Grain boundary α-phase precipitation and coarsening: Comparing laser powder bed fusion with as-cast Ti-6Al-4V

Grain boundary α-phase (GB-α) precipitation and coarsening behavior in Ti-6Al-4V (Ti-64) processed by laser powder bed fusion (LPBF) were investigated and compared with those in as-cast Ti-64. GB-α in LPBF processed Ti-64 tended to precipitate at the triple junctions (TJs) of the β-phase grain boundaries during the subsequent annealing process. Kinetic analysis on GB-α showed that the coarsening mechanisms varied in LPBF processed Ti-64 at different annealing temperatures (bulk diffusion at 750 °C and interface reaction at both 850 and 950 °C), while were consistent in as-cast Ti-64 (interface reaction at all the three temperatures). The inconsistent coarsening mechanisms were attributed to the GB-α curvature difference caused by the different α lamellae nucleation mechanisms. These findings could help rationalize the GB-α morphology evolution and benefit the further mechanical property manipulation in LPBF processed titanium alloys.