  Two phase titanium alloys are important for high performance engineering
components, such as aeroengine discs. The microstructures of these alloys are
tailored during thermomechanical processing to precisely control phase
factions, morphology and crystallographic orientations. In bimodal two phase
({\alpha} + {\beta}) Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloys there are often three
microstructural lengthscales to consider: large (~10 {\mu}m) equiaxed primary
{\alpha}; >200 nm thick plate {\alpha} with a basketweave morphology; and very
fine scaled (>50 nm plate thickness) secondary {\alpha} that grows between the
larger {\alpha} plates surrounded by retained {\beta}. In this work, we utilise
high spatial resolution transmission Kikuchi diffraction (TKD, also known as
transmission based electron backscatter diffraction, t-EBSD) and scanning
electron microscopy (SEM) based forward scattering electron imaging to resolve
the structures and orientations of basketweave and secondary {\alpha} in
Ti-6242. We analyse the {\alpha} variants formed within one prior {\beta}
grain, and test whether existing theories of habit planes of the phase
transformation are upheld. Our analysis is important in understanding both the
thermomechanical processing strategy of new bimodal two-phase titanium alloys,
as well the ultimate performance of these alloys in complex loading regimes
such as dwell fatigue. Our paper champions the significant increase in spatial
resolution afforded using transmission techniques, combined with the ease of
SEM based analysis using conventional electron backscatter diffraction (EBSD)
systems and forescatter detector (FSD) imaging, to study the nanostructure of
real-world engineering alloys.
