Role of ω phase in the formation of extremely refined intragranular a precipitates in metastable ß-titanium alloys

Coupling high-resolution characterization and phase-field microelasticity modeling, the non-conventional transformation pathway, mediated by omega precipitates, that leads to super-refined intragranular a precipitates in a metastable 13 Ti-alloy(Ti-5A1-5Mo-5V-3Cr, all in wt%) has been rationalized for the first time. Samples are initially solution treated above the 13 transus, rapidly cooled to room temperature, and then slowly heated (in the range 20 degrees C-1 degrees C/min) to 600 degrees 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 omega phase on the copious nucleation of a leading to super-refined precipitation. Thus, both stress and compositional variations associated with the development of isothermal omega during heating contribute to an extra driving force for nucleation. It is also found that the interfaces between the super-refined a and the (3 matrix are relatively coherent, unlike the case of such interfaces in alpha/beta 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. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.