The hot deformation behavior of a novel TNM-RE alloy (RE=Y,La,Ce) was studied using a hot simulation machine (Gleeble-3800), and microstructural evolution was also characterized. Finally, 3D forging was carried out on isothermal forging equipment. It is shown that the as-cast lamellar colony size is about 20~30 μm, which is refined by the formation of rare earth oxides and borides at grain boundaries inhibiting grain growth. The peak stress of the TNM-RE alloy deformed at 1200 ℃/0.01s-1 is about 97 MPa, which is governed by the lamellar colony size and the B2 phase. Based on microstructure observation, it is found that the lamellar is bent and elongated to coordinate plastic deformation, where dynamic recrystallization nucleates preferentially, and full dynamic recrystallization is obtained at 1220 ℃/0.01s-1. The TNM-RE alloy was forged by 3D isothermal forging method, and fine grains with a size of 10~20 μm were obtained by controlling the process parameters. The novel TNM-RE alloy shows an excellent hot workability.
In contrast to practical hot compression processes, the testing of the hot workability of TiAl alloys is usually conducted under the conditions of constant strain rates and constant temperatures. This work aims at investigating the microstructural evolution of TiAl alloys on a Gleeble-3800 thermomechanical simulator under a variable strain rate (0.1, 0.01 and 0.001 s−1) at 1200 °C. The results show that, after a holding time of 30 s, the abrupt change in the strain rate at ε = 0.3 (engineering strain) has a remarkable influence on the flow stress and dynamic recrystallization (DRX) behavior of the β-γ Ti-44Al-6Nb-1Mo-0.3 (B, Y, La, Ce) (at.%) alloy. The flow stress demonstrates a rapid decrease with a sudden reduction in the strain rate. A duplex microstructure of γ + B2/β can be obtained under a high strain rate or continuous medium strain rate. During the two-step deformation, however, both γ→α phase transformation and DRX exist, and the content of the α phase demonstrates a significant increase when the strain rate becomes lower. Finally, a fine-grained structure of γ + B2/β + α2 phases with low residual stresses can be obtained via the two-step heat treatment processes. This provides a promising approach to significantly improve the hot workability of β-γ TiAl alloys.
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