Designing a hybrid microstructure of Ti-43Al-9V-0.3Y alloy and its non-equilibrium phase transition mechanism via two-step forging

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 roomtemperature 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 fi0/gamma lamellar colony, a structure with inner alpha 2/gamma and outer fi0/gamma lamellae surrounded by fi0 phase, a structure of gamma grains embedded within alpha 2/gamma lamellar colony, and some granular fi0 within gamma 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 gamma growth interface induces the elemental reaction deviating from the equilibrium phase transition alpha ->alpha 2 + gamma, and alpha -> fi (fi0) + gamma transition occurs, resulting in the formation of fi (fi0)/gamma lamellar colony. During hot forging, alpha ->alpha 2 + gamma transition occurs to generate alpha 2/gamma 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 gamma lamellae increases to similar to 18.41 %, and alpha -> fi (fi0) + gamma transition occurs, so fi (fi0)/gamma lamellae are formed outside the alpha 2/gamma lamellar colony. In the final stage (III), the remaining alpha phase is less, and the diffusion of the V element is hindered, causing a sudden increase of the V element in alpha phase, resulting in the remaining alpha phase transformed into irregular fi (fi0) phase. Finally, the structure with inner alpha 2/gamma and outer fi0/gamma lamellae surrounded by fi0 phase is formed. Moreover, adjusting the cooling rate realizes the precise controlling of the alpha 2/gamma, fi0/gamma lamellar size and content of irregular fi0 phase based on the solute element distribution equation. Additionally, the structure of gamma grain embedded within alpha 2/gamma lamellar colony is obtained. fi (fi0) grains nucleate and grow within alpha 2/gamma lamellar colony through alpha 2 + gamma -> fi (fi0) + gamma phase transition and the coarse alpha 2 lamellae are decomposed into fine alpha 2 and gamma lamellae in parallel. Then, fi (fi0) ->gamma phase transition occurs, resulting in the formation of gamma grains. Finally, the structure of gamma grains embedded within alpha 2/gamma lamellar colony is formed, and some fi (fi0) phases are mixed. This work clearly reveals the mystery of various complex phase transition processes and results in fi-gamma 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. (c) 2024 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.