Multiscale microstructure regulation and strength–ductility response of a multi-element in-situ alloyed additively manufactured Ti-6Al-4V alloy

The local plastic response of additively manufactured Ti-6Al-4V (TC4) is strongly influenced by prior-β grain morphology, the hierarchical α/β microstructure, crystallographic texture, and local deformation constraints. Here, AlCoCrFeNi_2.1 assisted multi-element in-situ alloying was employed during laser-directed energy deposition to regulate the microstructure and mechanical response of TC4. Among the investigated compositions, TC4–5 % exhibits the most favorable strength–ductility balance, with a pronounced columnar-to-equiaxed transition, reduction of the average prior-β minor-axis length from approximately 314 μm to 78 μm, α-lath refinement, modified α/β phase constitution, and nanoscale α₂-Ti₃Al precipitation. The alloy achieves a yield strength of 1180.4 MPa, an ultimate tensile strength of 1239.1 MPa, and an elongation of 11.3 %. In-situ electron backscatter diffraction reveals progressive lattice rotation, increasing low-angle grain boundary fraction, and the development of local orientation gradients during tensile deformation. At high strain, high-KAM regions become increasingly concentrated near prior-β boundaries and triple junctions. Slip-trace analysis indicates local participation of basal, prismatic, and pyramidal candidate systems in representative regions, including local pyramidal 〈c+a〉 participation, while post-deformation transmission electron microscopy observations reveals dislocation activity in both α and β regions and local deformation interactions near α/β interfaces. Semi-quantitative strengthening analysis indicates that the high yield strength arises from the combined contributions of α-lath refinement, multi-element solid-solution strengthening, dislocation strengthening, and a supplementary contribution from nanoscale α₂ precipitation. These results establish a multiscale experimental correlation between alloying induced microstructural regulation, local deformation heterogeneity, and damage localization in the optimized alloy.