Vacancy-mediated ω-assisted α-phase formation mechanism in titanium-molybdenum alloy

Abstract

This study investigates the mechanism of the x-to-a phase transformation in a titanium–molybdenum alloy. Using aberration-corrected high annular angle dark-field scanning transmission electron microscopy and density functional theory (DFT), the authors demonstrate that the x-to-a phase transformation is mediated by vacancy ordering. Atomic resolution Z-contrast images display high- and low-contrast regions in a single x precipitate, and it was found that the low Z-contrast regions are composed of an unknown structure, which differs significantly from the x-phase crystal structure. DFT calculations show that the vacancies could be stabilized by Mo and should preferably be assembled with ordering. The relaxed x-phase structure with ordered defects explains the intricate atomic images in the low Z-contrast regions of the x phase. It was found that this distorted x phase with defects is similar to the hexagonal a-phase structure with vacancies. It was confirmed that these vacancies could easily be occupied by Ti atoms and could form a perfect a phase.