The effect of lanthanum boride on the sintering, sintered microstructure and mechanical properties of titanium and titanium alloys

An addition of ≤0.5 wt% lanthanum boride (LaB₆) to powder metallurgy commercially pure Ti (CP-Ti), Ti–6Al–4V and Ti–10V–2Fe–3Al (all in wt%) resulted in improved sintered density, substantial microstructural refinement, and noticeably increased tensile elongation. The addition of LaB₆ led to scavenging of both oxygen (O) and chlorine (Cl) from the titanium powder during sintering, evidenced by the formation of La₂O₃ and LaCl_xO_y. The pinning effect of La₂O₃, LaCl_xO_y and TiB inhibited prior-β grain growth and resulted in subsequent smaller α-laths. The formation of nearly equiaxed α-Ti phase is partially attributed to the nucleation effect of α-Ti on TiB. The improved sintered density was caused by B from LaB₆ rather than La, while excessive formation of La₂O₃ and TiB with an addition of >0.5 wt% LaB₆ resulted in a noticeable decrease in sintered density. The improved tensile elongation with an addition of ≤0.5 wt% LaB₆ was mainly attributed to the scavenging of oxygen by LaB₆, partially assisted by the improved sintered density. However, an addition of >0.5 wt% LaB₆ led to the formation of large La₂O₃ aggregates and more brittle TiB whiskers and therefore decreased tensile elongation. Balanced scavenging of O is thus important. The optimal addition of LaB₆ was 0.5 wt% but this may change depending on the powder size of the LaB₆ to be used