Achieving Bi-Lamellar Microstructure with Both High Tensile Strength and Large Ductility in Ti-6Al-4V Alloy by Novel Thermomechanical Processing

In this study, a novel through-β-transus processing followed by intercritical annealing was designed to obtain the bi-lamellar microstructure in Ti-6Al-4V alloy with refined colony sizes, by which both tensile strength and ductility were significantly improved. The colony size obtained in the through-β-transus processing was 60 μm, much smaller than the minimum colony size of 130 μm that can be achieved in the conventional β processing. The colony refinement was attributed to the decreased size of the grain boundary α phase with increased variety of crystallographic orientations, which acted as nucleation sites for subsequent colony structures. By intercritical annealing of the lamellar microstructures in α+β two-phase region followed by water quenching, bi-lamellar microstructures composed of primary α lamellae and transformed β regions composed of fine secondary α plates were obtained, maintaining the same colony size as the lamellar precursors. The total elongation of bi-lamellar microstructure significantly improved from 3.4% to 18.6% with decreasing the colony size, while the high yield and tensile strength was independent of the colony size. SEM-EBSD characterization of the bi-lamellar microstructures at interrupted tensile strains clarified that deformation behaviors of the bi-lamellar microstructures after yielding were mainly controlled by micro-shear bands across transformed β regions, which eventually evolved into micro-cracks at higher tensile strains. It was considered that the strain compatibility accommodated by the differently aligned micro-shear bands formed within different colonies was the main reason for delaying tensile fracture in the bi-lamellar microstructure with the smaller colony size.