Various transformation modes observed in two-phase γ+α₂ TiAl-based alloys

Abstract
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.