Using transmission Kikuchi diffraction to characterise α variants in an α + β titanium alloy

Abstract: 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.