Hot Deformation Behavior of Tc21 Titanium Alloy with Lamellar Microstructure Using Constitutive Analysis and Processing Map

The hot deformation behavior of TC21 titanium alloy with lamellar microstructure was investigated with the help of isothermal compression tests at the temperature range of 760-920 °C and the strain rate range of 0.001-10 s−1. The results showed that the flow stress curves show a dynamic softening characteristic, and the flow softening extent generally increases with the decrease of strain rate, but has no obvious relationship with temperature. The constitutive model in terms of Arrhenius kinetic rate equation was established at the strain rate ranges of 0.001-0.01and 0.1-10 s-1, respectively. In these two strain rate ranges, the activation energy were determined to be 400.635 and 734.430 kJ·mol−1, and they correspond to the main deformation mechanisms of dislocation glide/climb accompanied by cross slip and twinning of α phase, respectively. By the analyses of processing maps and the observation of microstructure evolution, the regime of flow instability is about 760-855 °C, 0.01-10 s-1 and flow instability phenomena include macrocrack, adiabatic shear bands and prior β boundary cavities. The preferable parameters for forging are at the temperature and strain rate ranges (760-850 °C, 0.001-0.005 s−1) and (850-920 °C, 0.001-0.01 s−1), the main microstructure change corresponding to these two domains are the globularization of α lamella and the combination of globularization of α lamella and transformation from α phase into β phase, respectively. The maximum value of power dissipation efficiency of about 0.63 occurs at 890 °C/0.001 s−1, and the microstructure with fine and globularized α phase can be obtained by deformation near this condition.