Influence of oxygen on the alpha ->gamma massive transformation in a Ti-48 at.% Al alloy

摘要

The structure and composition of a Ti-48 at.% Al alloy with various oxygen contents, quenched from a homogeneous alpha 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 gamma(m)-phase regions and regions having a two-phase (alpha(2) + gamma) ultrafine lamellar structure. gamma(m) regions are not of a single-phase nature but contain very thin alpha(2) plates. Analyses show that the precipitation of this phase is driven by the excess oxygen in the gamma(m) phase. Detailed structural analysis of the ultrafine lamellar structure reveals that it is not formed from either the alpha phase or the gamma(m) phase at high temperatures but at low temperatures from chemically ordered alpha(2) regions. When the oxygen content is higher than 1 at.%, only an ultrar ne lamellar structure is observed, meaning that the alpha --> gamma(m) massive transformation is suppressed and replaced by the alpha --> alpha(2) --> alpha(2) + gamma transformations paths. Oxygen is therefore found to favour a2 ordering, the driving force being the extreme difference of solubility of oxygen between alpha(2) and gamma. [References: 32]