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

Abstract

SummaryTwo phase titanium alloys are important for highperformance 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 (ß10 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.

Texturing and Phase Evolution in Ti-6Al-4V: Effect of Electron Beam Melting Process, Powder Re-Using, and HIP Treatment

“…Orientational analysis between β and α phases ( Figure 5 b,d,f,h) showed that the majority of β-phase demonstrate strict Burgers OR with α-phase crystallites [ 37 , 38 ]. Symmetry axis of Gauss distribution of the spectrum of misorientations is shifted by 1° from ideal BOR, while the deviations do not exceed ±3°.…”

Dislocation interactions and crack nucleation in a fatigued near-alpha titanium alloy

“…The phase map and the orientation map are also shown in Figures 13b-c. The orientation relation between secondary alpha plates α s1 , α s2 and β 1 is shown in Figure 13d The procedure for identifying these plane normals and directions is discussed in [29]. It is found that the expected (101) β ||(0001) α and [111] β ||[2110] α Burgers' orientation relation is obeyed [35].…”

“…The stage was tilted to 30 • to make the sample in the TKD holder normal to the electron beam. Detailed description of the orientation relations found by TKD can be found in Tong et al [29].…”

The Kinetics of Primary Alpha Plate Growth in Titanium Alloys

“…Where such a grain boundary film forms, growth of the Widmanstätten α colony then occurs from the grain boundary α. At higher cooling rates, a basketweave microstructure can be formed [18], where the growing α p plates branch, forming different crystallographic variants [19,20,21]. The formation of a variety of α p and α s variants is likely to be important, because slip bands can be easily transmitted between similarly oriented grains [22], particularly if the α phase is rich enough in Al that low temperature ageing precipitates the α 2 Ti 3 Al phase [23,24,25,26].…”