Hot deformation characteristic of TC21 titanium alloy with lamellar microstructure

Abstract

The hot deformation characteristic of TC21 titanium alloy with lamellar microstructure was investigated with the help of isothermal compression tests in the temperature and strain rate ranges of 760–920 ℃ and 0.001–10 s−1. The principal feature of true stress-true strain curves is flow softening. The flow softening extent increases with decreasing strain rate, but has no obvious relationship with temperature. Based on processing map and microstructure observation, the regime of flow instability was determined to be about 760–855 ℃, 0.01–10 s−1 with flow instability phenomena including macrocrack and adiabatic shear band. The preferable parameters for forging were in the temperature and strain rate ranges of (760–850 ℃, 0.001–0.005 s−1) and (850–920 ℃, 0.001–0.01 s−1), where the microstructure with higher globularization fraction can be achieved. In terms of Arrhenius kinetic rate equation, the activation energies were determined to be 400.635 and 734.430 kJ·mol−1 in the strain rate ranges of 0.001–0.01 s−1 and 0.1–10 s−1, respectively. TEM analysis showed that different activation energies correspond to different deformation modes, resulting in different globularization mechanisms. In the strain rate range of 0.001–0.01 s−1, the deformation is characterized by dislocation motion. The globularization mechanism involves the formation of groove, localized shear and subgrain in α lamellae, followed by β phase penetrating α lamellae along groove, shear interface and subgrain boundary. In the strain rate range of 0.1–10 s−1, the deformation is characterized by twinning besides dislocation motion. The globularization mechanism involves the formation of the twins of α phase and β phase penetrating α lamellae along the twinned region.