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 LaClxOy. The pinning effect of La2O3, LaClxOy 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.