Slip vs. Twin martensite in additively manufactured Ti-6Al-4V: A grain boundary perspective

There is an ongoing debate regarding whether martensitic transformation in Ti alloys is governed by slip or twin-based mechanisms. The mechanism that predominates depends on lattice parameter changes caused by composition. For instance, CP-Ti typically under- goes slip-based transformation, while Ti-6Al-4V tends to favor twin-based transformation. However, processing conditions, particularly in additive manufacturing (AM), can introduce lattice strains and alter lattice parameters, resulting in distinct martensitic transformation mechanisms. Both mechanisms influence the variant selection and the formation of character- istic variant cluster morphologies, such as triangular (slip-based) and V-shaped (twin-based), which in turn dictate the selection of intervariant boundaries (IBs). This study explores the characteristics and network of IBs in Ti-6Al-4V for two distinct AM processes: Laser-Powder Bed Fusion (L-PBF) and Wire Laser-Direct Energy Deposition (L-DED). The sympathetic approach to variant selection is extended to the selection of IB and twin boundaries. Key features of the IB network, including triple points and dihedral angles, are examined to pro- vide a deeper understanding of the stability, configuration, and correlation of grain boundary networks with variant selection mechanisms. It is observed that V-shaped clusters dominate in L-DED, while triangular clusters are more common in L-PBF. Furthermore, L-DED ex- hibits a higher fraction of Σ13b twin boundaries than L-PBF samples. These findings suggest that martensitic transformation in L-DED is primarily twin-based, whereas in L-PBF, it is more likely to be slip-based. The sympathetic approach provides valuable insights into the martensitic transformation mechanisms in Ti alloys.