On the Influence of Athermal ω and α Phase Instabilities on the Scale of Precipitation of the α Phase in Metastable β-Ti Alloys

Abstract

In the presentation and manuscript in the International Conference on Solid-Solid Phase Transformations in Inorganic Materials (PTM 2015 conference), the authors presented the preliminary results showing that the nano-scale compositional and structural instabilities in the parent β phase in titanium alloys have significant influence on the subsequent phase transformations, such as α phase precipitation, by a combination of advanced electron microscopy and phase field simulation. The aim of the current research described in this paper has been to develop an understanding of the role of two instabilities in metastable β Ti alloys which may influence the scale of refined distributions of the α phase. The first of these involves the role of athermal ω phase on the nucleation of the α phase, and the second is the stability of the α phase itself during up-quenching experiments performed as a part of heat-treatment schedules that lead to refined distributions of this phase. The first investigation has involved heat treatments designed to ensure that the athermal ω phase is present at a temperature at which α nucleation is known, from previous research, to occur. It was found that the scale of α precipitation produced was “refined”, rather than “super-refined”. Hence, it is concluded that athermal ω plays no role in the nucleation of the α phase. The second investigation involves up-quenching of samples already containing an incipient “super-refined” distribution of the α phase to determine the stability of these distributions against such up-quenching. It has been found that the stability of the α phase is dependent on the composition, C ₀(T), i.e., the alloy composition where, for a given temperature, T, the free energies of the α and β phases are equal.