SUPPRESSION OF THE MASSIVE TRANSFORMATION JN A Ti-48 AT. Al ALLOY: A DIRECT CONSEQUENCE OF THE OXYGEN INDUCED CHEMICAL ORDERING OF THE α-PHASE

摘要

Structure and composition of alloys with a Ti-48 at. % Al base composition and various oxygen contents, quenched from a homogeneous α 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 γ{sub}m structure regions and regions having a two-phase (α{sub}2 + γ) ultrafine lamellar structure. γ{sub}m regions contain very thin α{sub}2 plates. These plates are formed according α γ → α{sub}2 transformation involved in γ{sub}m regions after the α → γ{sub}m massive transformation. α{sub}2 precipitation in γ{sub}m lowers both Ti and O concentrations in the γ{sub}m structure. Ti and O in excess within γ{sub}m are hence found to favour α{sub}2 precipitation. Within ultrafine lamellar structure regions, oxygen is concentrated in α{sub}2 lamellae and is found to be responsible for the high volume fraction of α{sub}2-phase. Above a critical oxygen content (～1.2 at. %), the alloy exhibit a fully (α{sub}2 + γ) ultrafine lamellar structure. Analysis of the classical lamellar structure formed within the (a + Y) dual-phase field shows that the TO line for the α→α{sub}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 α→γ{sub}m massive transformation is suppressed and replaced by the α→α{sub}2 →α+ γtransformation paths. On the basis of these new results, hypotheses have been proposed to explain the divergences between former results, concerning the shape of the α{sub}2 single phase field and the composition dependence of the a α→γ{sub}m massive transformation.