Synergic strength-ductility enhancement in tri-modal Ti6321 alloy via αʹ martensite transformation and partial decomposition

Conventional homogeneous microstructures with a single-scale α phase struggle to overcome the strength-ductility trade-off in near α titanium alloys. In this study, a tri-modal microstructure was achieved in the Ti-6Al-3Nb-2Zr-1Mo (Ti6321) alloy through multiple heat treatments. The resulting architecture consists of micron-scale equiaxed α (α_p) and lamellar α phases (α_l), along with nano-scale αʹ martensite and/or partially decomposed martensite (α′_dec). This multi-scale heterogeneous microstructure, exhibiting morphological and compositional variations, enables a synergistic enhancement in both strength and ductility. A tensile strength of 1091 MPa with 16.9% elongation and 991 MPa with 19.3% elongation together represent the best strength-ductility balance reported to date for Ti6321. The enhanced strength primarily originates from the significant interface strengthening arising from the nano-scale αʹ/α′_dec phases. The superior ductility stems from the improved plastic compatibility between the α phase and the αʹ/α′_dec phases, facilitated by hetero-deformation induced (HDI) strain hardening. Furthermore, the variant selection of the αʹ phase within the tri-modal microstructure was examined, revealing that the α_l phase induces the formation of αʹ into a three-variant cluster structure characterized by a 63.26°/[10‾553‾] interface relationship. This microstructural design strategy not only provides a pathway for optimizing the strength-ductility balance in near α and (α+β) titanium alloys, but also sheds light on the mechanisms governing αʹ variant selection.