The strength of dual-phase α+α' Ti-6Al-4V sheet material can be significantly enhanced through additional short-time annealing lasting only a few minutes, as shown in previous work and attributed to nano-scale microstructural changes within the martensitically transformed β-phase. However, the microstructural mechanisms remained unclear. In this study, the microstructures of the as-received state, the solution heat treated state with α+α' microstructure and additionally (short-time) annealed states were compared to provide deeper insight into these microstructural processes. Advanced high-resolution techniques, including high resolution scanning transmission electron microscopy, atom probe tomography and high-energy X-ray diffraction, were combined with tensile testing for mechanical assessment. Short-time annealing of metastable Ti-6Al-4V α+α' microstructures at 570 °C for 180 s triggered an α' → α+β transformation, comprising: (i) chemical changes, involving V- and Fe-segregation to interfaces, and the formation of V-/Fe-enriched clusters; and (ii) crystallographic decomposition, manifested by α' lattice relaxation and lattice parameter changes. With prolonged annealing, element partitioning and β precipitation progressed from the clusters and nuclei located along interfaces, accompanied by slight changes in the lattice parameters of both phases. After 3 h, the microstructure approached equilibrium, with stabilized α and β phase fractions and lattice parameters. The strengthening achieved by short-time annealing is attributed to suppression of reorientation-induced plasticity in prior α'-martensite and dislocation-cluster/precipitate/solute interactions. In summary, this work reveals microstructural evolution and processes during martensite decomposition in dual-phase Ti-6Al-4V, including β-stabilizer segregation to interfaces. Further, it discusses their role in strength enhancement, providing guidance for developing effective heat treatment and processing routes.
Pfeffer et al. (Thu,) studied this question.