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 fractions, morphology and crystallographic orientations. In bimodal two phase (α + β) Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) alloys there are often three microstructural lengthscales to consider: large (∼10 μm) equiaxed primary α; >200 nm thick plate α with a basketweave morphology; and very fine scaled (<50 nm plate thickness) secondary α that grows between the larger α plates surrounded by retained β. 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 α in Ti-6242. We analyse the α variants formed within one prior β 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 as 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.
Titanium alloys are used to make jet engine parts, which operate under demanding service conditions, such as high temperatures combined with cyclic mechanical and thermal loading.
To optimise performance in these demanding environments, it is important to precisely control both grain morphologies and crystallographic orientations, especially for dual phase titanium alloys. In this paper, we study the Ti‐6Al‐2Sn‐4Zr‐2Mo (Ti‐6242) alloy where the α phase has a hexagonal close‐packed crystal structure, and the β phase has a body‐centred cubic crystal structure. In these alloys, the spatial distribution, shape and orientations of the α and β phases are related due to the processing history. We find that the α grains are present in the form of either equiaxed α grains, larger (>200 nm) plate‐shaped grains called basketweave α, or very small (<50 nm) plate‐shaped grains called secondary α.
In this paper, we exploit the new transmission Kikuchi diffraction (TKD) technique to characterise the very fine secondary α in a scanning electron microscope. Use of TKD is essential for this application, as we can map relatively large areas quickly using automated electron backscatter diffraction (EBSD) algorithms within the easy to use scanning electron microscope (SEM). The TKD technique is exciting to use for this characterisation challenge in that it affords a spatial resolution improvement of one order of magnitude compared to regular EBSD and automated orientation analysis is trivial in comparison to techniques available within the transmission electron microscope (TEM).
Repeat mapping of large areas of the fine scale dual phase microstructure is important, as we want to know which orientation relationships are found in this material and how the microstructure is generated during thermomechanical processing. In particular, we are concerned with how the α‐phase grains form with respect to the parent β grain, and the size, shape and orientations of each ‘daughter’ α grains. Using mapping of a large number of daughters within this region, focussing on both the large scale and fine scale α reveals that all 12 daughter grains are formed and that there is a particular habit plane (the intersecting plane between α and β) which is reasonable for all the variants. This area of variant selection analysis, afforded through automated mapping of crystal orientations at a very fine lengthscale, is critical to inform the next generation of aerospace alloys.
Keywords: forescatter electron imaging; metallurgy; microscopy; titanium; transmission Kikuchi diffraction; variant selection.
© 2017 The Authors. Journal of Microscopy published by JohnWiley & Sons Ltd on behalf of Royal Microscopical Society.