Dominating effects of vanadium diffusion on β↔α phase transformation in LDED Ti–6Al–4V alloy revealed by multiscale in-situ electron microscopy

Additive manufacturing-driven vanadium (V) segregation fundamentally alters β↔α transformation pathways, thereby governing non-equilibrium microstructure evolution in Ti–6Al–4V alloys. Under moderate cooling rates in laser-directed energy deposition (LDED), partial V diffusion results in lamellar α structures with localized ω phase formation in the as-built Ti–6Al–4V. In situ EBSD and TEM reveal that upon heating, progressive V depletion from β-nanolayers via the Burgers orientation relationship: (0001) α //(110) β and <11 2 ¯ 0> α //<111> β to drive β→α transformation. Coarse α lamellae subsequently consume finer variants through interface migration, achieving a dual optimization of chemical homogeneity and crystallographic compatibility. At 900 °C, competitive α growth induces crystallographic tilting and surface relief, accompanied by V accumulation that promotes localized α→β reversion. Meanwhile, elemental partitioning decomposes β-nanolayers into α + ω phases, revealing an ω-mediated transformation path. This study identifies spatially heterogeneous V diffusion as the central mechanism driving phase transformations and interface dynamics in non-equilibrium AM Ti alloys.