Study on Rheological Stress of Titanium Elbow under Different Thermal Deformations

At present, the design concept of aerospace structural materials is gradually shifting from simple static strength design to modern damage tolerance design. It requires that the titanium elbows, under certain strength conditions, should also have high fracture toughness and low fatigue crack growth rate. TC4-DT titanium alloy is a new type of damage tolerance titanium alloy independently developed by China under this concept. Currently, the research on TC4-DT titanium alloy mainly focuses on its damage tolerance performance, while there is less research on its hot forming behavior. Since the microstructure has a significant impact on damage tolerance performance, it is meaningful to study the deformation mechanism of TC4-DT titanium alloy under high-temperature conditions. This paper mainly studies the effects of deformation temperature, strain rate and deformation degree on rheological stress and microstructure during the hot compression deformation of TC4-DT titanium alloy, establishes the Arrhenius-type thermal deformation constitutive equation of titanium alloy, and analyzes the dynamic recrystallization behavior, providing theoretical reference for actual production.

Through relevant experiments, from the true stress - true strain curves of the TC4-DT titanium elbow alloy under different thermal deformation conditions, it can be seen that in the initial stage of deformation, the titanium alloy exhibits a work hardening effect. The rheological stress increases at a faster rate as the strain increases, and the rheological stress reaches its peak at a very small strain. Then, the softening mechanism takes the dominant position. The rheological softening of the stress becomes more obvious at lower strain rates. The deformation resistance of the titanium alloy decreases with the increase in temperature. At lower temperatures (such as 850°C and 900°C), the stress softening gradually decreases with the increase in strain, and the softening phenomenon occurs. Additionally, the phenomenon is quite obvious at high strain rates. After the stress peak, the rheological stress decreases at a slower rate as the strain increases, and the decrease in rheological stress tends to be more gentle when the strain reaches a certain level. When the deformation temperature is 950°C and 1000°C and the strain rate is lower than 10s-1, the rheological stress fluctuates in a steady sawtooth pattern, indicating a continuous softening process. When the deformation temperature is 950°C and 1000°C and the strain rate is 10s-1, the stress increases with the strain, indicating that the work hardening still dominates.

The experimental measurement of the thermal excitation force of the TC4-DT titanium elbow alloy was 971.67 kJ·mol⁻¹, which was much greater than the self-diffusion excitation force of pure a and B titanium alloys. The reason might be related to the simultaneous phase transformation behavior during the thermal deformation. At low temperatures, there are fewer movable slip systems in the titanium alloy, and dislocations accumulate at defects such as grain boundaries, which cannot be effectively released through the diffusion-controlled recovery mechanism. This indicates that the thermal deformation of the titanium alloy is controlled by processes other than high-temperature diffusion under this condition. At the same time, observing the rheological stress curve of the titanium alloy, it was found that its change process has the characteristics of a dynamic recrystallization type curve, indicating that the dynamic recrystallization softening mechanism plays a dominant role in the thermal deformation process of the alloy. Therefore, it is believed that dynamic recrystallization occurs during the thermal deformation of the titanium alloy.