Crystallographic ordering of Al and Sn in {\alpha}-Ti

Increasing attention is being paid to $\alpha$$_2$ Ti$_3$(Al,Sn) precipitation from the $\alpha$ phase of titanium alloys owing to its effect on slip band formation, localisation and the implications for fatigue performance in jet engine titanium. However, the early stages of $\alpha$$_2$ precipitation have historically been difficult to observe in TEM, neutron diffraction or atom probe analysis. Here, small angle X-ray scattering is used to reexamine the phase boundary in binary Ti-Al and Ti-Sn alloys with around 500 ppmw O. It is found that the phase boundaries in the literature are approximately correct, at 6.2 wt.% Al and 16.9 wt.% Sn, and that this favours the use of Al as a solid solution strengthener over Sn for ambient temperature applications. However, once O content and phase partitioning in $\alpha$+$\beta$ alloys are taken into account, this implies that Al$_{eq}$ limits for future alloy design of critical rotating parts should be lowered substantially.