Annealing microstructure evolution and strength-plasticity enhancement mechanism of a novel titanium alloy Ti6422.

The influence of annealing temperature (450 °C ∼ 800 °C) on the microstructure and mechanical properties of the novel α + β titanium alloy Ti-6Al-4V-2Mo-2Fe (Ti6422) designed by the authors was investigated systematically in this paper, and the strength-plasticity enhancement mechanism was addressed comprehensively. The results showed that the microstructure of the samples annealed at 450 °C ∼ 700 °C was composed of primary α p phase and β trans structure. And as the annealing temperature increased, the size of α p and α s phase, as well as, the content of β phase gradually increased. When annealed at 750 °C and 800 °C, the microstructure consisted of equiaxed α p phase and irregular β phase without α s precipitates. As the annealing temperature increased from 450 °C to 800 °C, the yield strength of the samples climbed and subsequently declined, while the elongation after fracture kept rising. Ultrahigh strength (σ y ≥ 1340 MPa, σ b ≥ 1390 MPa) and considerable plasticity (δ ≥ 11.8%) were obtained under the annealing temperature of 500 °C and 550 °C. During deformation, annealed samples with higher content and slighter size of nanoscale α s and microscale α p phase accumulated an abundance of dislocation at the α p / β and α s / β interfaces. This resulted in notable interface strengthening and improved the strength. Moreover, the considerable plasticity was attributed to the superior coordinated deformation ability of the α p phase and certain coarse α s phase, which was conducive to delaying the reduction of the work-hardening rate and thus postponing the occurrence of necking. The microscale α p phase could withstand most of the plastic deformation. While high-density dislocation entanglement and local kink occurred in certain coarse α s phase, which alleviated the strain incompatibility in the hard β trans structure. • Ti6422 alloy exhibits an improved strength-plasticity combination through simple annealing. • Ultrahigh strength of ∼1340 MPa together with considerable plasticity of ∼12% is achieved. • Notable interface strengthening is the primary source of the ultrahigh strength. • The considerable plasticity is attributed to the superior deformation ability of the α p and coarse α s phase. [ABSTRACT FROM AUTHOR]