Uncovering the deformation pathways of additive manufactured Ti-6Al-4V with engineered duplex microstructure

Directional thermal gradients, high cooling rates and stochastic powder-laser interactions during additive manufacturing (AM) result in printed Ti alloys with anisotropic properties, high strength but reduced ductility. Harnessing the inherent AM defects and using a standardized thermomechanical process, the authors designed a Ti alloy with duplex microstructure that overcomes the strength-ductility trade-off while reducing anisotropy. Here, we uncover the deformation mechanisms of the Ti-6Al-4V duplex microstructure consisting of defect-free globular α-grains and hierarchical α-laths. Evaluation of the tensile surface reveals that the deformation begins with the early saturation of work hardening in α-laths, followed by the emergence of interfacial back stress hardening, and finally the transfer of strains to globular α-grains. Plastic strains partaken by the globular α-grains prompt crystal rotations to “softer” orientations (basal, and prismatic) and generate a fine network of dislocation cells. These findings suggest the ability to push property limits of structural materials by microstructure engineering during AM.