Crystallographic ordering of Al and Sn in α-Ti

Increasing attention is being paid to α2 Ti3(Al,Sn) precipitation from the α 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 α2 precipitation have historically been difficult to observe by electron microscopy, 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 α+β alloys are taken into account, this implies that Aleq limits for future alloy design of critical rotating parts should be lowered substantially.