In-situ alloying-enabled martensitic metastability engineering for enhanced strain hardening and strength-ductility synergy in additively manufactured titanium alloys

Insufficient strain-hardening capability remains a major limitation for high-strength titanium (Ti) alloys, often leading to plastic instability and compromised strength-ductility synergy. Here, we demonstrate that by employing in-situ alloying during additive manufacturing (AM), strong and sustained strain hardening can be achieved via tuning martensitic phase stability through an alloy-on-alloy metastability engineering strategy. Specifically, multi-component metastable β-Ti alloy was introduced as a moderated β-stabilizer carrier to tailor the martensitic metastability of additively manufactured Ti-6Al-4V across different stability regimes. The intermediate-stability alloy unlocked pronounced reorientation-induced plasticity (RIP) upon deformation, enabling an outstanding strain hardening response and resulting in an excellent combination of ultimate tensile strength (1162.0 MPa) and fracture elongation (15.8%). In-situ tensile testing indicated that the RIP-driven reconstruction was governed by the growth of mechanically favourable α′ variants at the expense of unfavourable variants. Mechanistically, this process proceeds through multiple crystallographic routes, including intervariant-boundary migration associated with a {134¯1}-type twin-related interface, deformation-twinning-assisted reorientation involving {101¯2} extension nanotwins and {101¯1} contraction nanotwins, and cross-variant twin-mediated replacement where {101¯1}-related fronts transmit into neighbouring variants. These findings establish AM-enabled martensitic metastability engineering as an effective strategy for designing advanced structural Ti alloys with sustained strain hardening and superior mechanical performance.