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Study on flow stress of titanium elbow under different thermal deformation

At present, the design concept of aeronautic structural materials has gradually changed from simple static strength design to modern damage tolerance design, which requires titanium elbow to have higher fracture toughness and lower fatigue crack growth rate under certain strength conditions. TC4-DT titanium alloy is a new type of damage tolerance titanium alloy developed by our country under this concept. At present, the research of TC4-DT titanium alloy mainly focuses on the damage tolerance performance, and the hot forming behavior of TC4-DT titanium alloy is less studied. Due to the great influence of microstructure on the damage tolerance, it is of great significance to study the deformation mechanism of TC4-DT titanium alloy at high temperature. In this paper, the effects of deformation temperature, strain rate and deformation degree on flow stress and microstructure during hot compression deformation of TC4-DT titanium alloy were studied, and the Arrhenius type hot deformation constitutive equation of titanium alloy was established to analyze the dynamic recrystallization behavior, providing a theoretical reference for practical production.

According to the true stress-strain curves of TC4-DT titanium elbow alloy under different thermal deformation conditions, it can be seen from the relevant experiments that work hardening effect occurs in titanium alloy at the initial deformation stage, and the flow stress increases with the increase of strain, and the flow stress reaches its peak at a very small strain. The rheological softening mechanism is more obvious than that at low strain rate, and the deformation resistance of titanium alloy decreases with increasing temperature. At lower temperatures (such as 850℃ and 900℃), the stress softening gradually decreases as the strain increases, and the softening phenomenon occurs. In addition, the phenomenon is quite obvious at high strain rate. After the stress peak, the flow stress decreases with the increase of strain, and the decline of flow stress tends to moderate when the strain reaches a certain degree. When the strain rate is lower than 10s-1 at a higher temperature (such as 950~1000℃), the flow stress fluctuates in a steady-state zigzag pattern, showing a continuous softening process. When the deformation temperature is 950% and the strain rate is 10s1 at 1000°, the stress increases with the strain, indicating that the work hardening has been dominant.

The thermal excitation force of TC4-DT titanium elbow alloy is 971.67kJ? The self-diffusion excitation force of mol- is much higher than that of pure a and B titanium alloys. The reason may be related to the phase transition during thermal deformation. At low temperature, fewer slip systems can be activated in titanium alloy, and the dislocations produce plug deposits at grain boundaries and other defects, which cannot be effectively released through the recovery mechanism controlled by diffusion, indicating that the thermal deformation of titanium alloy under such conditions is controlled by processes other than diffusion at high temperature. At the same time, the flow stress curve of titanium alloy was observed, and it was found that the dynamic recrystallization curve at low temperature showed the characteristics of dynamic recrystallization curve, indicating that the dynamic recrystallization softening mechanism played a dominant role in the thermal deformation process of titanium alloy. Therefore, it was considered that dynamic recrystallization occurred in the thermal deformation process of titanium alloy.

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