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

The influence of annealing temperature (450 degrees C similar to 800 degrees C) on the microstructure and mechanical properties of the novel alpha+beta 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 degrees C - 700 degrees C was composed of primary alpha(p) phase and beta(trans) structure. And as the annealing temperature increased, the size of alpha(p) and alpha(s) phase, as well as, the content of beta phase gradually increased. When annealed at 750 degrees C and 800 degrees C, the microstructure consisted of equiaxed alpha(p) phase and irregular beta phase without alpha(s) precipitates. As the annealing temperature increased from 450 degrees C to 800 degrees C, the yield strength of the samples climbed and subsequently declined, while the elongation after fracture kept rising. Ultrahigh strength (sigma(y) >= 1340 MPa, sigma(b) >=; 1390 MPa) and considerable plasticity (delta >= 11.8%) were obtained under the annealing temperature of 500 degrees C and 550 degrees C. During deformation, annealed samples with higher content and slighter size of nanoscale alpha(s) and microscale alpha(p) phase accumulated an abundance of dislocation at the alpha(p)/beta and alpha s/beta 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 alpha(p) phase and certain coarse alpha(s) phase, which was conducive to delaying the reduction of the work-hardening rate and thus postponing the occurrence of necking. The microscale alpha(p) phase could withstand most of the plastic deformation. While high-density dislocation entanglement and local kink occurred in certain coarse alpha(s) phase, which alleviated the strain incompatibility in the hard beta(trans) structure.