Investigation Of Microstructure Evolution And Thermal Deformation Behavior Of TC11Titanium Alloy

TC11titanium alloy becomes the ideal aerial material due to its low density, high specific strength, corrosion resistance and good high temperature strength. TC11also has some excellent features of engineering materials like steel and aluminum alloy etc. The product quality of TC11is influenced seriously because uniformity structure, interior cracking, fracture or crinkle are usually caused during the thermal deformation process. An investigation of thermal deformation behavior and microstructure evolution will be of importance for understanding of thermal deformation mechanism of TC11, and provide a guideline of hot processing.In this paper, TC11with α/β lamellar structure was selected. Thermal compressive tests were conducted to investigate deformation mechanism and microstructure evolution of TC11at different temperatures (from room temperature to950℃), strain rates (0.1s-1、0.01s-1、0.001s-1) and height reduction (10%～80%). The conclusions can be obtained as follows.The compressive tests at temperature from room temperature to950℃reveal that flow softening of the material began when the deformation temperature is at600℃～700℃. Peak flow stress decreases with increasing the compressive deformation temperature. The peak flow stress increases with increasing strain rate when T>700℃. The calculation of thermal activation energy indicates that the deformation at T=800℃is controlled by dislocation motion, twinning is dominant at850℃～900～and the deformation at T=950～is dominated by diffusion.The characterization of microstructure evolution reveals that inner cracking in TC11initiates more easily with increasing the deformation temperature, decreasing strain rate and height reduction. At the temperature ranging from room temperature to500～, lamellar structure was sheared locally by shear bands and some was sheared off, while at that from600to700℃, both shear banding and α/β lamella kinking occurred accompanied with fragmentation of α/β lamella. At temperature from800to950℃, a/p lamella kinking and fragmentation of α/β lamella appeared. Based on the competition between the barrier strength of α/β lamella and the stress of dislocation pile-up at the interface, the theoretical calculation demonstrates that the shear banding easily occurs at low temperature, while the α/β lamella kinking is preferred at high temperature, as observed in this investigation.The EBSD analysis of microstructures in the samples deformed at600℃and950℃shows that the microstructures deformed at600℃was stable, and the orientation relationship of α/β lamellae was not changed in shear deformation region. The Burger orientation was destroyed slightly. While the microstructure at950℃was unstable and the orientation relationship of α/β lamellae in severe shear strain region where globalization, α/β lamella kinking, fragmentation of kinks was changed greatly. The Burger orientation was destroyed severely. Statistical analysis of five slip systems indicates that the slip systems did not change at600℃deformation, on the contrary prismatic slip was extensively activated and became dominant at950℃. The analysis based on EBSD characterization proves that the flow softening is a result of the transition that hard slip system was destroyed.