Oxygen induced chemical ordering and ultrafine lamellar structure formation in a Ti-48 at. Al alloy
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Oxygen induced chemical ordering and ultrafine lamellar structure formation in a Ti-48 at. % Al alloy Williams Lefebvre, Annick Loiseau1 and Alain Menand Groupe de Physique des Materiaux, UMR CNRS 6634, Institut des Matériaux de Rouen, UFR Sciences et Techniques, Technopôle du Madrillet 76801 Saint Etienne du Rouvray, FRANCE 1 Laboratoire d'Etude des Microstructures, UMR CNRS-ONERA, ONERA, BP 72, 29 av. de la Division Leclerc, 92322 Châtillon, FRANCE ABSTRACT Influence of oxygen on the microstructure development of a Ti-48 at. % Al alloy has been investigated by means of transmission electron microscopy and 1D atom probe. Oxygen is found to significantly increase the temperature of the α → α2 chemical ordering reaction. As a consequence, above a critical oxygen content, the α → α2 transformation is substituted to the α → γm massive transformation when the Ti-48 Al alloy is quenched from the single α-phase field. In such a case, our pervious work has shown that the alloy exhibits a fully (α2 + γ) ultrafine lamellar structure. The present work gives a complete description of the ultrafine lamellar structure formation which, in opposition to the classical lamellar structure formation, involves an intragranular nucleation and growth of the γ phase within the α2 matrix. INTRODUCTION Oxygen is an unavoidable interstitial impurity in γ-based titanium aluminides, insofar as it is already contained in the commercial titanium used to elaborate these intermetallic materials. The average content of oxygen in γ-based TiAl alloys is 2000 at. ppm.. Furthermore, the amount of oxygen can rapidly increase during long time exposure at high temperature. The bad influence of oxygen on mechanical properties has been demonstrated for a long time [1]. In the past, oxygen has also been supposed to play a main role in the mechanical behaviour of the γ-TiAl compound by disturbing the slip of some dislocations [2-4]. The slight amount of α2 lamellae in the twophase (α2 + γ) lamellar structure was supposed to lower the oxygen concentration within γ. However, Menand et al. shown that the γ-phase is ever saturated with oxygen in both the single phase alloys and the (α2 + γ) lamellar alloys [5]. The change of mechanical properties was later attributed to the intrinsic properties of the lamellar structure [6]. Nowadays, the most clearly identified effect of oxygen on the mechanical properties is the hardening of α2 phase [7]. Because of the high difference in the solubilities of oxygen in the α, α2 and γ phases [5, 8], this interstitial element can be supposed to have an influence on the microstructure development of two-phase (α2 + γ) alloys, especially when non equilibrium transformations are involved. In a previous study, we have been able to demonstrate that, above a critical oxygen concentration (~ 1.2 at. %), the α → γm massive transformation is suppressed when a Ti-48 at. % Al alloy (concentrations are in at. % unless otherwise mentioned) is quenched from the single α-phase field. In such a case, the Ti-48Al based alloy exhibits a f
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