A novel high-strength β-Ti alloy with hierarchical distribution of α-phase: The superior combination of strength and ductility

A novel high-strength metastable β-Ti alloy Ti-5Al-4Zr-8Mo-7V has been successfully designed utilizing the “d-electron theory” combined with the semi-empirical element-hardening criteria. The excellent combination of strength σ _b ~ 1460 MPa and ductility ɛ _f ~ 10% is achieved by tailoring hierarchical distribution of α-phase in β-matrix. This hierarchical microstructure consisting of elongated primary α-phase (α _p ), sub-micro α-rods (α _r ) and nano-scale α-platelets (α _s ) was produced by β-transus forging together with heat treatment. Detailed TEM analysis shows that both the α _p and α _r could deform plastically, resulting in dense tangled dislocations. Furthermore, dislocation features of α _p show that the α _p /β interface is effectively hardened whilst the entire α _p presents prominent deformation capability. Meanwhile, the plastic strain is partitioned among α _p , α _r and β-matrix compatibly, which is beneficial to improve the ductility. As for the α _s , HRTEM observation shows highly defected substructure and local lattice rotation. This highly defected substructure and nano-scale α-distribution effectively block dislocation motion and ultimately strengthen the alloy. The approach presented in this study provides a guideline for the design and fabrication of other β-Ti alloys with hierarchical structure and superior mechanical properties.