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

This study investigates the mechanism of the ω-to-α 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 ω-to-α phase transformation is mediated by vacancy ordering. Atomic resolution Z-contrast images display high- and low-contrast regions in a single ω precipitate, and it was found that the low Z-contrast regions are composed of an unknown structure, which differs significantly from the ω-phase crystal structure. DFT calculations show that the vacancies could be stabilized by Mo and should preferably be assembled with ordering. The relaxed ω-phase structure with ordered defects explains the intricate atomic images in the low Z-contrast regions of the ω phase. It was found that this distorted ω phase with defects is similar to the hexagonal α-phase structure with vacancies. It was confirmed that these vacancies could easily be occupied by Ti atoms and could form a perfect α phase.