1. Designing a hybrid microstructure of Ti-43Al-9V-0.3Y alloy and its non-equilibrium phase transition mechanism via two-step forging.
- Author
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Ye, Yuan, Zhang, Yu, Zhang, Shuzhi, Chen, Yuyong, and Sun, Jianfei
- Subjects
PHASE transitions ,MICROSTRUCTURE ,PHASE equilibrium ,ALLOYS ,TENSILE strength - Abstract
• The best ductility of Ti-43Al-9V-Y alloys with total elongation to failure of 2.15 % is obtained ever reported. • A new hybrid microstructure of TiAl alloy is obtained through non-equilibrium phase transition. • The formation mechanism of β 0 /γ lamellar colony and a structure with inner α 2 /γ and outer β 0 /γ lamellae are clarified from the perspective of solute element distribution in front of the phase transition interface. 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 β 0 /γ lamellar colony, a structure with inner α 2 /γ and outer β 0 /γ lamellae surrounded by β 0 phase, a structure of γ grains embedded within α 2 /γ lamellar colony, and some granular β 0 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 → α 2 + γ, and α → β (β 0) + γ transition occurs, resulting in the formation of β (β 0)/γ lamellar colony. During hot forging, α → α 2 + γ transition occurs to generate α 2 /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 (β 0) + γ transition occurs, so β (β 0)/γ lamellae are formed outside the α 2 /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 (β 0) phase. Finally, the structure with inner α 2 /γ and outer β 0 /γ lamellae surrounded by β 0 phase is formed. Moreover, adjusting the cooling rate realizes the precise controlling of the α 2 /γ, β 0 /γ lamellar size and content of irregular β 0 phase based on the solute element distribution equation. Additionally, the structure of γ grain embedded within α 2 /γ lamellar colony is obtained. β (β 0) grains nucleate and grow within α 2 /γ lamellar colony through α 2 + γ → β (β 0) + γ phase transition and the coarse α 2 lamellae are decomposed into fine α 2 and γ lamellae in parallel. Then, β (β 0) → γ phase transition occurs, resulting in the formation of γ grains. Finally, the structure of γ grains embedded within α 2 /γ lamellar colony is formed, and some β (β 0) 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. [Display omitted] [ABSTRACT FROM AUTHOR] more...
- Published
- 2024
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