Role of ω phase in the formation of extremely refined intragranular α precipitates in metastable β-titanium alloys

Coupling high-resolution characterization and phase-field microelasticity modeling, the non-conventional transformation pathway, mediated by ω precipitates, that leads to super-refined intragranular α precipitates in a metastable β Ti-alloy (Ti-5Al-5Mo-5V-3Cr, all in wt%) has been rationalized for the first time. Samples are initially solution treated above the β transus, rapidly cooled to room temperature, and then slowly heated (in the range 20°C-1 °C/min) to 600 °C. Interrupted tests, coupled with characterization involving scanning electron microscopy (SEM), transmission electron microscopy (TEM), high angle annular dark field-high resolution scanning transmission electron microscopy (HAADF-HRSTEM) and atom probe tomography (APT), have revealed the role of the ω phase on the copious nucleation of α leading to super-refined precipitation. Thus, both stress and compositional variations associated with the development of isothermal ω during heating contribute to an extra driving force for nucleation. It is also found that the interfaces between the super-refined α and the β matrix are relatively coherent, unlike the case of such interfaces in α/β Ti alloys. The experimental results have been combined with CALPHAD and phase field microelasticity modeling to assist in developing a robust notion of the factors influencing copious nucleation in these samples.