Key Takeaways & Executive Findings
- •• • The pre-deformation heat treatment reduces average lamellar colony size to 5.3% (DP) and 20% (NL) of the as-cast microstructure, directly addressing the coarse-grain bottleneck that limits mechanical performance in TiAl alloys. • • At 800°C, the DP structure achieves UTS of 537 MPa and elongation of 6.8%, representing a 44.7% increase in strength and a 518.2% increase in ductility compared to as-cast alloy, critical for high-temperature aerospace components. • • The process eliminates remnant colonies after deformation via heat treatment, ensuring microstructural homogeneity, which is essential for reliable and predictable mechanical behavior in service. • • The refinement mechanism is dominated by γ recrystallization, γ→α phase transformation, and α recrystallization, providing a scientific basis for optimizing thermomechanical processing of TiAl alloys.
Abstract
The mechanical properties of as-cast TiAl alloys are severely limited by their coarse as-cast microstructure. This study proposes a low-temperature pre-deformation heat treatment to refine the microstructure of an as-cast Ti-47Al-2Nb-2Cr alloy. The process involves pre-deformation below the eutectoid temperature followed by heat treatment in the α+γ phase field, yielding two distinct microstructures: duplex (DP) and nearly lamellar (NL). The average lamellar colony size was reduced to 5.3% and 20% of the as-cast size for DP and NL structures, respectively. Microstructural evolution analysis reveals that the decomposition of coarse lamellar colonies and the formation of fine grains are governed by γ recrystallization, γ→α phase transformation, and α recrystallization. Remnant colonies after deformation are completely eliminated by subsequent heat treatment, enhancing microstructural homogeneity and mechanical properties. The DP alloy exhibits an ultimate tensile strength (UTS) of 537 MPa and an elongation (EI) of 6.8% at 800°C, representing increases of 44.7% and 518.2% over the as-cast alloy, respectively. Analysis of crack propagation and deformation mechanisms indicates that grain refinement and lamellar structure refinement contribute to improved deformation ability. This work provides a viable pathway for optimizing as-cast TiAl alloys for high-temperature applications.
1. Introduction
TiAl alloys are prime candidates for high-temperature structural applications in aerospace, offering low density, high specific strength, and excellent oxidation resistance. However, their widespread adoption is hindered by poor room-temperature ductility and the presence of coarse, inhomogeneous as-cast microstructures that degrade mechanical properties. Conventional refinement strategies, such as trace element addition (B, C, Y) or multi-step cyclic heat treatment, have limitations: elemental additions can introduce crack-initiation sites, while cyclic heat treatments are time-consuming and often ineffective on coarse initial lamellar colonies.
This study introduces a low-temperature pre-deformation heat treatment that directly tackles the coarse microstructure bottleneck. By deforming below the eutectoid temperature and subsequently annealing in the α+γ phase field, the process promotes recrystallization and phase transformation, achieving significant grain refinement without altering alloy composition. The resulting microstructures exhibit substantially improved tensile properties at elevated temperatures, offering a practical and efficient route to enhance the performance of as-cast TiAl alloys for demanding engineering applications.
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Chuan Rong, Xin Du, Zedong Liu, Yunlu Ma, Jieren Yang, Ruirun Chen (2026). Grain refinement and improved properties of an as-cast Ti-47Al-2Cr-2Nb alloy by pre-deformation heat treatment. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3927-9
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Frequently Asked Questions
What is the specific pre-deformation temperature and strain rate used in this study, and how do they influence the final microstructure?
The paper indicates pre-deformation is performed below the eutectoid temperature, but exact parameters (temperature, strain rate) are not specified in the provided text. The resulting microstructures (DP and NL) depend on the heat treatment schedule in the α+γ phase field. For precise parameters, refer to the full paper.
How does the pre-deformation heat treatment compare to conventional multi-step cyclic heat treatment in terms of processing time and effectiveness on coarse as-cast microstructures?
The proposed method is more time-efficient and directly addresses coarse lamellar colonies, which limit cyclic heat treatment effectiveness. It achieves colony size reduction to 5.3% (DP) and 20% (NL) of as-cast, with complete elimination of remnant colonies after heat treatment, ensuring homogeneity.
What are the underlying mechanisms for grain refinement during the pre-deformation and heat treatment process?
The refinement is dominated by γ recrystallization, γ→α phase transformation, and α recrystallization. Deformation introduces defects that drive recrystallization, while heat treatment in the α+γ field promotes phase transformation and further refinement.
What are the specific tensile properties at 800°C for the DP and NL structures, and how do they compare to the as-cast alloy?
At 800°C, the DP structure exhibits UTS of 537 MPa and elongation of 6.8%, representing increases of 44.7% and 518.2% over as-cast, respectively. The NL structure's properties are not detailed in the provided text but are likely lower than DP due to coarser lamellae.
What is the industrial significance of achieving a 518.2% increase in elongation at 800°C?
This substantial ductility improvement at high temperature is critical for components subjected to thermal and mechanical stresses, such as turbine blades, where brittle failure is a major concern. Enhanced ductility allows for better damage tolerance and formability, expanding the operational envelope of TiAl alloys.
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