Tailoring multi-scale hierarchical microstructure parameters to achieve super-high strength and considerable plasticity in a dual-phase Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy

In the present study, traditional treatment was designed specially and carried out on a dual-phase Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy, and a multi-scale hierarchical microstructure differed from the ordinary microstructure of titanium alloys was obtained. During the solution-treatment at 900 ℃ for 1 h followed by furnace cooling (FC), air cooling (AC), water cooling (WC) and at 920℃for 1 h followed by WC, three equiaxed microstructures composed of primary equiaxed α phase (αp) and β phase with different fractions, and a typical bimodal microstructure containing primary αp and acicular secondary α phase (αs) were formed. During the aging-treatment at 550 ℃ for 6 h followed by AC after solution-treatment at 900 ℃ for 1 h followed by WC, a multi-scale hierarchical microstructure composed of equiaxed αp, acicular αs with a thickness of dozens of nanometers, smaller acicular α phase (αs-s) with a thickness of ten nanometers, distributed in the space of two αs, and whisker β phase (βw) in the transformed β (βt) regions, was obtained. Moreover, with the solution temperature increased to 920 ℃, it was deliberately designed that equiaxed αp was more refined, and finer and denser acicular αs-s precipitated in the alloy. Contributing to the equiaxed αp refinement and nano-scale acicular α phases, the hierarchical microstructure exhibited super-high yield strength of 1228 MPa and ultimate tensile strength of 1389 MPa. Meanwhile, the refined equiaxed αp could also offset the negative effect of acicular α phases on plasticity, and the plasticity was maintain at an considerable level of 8%.