Atomic-scale investigation on collapse mechanism of athermal ω-phase transformation in β-type titanium alloys

The collapse mechanism of athermal ω-phase (ωath) transformation in β-type titanium alloys was investigated at atomic-scale by ab initio molecular dynamics simulation and high-angle annular dark-field scanning transmission electron microscopy. Calculated pair correlation functions and structural descriptors in supercell showed the structural characteristics corresponding to ωath transformation, which were consistent with the experimental result that ωath tended to form in Mo-depleted region. The evolution of Honeycutt-Anderson pairs and collapse degrees during rapid cooling revealed the temperature dependence of ωath transformation, demonstrating a more active ωath transformation within the temperature range of 700-500K. From the electronic basis, the higher charge density differences around Mo atoms than those around Ti atoms reflected the strong Mo-Ti bonds and weak Ti-Ti bonds. A positive value of integral charge density differences for Mo-Ti bonds along <111>β direction demonstrated that the strong Mo-Ti bonds suppressed the collapse of local structure around Mo atom. In contrast, the weak Ti-Ti bonds promoted the ωath transformation in Mo-depleted region.