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

TL;DR

This study uses small angle X-ray scattering to clarify the phase boundaries of Al and Sn in titanium alloys, revealing implications for alloy design and strengthening strategies in jet engine applications.

Contribution

It provides new insights into the phase boundaries of Al and Sn in Ti alloys, informing alloy design and strengthening approaches.

Findings

Phase boundaries are approximately 6.2 wt.% Al and 16.9 wt.% Sn.

Al is preferable over Sn as a solid solution strengthener at ambient temperature.

Lower Al_eq limits are recommended for future alloy design considering oxygen content.

Abstract

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.