Hot deformation behavior and microstructure evolution of SP700 titanium alloy

The hot compression test of SP700 titanium alloy was performed using a Gleeble3800 thermal simulation test machine, and the thermal deformation behavior and microstructure evolution were examined in the temperature range of 800–880 °C, strain rate range of 1–10 s−1, and compression deformation of 30%–50%. The findings reveal that the peak flow stress of the SP700 titanium alloy decreases with increasing deformation temperature but increases with increasing strain rate. At a deformation temperature of 800 ℃, the flow stress curves demonstrate evident dynamic softening features with a rapid decrease in flow stress after the peak stress. By metallographic and scanning electron microstructure observations of the deformed microstructure, the α lamellar is gradually broken and spheroidized, and dynamic recrystallization occurs. With increasing deformation temperature, the induced phase transformation occurs, which leads to the dissolution of the α phase and an increase in the volume fraction of the β phase. The degree of recrystallization of the β phase increases with several β recrystallization grains at the grain boundaries, whereas the degree of globularization of the α lamellae decreases with increasing temperature. As the deformation temperature increases to 880 ℃, the flow stress curves exhibit steady flow. Recrystallization behavior preferentially occurs in the β grains, while the α lamellar remains flat without globularization behavior. That is, recrystallization of the β phase occurs under the test deformation conditions. For the α lamellae, when the deformation temperature is constant, the degree of spheroidization of the α lamellae increases with strain rate and compression deformation. During the hot deformation process, the α lamellae parallel to the compression axis kink, and the cumulative misorientation is discontinuous inside the α lamellae. At the discontinuous points, the new α/α interface boundary is produced, which causes the formation of unstable dihedral angles. To lower the surface tension energy, the β phase wedges into the α lamellae, which eventually results in the break of the α lamellae. For the α lamellae perpendicular to the compression axis, the interface fluctuates, resulting in continuous cumulative misorientation inside the α lamellae. When the rotation axis of the lamellae changes, a new α/α interface boundary is produced. At the interface fluctuation or the new α/α interface, the β phase easily wedges into the α lamellae by element diffusion, which finally causes fragmentation and spheroidization. Moreover, some of the α lamellae experience a shear deformation, leading to fragmentation under compression.