AbstractThe most efficient way to tune microstructures and mechanical properties of metallic alloys lies in designing and using athermal phase transformations. Examples are shape memory alloys and high strength steels, which together stand for 1,500 million tons annual production. In these materials, martensite formation and mechanical twinning are tuned via composition adjustment for realizing complex microstructures and beneficial mechanical properties. Here we report a new phase transformation that has the potential to widen the application window of Ti alloys, the most important structural material in aerospace design, by nanostructuring them via complexion-mediated transformation. This is a reversible martensitic transformation mechanism that leads to a final nanolaminate structure ofα″ (orthorhombic) martensite bounded with planar complexions of athermalω(a–ω, hexagonal). Both phases are crystallographically related to the parentβ(BCC) matrix. As expected from a planar complexion, thea–ωis stable only at the hetero-interface.