Strength–Ductility Enhancement in Additively Manufactured Titanium Alloys via Neutral Element-Mediated Microstructural Control

To overcome the challenges of the strength-ductility trade-off in laser additively manufactured (LAM) titanium alloys, and to avoid the intragranular microstructure degradation often caused by an exclusive focus on grain equiaxialization, this work proposes a novel strategy of “grain morphology-intragranular microstructure co-optimization” guided by the addition of the neutral element Zr. Two alloys with distinct Zr contents, Ti-6Al-4V-2Cr-1Mo-xZr (Ti-6421-xZr; x = 3, 25), were developed and prepared by laser direct energy deposition (DED). The results demonstrate that Zr not only enhances the columnar-to-equiaxed transition (CET) capability and induces lattice expansion to achieve fully equiaxed fine grains, but also exhibits β-like stabilizing effects that refine and homogenize the intragranular α-laths, thereby enhancing the overall mechanical performance. Compared with the Zr-free DED Ti-6Al-4V-2Cr-1Mo (Ti-6421) alloy, the Ti-6421-3Zr alloy exhibits a ∼15% increase in yield strength (∼1025 MPa) while maintaining a comparable excellent elongation (∼15.7%), whereas the Ti-6421-25Zr alloy exhibits an ultrahigh tensile strength exceeding 1300 MPa (ultra-high strength), coupled with a substantial ∼42% enhancement in yield strength (∼1258 MPa), while maintaining a considerable ductility of ∼10.5%, demonstrating excellent strength-ductility matching in both cases. This work highlights the distinctive role of neutral Zr in enabling microstructural synergy and strength-ductility integration, offering a promising pathway for high-performance LAM titanium alloys.