Effect of strain rate on microstructure evolution of a fine-grained γ + γ' Ni-Co-base superalloy during superplasticity

The effect of strain rate on microstructure evolution of a Ni-Co-base superalloy during superplasticity was examined in this study. The experiments were conducted at 1050 °C with various strain rates. The strain rate had profound impacts on flow behavior and elongation that were connected to the characteristics of dynamic recrystallization (DRX). A comparatively high strain rate promoted the occurrence of DRX, and the elongation of >1335% prior to failure was obtained at 0.01/s. Scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) measurements were used to investigate the deformed microstructures. A 0.01/s strain rate accelerated the recovery and dissolution within the primary γ'-phase, resulting in a reduction in its volume fraction and size, which enhanced the DRX process of the γ-matrix. The primary γ'-phase partly retarded the DRX process of γ-matrix grains, which was expedited by twin boundaries. Regardless of strain rate, discontinuous-DRX (DDRX) dominated the superplastic deformation. Meanwhile, continuous-DRX (CDRX) occurrence increased slightly at 0.1/s, which had a minor effect during the superplastic deformation of the Ni-Co-base superalloy.