Hot deformation characteristics and dynamic recrystallization mechanisms of a Co-Ni-based superalloy

The hot deformation behavior of a Co-Ni-based superalloy was systematically investigated using thermal compression tests. Stress-strain curves showed a typical dynamic softening after peak stress, especially at high temperatures and low strain rates. An Arrhenius-type constitutive equation was developed to reveal the relationship between the flow stress, strain rate, and temperature, while a processing map was constructed based on the calculations from the stress-strain curves combined with microstructural observations to determine the optimum thermal deformation conditions. The extent of recrystallization was found to increase with increasing temperature, a decreasing strain rate, or an increasing strain. A complete dynamic recrystallization (DRX) condition was reached at 1050 degrees C/0.01 s(-1)/0.7. In addition, pre-existing annealing twins were replaced by discontinuous dynamic recrystallization (DDRX) grains along the twin boundaries and the twin-DRX (TDRX) grains in the twin interior. In the case of an un-twinned matrix, a combined DDRX and continuous DRX (CDRX) process occurred at high strain rates, in contrasted with a single DDRX process taking place at low strain rates.