B alloys were designed.
• Microstructure consists of β-Ti dendrites and ultrafine-structured eutectic.
• Minor boron additions promote a distinct refinement of β-Ti dendrites.
• Correlation between microstructure and mechanical properties was studied.
• Minor boron additions promote a pronounced strengthening of the alloys.

Boron refinement is an established method to refine the grain structure of titanium alloys. The effect of boron on microstructure of multicomponent Ti–Nb–Cu–Co–Al–B alloys was studied. These as-cast alloys exhibit a composite-like microstructure consisting of about 92 vol% of β-Ti dendrites surrounded by an ultrafine-structured eutectic composed of β-Ti and B2 TiCo. It was found that significant additions (up to 1 at%) of boron do not result in a pronounced refinement of microstructure in these alloys. However, noticeable strengthening and stiffening effects are observed for the alloys with increasing boron concentration. In particular, the yield strength of Ti–13.6Nb–6Co–5.1Cu–6.5Al (at%) improves from 1110±30 MPa to 1200±30 MPa with addition of 1 at% of boron. Additionally, increasing boron content affects the morphology of the eutectic structure causing its degradation.