Experimental and first-principles novel insights of atomic collapsed structure and composition-driven ω-precipitates

A complex ω phase transition is often observed in titanium alloys; nevertheless, the underlying knowledge of ω-embrittlement are not yet fully understood and require deeper insight. In this study, the ωT and ωH phases, as well as their interfacial structures, have been systematically investigated to examine compositional partitioning and to estimate Z values. A combination of aberration-corrected HAADF-S/TEM advanced characterization techniques, together with first-principles density functional theory calculations were performed under various heat treatment conditions. The results revealed that the low-temperature ageing process suppresses ω-assisted secondary α precipitations at ωT/β interfaces, whereas high-temperature ageing process promotes ω-assisted secondary α precipitations at ωH/β interface, due to relatively higher concentrations of Mo and Al contents. The interfacial energies (γ{ωβ}) between β and ω phases increase with decreasing local compositions of Mo and Al, suggesting increased stability of the ω phase and decreased stability of the β phase. Furthermore, the formation energies of β and ω phases were computed with Mo content at different Al composition, results indicate that the ω phase transition process is affected by the reduced Al content. The observations of this work provide deeper understanding of the ω phase transition mechanism in titanium alloys.