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

Abstract

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 σb ~ 1460 MPa and ductility ɛf ~ 10% is achieved by tailoring hierarchical distribution of α-phase in β-matrix. This hierarchical microstructure consisting of elongated primary α-phase (αp), sub-micro α-rods (αr) and nano-scale α-platelets (αs) was produced by β-transus forging together with heat treatment. Detailed TEM analysis shows that both the αp and αr could deform plastically, resulting in dense tangled dislocations. Furthermore, dislocation features of αp show that the αp/β interface is effectively hardened whilst the entire αp presents prominent deformation capability. Meanwhile, the plastic strain is partitioned among αp, αr and β-matrix compatibly, which is beneficial to improve the ductility. As for the αs, 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. Keywords: Titanium alloys, High strength, Ductility, Hierarchical structure