Achieving super-high strength and acceptable plasticity for a near β-type Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy through manipulating hierarchical microstructure

In the present study, a hierarchical microstructure, which was significantly different from the traditional microstructure of titanium alloys, was prepared by specially designing the solution and aging treatment parameters of a near β-type hot-rolled Ti-4.5Mo-5.1Al-1.8Zr-1.1Sn-2.5Cr-2.9Zn alloy: (1) Firstly, in the process of solution-treatment (920 °C/1 h/WQ), a hierarchical microstructure in equiaxed primary α grains (α_p) composed of nano-scale equiaxed α grains (α_ps) and β phase embedded between α_ps was formed by controlling the diffusion rate of β stable elements in α_p regions. (2) Then, in the process of aging-treatment (550 °C/6 h/AC), a hierarchical microstructure composed of acicular secondary α phase (α_s) with a thickness of dozens of nanometers, smaller acicular α phase (α_ss) with a thickness of 10 nm, distributed in the space of two α_s, and whisker β phase (β_whisker), was observed in transformed β (β_t) regions. Comparing to the solution-treatment of 900 °C/1 h/WQ followed by the same aging-treatment, smaller α_p, a large number of nano-scale equiaxed α_ps, and denser and finer acicular α phases were found in the titanium alloy. Due to the combined strengthening effect of equiaxed α_p refinement, nano-scale equiaxed α_ps and acicular α phases, the hierarchical microstructure exhibited super-high yield strength of 1255 MPa and ultimate tensile strength of 1420 MPa. Meanwhile, the refined equiaxed α_p and the nano-scale equiaxed α_ps could offset the negative effect of acicular α phases on plasticity, hence to maintain the plasticity at an acceptable level (elongation: 6%). Such hierarchical microstructure in the titanium alloy overcame the limitation of the strength-ductility trade-off to a certain extent.