Influence of oxygen on the α → γ massive transformation in a Ti-48 at.% Al alloy

Abstract

The structure and composition of a Ti-48 at.% Al alloy with various oxygen contents, quenched from a homogeneous ci state, have been studied by coupling a one-dimensional atom probe and transmission electron microscopy. When the global oxygen content is lower than 1 at.%, the alloys display massive y_m-phase regions and regions having a two-phase (α₂ + γ) ultrafine lamellar structure. γ_m regions are not of a single-phase nature but contain very thin α₂ plates. Analyses show that the precipitation of this phase is driven by the excess oxygen in the γ_m phase. Detailed structural analysis of the ultrafine lamellar structure reveals that it is not formed from either the α phase or the γ_m phase at high temperatures but at low temperatures from chemically ordered α₂ regions. When the oxygen content is higher than 1 at.%, only an ultrafine lamellar structure is observed, meaning that the α → γ_m massive transformation is suppressed and replaced by the α → α₂ → α₂ + γ transformations paths. Oxygen is therefore found to favour α₂ ordering, the driving force being the extreme difference of solubility of oxygen between α₂ and γ.