  Twinning in crystalline materials plays an important role in many
transformation and deformation processes, where underlying mechanisms can be
strongly influenced by the structural, energetic and kinetic properties of
associated twin boundaries (TBs). While these properties are well characterized
in common cases, the possibility that TBs can display multiple complexions with
distinct properties, and phase transitions between them, has not been widely
explored, even though such phenomena are established in a few more general
grain boundaries. We report experimental findings that {11-24} TBs in titanium
display a thick interfacial region with crystalline structure distinct from the
bulk. First-principles calculations establish that this complexion is linked to
a metastable polymorph of titanium, and exhibits behavior consistent with a
solid-state wetting transition with compressive strain, and a first-order
structural transition under tension. The findings document rich TB complexion
behavior in an elemental metal, with important implications for mechanical
behavior and phase-transformation pathways.
