Abstract: Understanding the non-equilibrium phase transition mechanism is critical to controlling the transforming microstructures and thus material performance. In order to improve the problem of low room-temperature ductility of TiAl alloys with traditional microstructures, a two-step forging with an intermediate heat preservation process is proposed to prepare a hybrid microstructure via non-equilibrium phase transition in this study. This hybrid microstructure is composed of β₀/γ lamellar colony, a structure with inner α₂/γ and outer β₀/γ lamellae surrounded by β₀ phase, a structure of γ grains embedded within α₂/γ lamellar colony, and some granular β₀ within γ phase. This hybrid microstructure exhibits excellent room-temperature mechanical properties with a total elongation to failure of 2.15 % and tensile strength of 920 MPa. Furthermore, the evolution mechanisms of these various structures are analyzed from the perspective of solute element diffusion and distribution in front of the phase transition interface. Aggregation of V element in front of the γ growth interface induces the elemental reaction deviating from the equilibrium phase transition α → α₂ + γ, and α → β (β₀) + γ transition occurs, resulting in the formation of β (β₀)/γ lamellar colony. During hot forging, α → α₂ + γ transition occurs to generate α₂/γ lamellae in the initial transition stage (I) of solute diffusion. In the stable stage (II), the content of V element in front of the growth interface of γ lamellae increases to ∼18.41 %, and α → β (β₀) + γ transition occurs, so β (β₀)/γ lamellae are formed outside the α₂/γ lamellar colony. In the final stage (III), the remaining α phase is less, and the diffusion of the V element is hindered, causing a sudden increase of the V element in α phase, resulting in the remaining α phase transformed into irregular β (β₀) phase. Finally, the structure with inner α₂/γ and outer β₀/γ lamellae surrounded by β₀ phase is formed. Moreover, adjusting the cooling rate realizes the precise controlling of the α₂/γ, β₀/γ lamellar size and content of irregular β₀ phase based on the solute element distribution equation. Additionally, the structure of γ grain embedded within α₂/γ lamellar colony is obtained. β (β₀) grains nucleate and grow within α₂/γ lamellar colony through α₂ + γ → β (β₀) + γ phase transition and the coarse α₂ lamellae are decomposed into fine α₂ and γ lamellae in parallel. Then, β (β₀) → γ phase transition occurs, resulting in the formation of γ grains. Finally, the structure of γ grains embedded within α₂/γ lamellar colony is formed, and some β (β₀) phases are mixed. This work clearly reveals the mystery of various complex phase transition processes and results in β-γ TiAl alloy. Moreover, this design strategy of forging process and controlling the microstructure should be extendable to other TiAl systems and provides a promising new route to solve the low room-temperature ductility of TiAl alloy.

Key words: TiAl alloy, Hybrid microstructure, Lamellar structures, Non-equilibrium phase transition, Two-step forging