Quantifying the dislocation structures of additively manufactured Ti–6Al–4V alloys using X-ray diffraction line profile analysis

Ti–6Al–4V alloy is widely used in aerospace and biomedical industries, and its preparation using additive manufacturing techniques has recently attracted considerable attention. Herein, the dislocation structures developed during electron beam and laser beam powder-bed fusion (EB-PBF and LB-PBF, respectively) of the Ti–6Al–4V alloy were quantitatively examined via X-ray diffraction (XRD) line profile analysis. The microstructures of both as-built samples were characterized, revealing fine acicular microstructures attributable to a β → α' martensitic transformation. While a fully α'-martensite matrix with a high dislocation density was formed and preserved during the LB-PBF process, the decomposition of the α'-martensite toward the thermodynamically stable α + β microstructure occurred during EB-PBF as a result of post-solidification exposure to high temperatures. Accordingly, a higher dislocation density and finer crystallite size were observed at the top cross-section from the XRD line profile analysis, suggesting that the extent of phase decomposition depended on the duration of the exposure to the elevated temperature. Nonetheless, the saturated dislocation density was as high as 10¹⁴ m⁻², where dislocation strengthening affected the overall strength of the EB-PBF specimen. Diffraction peaks of sufficient intensity that enabled the analysis of the dislocation structures in both the α (α')-matrix and the nanosized β-phase precipitates at the α (α')-laths were obtained under high-energy synchrotron radiation; this revealed that the β-phase had a much higher dislocation density than the surrounding α (α')-matrix. The enhanced dislocation accumulation in the nanosized β-phase precipitates probably reflects the elemental partitioning that occurred during post-solidification cooling. The valuable insights provided in this study are expected to promote further development of alloy preparation using additive manufacturing processes.