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

Abstract

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 αp phase and βtrans structure. And as the annealing temperature increased, the size of αp and αs phase, as well as, the content of β phase gradually increased. When annealed at 750 °C and 800 °C, the microstructure consisted of equiaxed αp phase and irregular β phase without αs 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 (σy ≥ 1340 MPa, σb ≥ 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 αs and microscale αp phase accumulated an abundance of dislocation at the αp/β and αs/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 αp phase and certain coarse αs phase, which was conducive to delaying the reduction of the work-hardening rate and thus postponing the occurrence of necking. The microscale αp phase could withstand most of the plastic deformation. While high-density dislocation entanglement and local kink occurred in certain coarse αs phase, which alleviated the strain incompatibility in the hard βtrans structure.