Experimental evidence of concurrent compositional and structural instabilities leading to ω precipitation in titanium-molybdenum alloys

The ω phase is commonly observed in many commercial β or near-β titanium alloys on rapidly cooling from the single β phase field and also during subsequent isothermal annealing. However, the crystallographic formation mechanism for the ω particles is hitherto unclear/under discussion. The present study primarily focuses on ω precipitation within the β (body-centered cubic (bcc)) matrix of simple model binary titanium-molybdenum (Ti-Mo) alloys. It provides direct experimental evidence of the formation of ω-like embryos from competing compositional and structural instabilities arising in the bcc lattice of Ti-Mo alloys during rapid cooling from the high-temperature single β phase field. The displacive partial collapse of the {1 1 1} planes of the parent bcc structure within compositionally phase-separated regions containing several at.% less of Mo, forming ω-like embryos, has been conclusively shown by coupling aberration-corrected high-resolution scanning transmission electron microscopy with atom probe tomography observations. Growth and coarsening of these ω-like embryos take place during subsequent isothermal annealing, accompanied with both a completion of the collapse of the {1 1 1} β planes leading to a fully developed ω structure as well as rejection of Mo from these precipitates, resulting in near-equilibrium compositions.