Optimising large-area crystal orientation mapping of nanoscale β phase in α + β titanium alloys using EBSD

Abstract

α + β titanium alloys, such as the commercially important alloy Ti-6Al-4 V (Ti64), develop complex textures during thermomechanical processing due to the allotropic phase transformation between the β and α phases. These phases are crystallographically related through the Burgers orientation relationship (BOR) and are often characterised by electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM). However, the BOR can be destroyed for the primary α in wrought Ti64, and techniques that utilise the BOR to reconstruct the β phase from the room temperature α-phase data cannot be used. Instead, the β texture must be measured from the residual, nanoscale β ligaments in the room temperature microstructure, which are challenging to index because of orientation and phase overlap in the EBSD Kikuchi patterns. In this work, the SEM-EBSD acquisition and processing parameters were systematically varied to determine how best to index the residual β in Ti64, and an experimental methodology was thus developed to measure the β-phase texture efficiently and reliably after thermomechanical processing. The best compromise for maximising indexing of the residual β was achieved with a low current (~1 nA in this case), and additional indexing was achieved by increasing the sample stage tilt (to 75° in this case) and by limited the scanning frame size. It was shown that the β-phase texture could be reliably measured with the optimised beam and tilt settings using a relatively coarse step size (3 μm), but this approach does not yield any morphological or spatially relevant microstructure data. Thus, it was proposed that numerous multi-scale scans be performed with different settings to characterise the residual β phase in T64, each optimised to acquire either bulk texture analysis or microstructure spatial and morphological detail. • Residual β in Ti64 can be detected with EBSD using carefully selected parameters. • A ~ 1 nA current was the best compromise for maximising β indexing and EBSP intensity. • Additional β indexing was also achieved by increasing the sample tilt to 75°. • Dictionary Indexing can use biased pattern solutions to boost β-phase indexing.