Making martensitic structures strong and ductile via periodic solute segregation in an additively manufactured titanium alloy

Martensite, a common product in additively manufactured titanium alloys, is typically strong but brittle. Here, we report an additively manufactured Ti-5Cu alloy that overcomes this intrinsic trade-off, achieving an exceptional combination of high tensile strength (1130 MPa) and ductility (15% elongation) within a fully martensitic microstructure. We attribute this property synergy to a high density of nanoscale {10-11} transformation twin boundaries between martensite variants, which facilitate dislocation transmission across variant boundaries. Atomic-scale characterization and first-principles calculations reveal that rapid diffusion of Cu during cyclic thermal excursions in the laser powder bed fusion process drives periodic segregation of Cu at compression sites along these twin boundaries. This segregation reduces interfacial energy and enhances boundary stability, thereby promoting twin formation and enabling simultaneous strength and ductility. Our work demonstrates a pathway to design strong and ductile martensitic alloys via in situ solute segregation at boundaries, offering a paradigm for enhancing the mechanical performance of materials produced by additive manufacturing.