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 ym-phase regions and regions having a two-phase (α2 + γ) ultrafine lamellar structure. γm 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 γm phase. Detailed structural analysis of the ultrafine lamellar structure reveals that it is not formed from either the α phase or the γm 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 → γm 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.