  Nucleation and growth of Ti$_3$Al \textalpha{}$_2$ ordered domains in
\textalpha{}-Ti--Al--X alloys were characterised using a combination of
transmission electron microscopy, atom probe tomography and small angle X-ray
scattering. Model alloys based on Ti--7Al~(wt.\%) and containing O, V and Mo
were aged at \SI{550}{\celsius} for times up to \SI{120}{\day} and the
resulting precipitate dispersions were observed at intermediate points.
Precipitates grew to around \SI{30}{\nano\metre} in size, with a volume
fraction of 6--10\% depending on tertiary solutes. Interstitial O was found to
increase the equilibrium volume fraction of \textalpha{}$_2$, while V and Mo
showed relatively little influence. Addition of any of the solutes in this
study, but most prominently Mo, was found to increase nucleation density and
decrease precipitate size and possibly coarsening rate. Coarsening can be
described by the Lifshitz-Slyozov-Wagner model, suggesting a matrix
diffusion-controlled coarsening mechanism (rather than control by interfacial
coherency). Solutionising temperature was found to affect nucleation number
density with an activation energy of $E_{\mathrm{f}} = 1.5\pm{}0.4$~eV,
supporting the hypothesis that vacancy concentration affects \textalpha{}$_2$
nucleation. The observation that all solutes increase nucleation number density
is also consistent with a vacancy-controlled nucleation mechanism.
