Suppression of the massive transformation in a Ti-48 at. % Al alloy:: A direct consequence of the oxygen induced chemical ordering of the α-phase

Structure and composition of alloys with a Ti-48 at. % Al base composition and various oxygen contents, quenched from a homogeneous cc state have been studied by coupling TEM (Transmission Electron Microscopy) and atom probe techniques (1DAP: one dimensional atom probe and TAP: tomographic atom probe). In alloys containing oxygen contents lower than 1.2 at. %, alloys display massive gamma(m) structure regions and regions having a two-phase (alpha(2) + gamma) ultrafine lamellar structure. gamma(m) regions contain very thin alpha(2) plates. These plates are formed according a gamma --> alpha(2) transformation involved in gamma(m) regions after the alpha --> gamma(m) massive transformation. alpha(2) precipitation in gamma(m) lowers both Ti and O concentrations in the gamma(m) structure. Ti and O in excess within gamma(m) are hence found to favour alpha(2) precipitation. Within ultrafine lamellar structure regions, oxygen is concentrated in alpha(2) lamellae and is found to be responsible for the high volume fraction of alpha(2)-phase. Above a critical oxygen content (similar to1.2 at. %), the alloy exhibit a fully (alpha(2) + gamma) ultrafine lamellar structure. Analysis of the classical lamellar structure formed within the (alpha + gamma) dual-phase field shows that the To line for the alpha --> alpha(2) chemical ordering reaction is shifted towards high temperature when oxygen content is increased. It has therefore been inferred that, above 1.2 at. % the alpha --> gamma(m) massive transformation is suppressed and replaced by the alpha --> alpha(2) --> alpha + gamma transformation paths. On the basis of these new results, hypotheses have been proposed to explain the divergences between former results, concerning the shape of the alpha(2) single phase field and the composition dependence of the alpha --> gamma(m) massive transformation.