Compositional architecting by additive manufacturing enables hardenable titanium alloys.

Titanium alloys (Ti-alloys) often suffer from limited strain-hardening capacity (typically <250 MPa), hindering broader adoption in demanding structural applications. This study reports a Ti-alloy with ultrahigh strain-hardening capability, enabled by a unique three-dimensional compositional architecture obtained by in-situ alloying via additive manufacturing (AM) using a mixture of Mo and Ti-6Al-4V (Ti64) powders. In particular, the 3D interconnected compositional waves generate three distinct microstructural regions in as-built components: α' martensite in low-Mo regions, metastable β phase in medium-Mo regions, and stable β phase in high-Mo regions. Each region activates unique deformation mechanisms - detwinning/retwinning, stress-induced martensitic transformation, and multiple systems-involved slipping, respectively - which together contribute to a synergistic enhancement in strain-hardening. The mechanical contrast among different regions generates heterogeneous deformation-induced (HDI) stresses, prompting a progressive, stepwise increase in hardening rate. This microstructural architecture enables an exceptional hardening increment of ~557 MPa (exceedingly twice the conventional limit in Ti-alloys (<250 MPa)), along with a remarkable combination of tensile strength (1236 MPa) and ductility (uniform elongation:11.5%). This work presents a powerful AM-enabled strategy for harnessing bulk compositional modulations for designing next-generation strain-hardenable Ti-alloys.