Influence of oxygen on phase transformations in a Ti-48 at. pct Al alloy

The composition and structure of Ti-48 at. pct Al alloys with various oxygen contents, quenched from a homogeneous alpha state, have been studied by means of one-dimensional atom-probe (1DAP) and transmission electron microscopy (TEM) analysis. Two regimes are observed. The change from one regime to the other depends on the global oxygen content. If the oxygen content is lower than 1.2 at. pct, the alpha-->gamma(m) massive transformation is involved during the quench. The alloys, hence, exhibit massive gamma(m)-structure regions and regions having a two-phase (alpha(2) + gamma) ultrafine lamellar structure. Very thin alpha(2) plates, saturated with oxygen, are observed in gamma(m) regions. The precipitation of these alpha(2) plates is promoted by excess oxygen in the gamma(m) structure. Within ultra-fine lamellar-structure regions, oxygen is concentrated in alpha(2) lamellae (not saturated with oxygen) and is found to be responsible for the high volume fraction of alpha(2) phase. When the oxygen content is larger than 1.2 at. pct, the massive transformation is suppressed and the ultrafine lamellar structure is only observed in quenched samples. Analysis of the alpha-->alpha(2) chemical ordering in the classical lamellar structure, formed within the (alpha + gamma) dual-phase field, shows that high oxygen contents favor the chemical-ordering reaction of a phase at high temperatures (e.g., 1423 and 1523 K). It has, hence, been inferred that, above 1.2 at. pct O, the alpha-->gamma(m) massive transformation is suppressed and replaced by the alpha-->alpha(2)-->alpha + gamma transformation paths.