Analysis of hierarchical microstructural evolution in electron beam powder bed fusion Ti-6Al-4V alloys via time-of-flight neutron diffraction

Ti-6Al-4V alloys prepared via electron beam powder bed fusion (EB-PBF) generally exhibit a hierarchical microstructure consisting of columnar fl-grains, an acicular a-matrix, and nanoscale fl-precipitates at a-lath interfaces. In this study, we performed time-of-flight neutron diffraction measurements to investigate the texture and volume fraction of EB-PBF Ti-6Al-4V alloys via Rietveld texture analysis (RTA). The high-intensity neutron source enabled the analysis of the nanosized fl-phase precipitates at the a-lath interfaces. The results indicate that the a and fl-textures of the as-built specimen can be ascribed to multiple Burgers orientation relationships among the prior fl-phase, a-laths, and nanosized fl-phase precipitates, which are not clearly understood. The as-built texture was maintained during post-processing, suggesting that texture manipulation in EB-PBF fabrication is important for controlling the crystallographic orientations of the built components, even after post-processing. RTA allowed a precise quantification of the fl-phase fraction, which was found to have increased in post-processed specimens. The results would contribute to a comprehensive understanding of the microstructural evolution of such alloys, which is vital for the modeling and optimization of alloy performance.