Troisiéme Conférence Européenne sur les Matériaux et les Procédés Avancés
J. Phys. IV France 03 (1993) C7-383-C7-388
Various transformation modes observed in two-phase γ+α2 TiAl-based alloysA. DENQUIN and S. NAKA
Office National d'Etudes et de Recherches Aérospatiales, 29 avenue de la Division Leclerc, 92322 Châtillon cedex, France
This paper deals with various transformation modes taking place in two-phase γ+α2 TiAl-based alloys. These transformation modes are : (1) decomposition of the α phase leading to the precipitation of the γ lamellae (α→α+γ), (2) ordering reaction of the α phase (α→α2), (3) massive transformation (α→γ), (4) formation of monolithic γ grains and (5) discontinuous coarsening. By analyzing their kinetics, three types of competitions were identified between these transformation modes : (1) vs. (2), (2) vs. (3) and (4) vs. (5). The occurrence of (2) or (3) and (4) or (5) in binary alloys is strongly dependent on the alloy composition. Therefore, by examining these two competitions in transformation modes, it is possible to evaluate the influence of small additions of ternary and quaternary elements on the response to heat treatments without a precise knowledge of the phase diagrammes of complex alloy systems. Another important finding of the present study is related to the formation of the γ phase from the α phase ; this can occur through either lamellar precipitation (α→α2→α2+γ, or α→α+γ→α2+γ) or massive reaction (α→γ). The mechanisms involved during these two transformations are investigated and some of the key issues are discussed in the present paper.
© EDP Sciences 1993