Hot deformation behavior of Ti-22Al-25Nb alloy by processing maps and kinetic analysis
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tudy the hot deformation behavior of the Ti–22Al–25Nb alloy, isothermal compression tests were conducted at the temperature range of 930–1080 °C with strain rates of 0.001–1.0 s1. Both the strain rate and the deformation temperature have a significant influence on the stress–strain behavior of the Ti–22Al–25Nb alloy. A hyperbolic–sine constitutive equation is established to quantitatively demonstrate the relationship between the parameters involved, and the hot deformation activation energy Q is determined as 621 kJ/mol. To optimize the processing window, a hot processing map is established, which is related to the microstructure evolution in hot working. The lamellar globularization as well as the dynamic recrystallization (DRX) would contribute to the stable regions with high power dissipation, while the adiabatic shear bands would lead to unstable regions. Moreover, an Avrami-type kinetics model is applied to examine the evolution of DRX during isothermal deformation process.
I. INTRODUCTION
Based on the ordered orthorhombic phase discovered by Banerjee et al.,1,2 the Ti2AlNb alloy exhibits great advantages over other traditional titanium–aluminum alloys in high-temperature performances like creep resistance and fatigue resistance.3,4 On the other hand, due to the brittleness and poor processability of the alloy at room temperature, the applications and the development of the Ti2AlNb alloy have been impeded. Thus, considerable attention is focused on the adjustment of the processing parameters of the Ti2AlNb alloys at elevated temperature, as well as the mechanism of the phase transformation.5–8 Thermal deformation process is an effective way to tailor microstructure and eliminate internal defects. With respect to the Ti2AlNb alloy, isothermal compression would promote lamellar globularization and dynamic recrystallization (DRX),9 which is beneficial to mechanical properties.10 Prasad et al.11 developed a dynamic material model (DMM), by which a processing map can be constructed to provide information of workability. Combining the material deformation mechanics with microstructure evolution, the material is treated as a nonlinear energy dissipation unit. According to the DMM, the stress–strain curve has a close relationship with the material’s power dissipation (g), and the processing map can be constructed from the data of the curve. As the
following equation indicates, the power dissipation consists of two complementary parts12: Z
e_
P ¼ r_e ¼ 0
Z
r
rd_e þ
e_ dr ¼ G þ J
;
ð1Þ
0
where P is the power loaded on the material, r is the flow stress, e_ is the applied strain rate, G is the major power input dissipated in the form of the temperature, and J is the dissipation through metallurgical process. In a typical titanium–aluminum alloy, DRX has been considered as one of the most important microstructural evolution mechanisms.13,14 Therefore, the significance of concerned quantitative calculation should be underlined. The DRX kinetics model is an intuitive and practical way to reveal the evolution process
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