Hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy with columnar grains

Abstract

In this work, hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy prepared using directional solidification (DS) were investigated by carrying out the isothermal compression tests within the temperature range of 1080–1190 °C and the strain rate range of 0.001–1 s-1 under the true strain of 0.693. The effects of temperature and strain rate on dynamic recrystallization (DRX) were analyzed, and constitutive equations were established. The activation energies for γ+γʹ dual-phase and the γ single-phase regions of Ni–Co-based superalloy with columnar grains were 1190 kJ mol-1 and 415 kJ mol-1, respectively. The results showed that the dominant deformation mechanisms were closely related to the strain rate and temperature. DRX was accelerated at low strain rate conditions, while the DRX process was apparently sluggish at strain rates higher than 1 s-1. The undissolved γʹ phase interacted with dislocations and accelerated the DRX of the matrix during γʹ sub-solvus temperature (1080–1120 °C) deformation. Continuous dynamic recrystallization (CDRX) nucleation proved to play a dominant role. During γʹ super-solvus temperature (1150–1190 °C) deformation, the dominant dynamic softening mechanism of the alloy is discontinuous dynamic recrystallization (DDRX). Local migration of grain boundaries (GBs) causes an increase of misorientation within the deformed grains and induced the formation of subgrain boundaries near the GBs, which subsequently evolved into high angle grain boundaries (HAGBs) and participated in DDRX. Furthermore, during high-temperature deformation in Ni–Co-based superalloys, the relatively rare microtwins (MTs) were detected under compression at 1100 °C/1 s-1.