Effect of Zener-Hollomon parameter on microstructure evolution of a HEXed PM nickel-based superalloy

Highlights: • Four nucleation mechanisms of DRX in the PM superalloy are clarified. • DDRX is the principal nucleation mechanism of DRX for all Z parameters. • Twins and γ′ phases play significant roles in the nucleation of DRX at lower Z value. • CDRX is also an important nucleation mechanism of DRX at higher Z value. -- Abstract: The microstructure evolution of a hot extrusion (HEXed) powder metallurgy (PM) nickel-based superalloy was investigated by hot compression tests under different deformation conditions. Optical microscope (OM), electron backscatter diffraction (EBSD) technique and transmission electron microscope (TEM) were applied to investigate the effect of Zener-Hollomon (Z) parameter on the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). The results show the stress-strain curves of the experimental superalloy exhibit obvious characteristics of DRX. Microstructure observations reveal that with the decrease of the Z parameter, both the degree of DRX and the size of DRX grains for the experimental superalloy gradually increase. The nucleation mechanism of DRX of the experimental superalloy is related to the Z parameter. Under the condition of lower Z value, discontinuous dynamic recrystallization (DDRX) characterized by bulging of grain boundaries is the main mechanism of DRX, while continuous dynamic recrystallization (CDRX) is considered to be an auxiliary mechanism for DRX nucleation in hot deformation. The existence of twins and γ' phase also has a major effect on the DRX nucleation mechanism of the experimental superalloy. Under the condition of higher Z value, CDRX and DDRX occur simultaneously, while CDRX characterized by progressive subgrain rotation can also be regarded as an important nucleation mechanism of DRX.