Hot compressive deformation behaviors of the aged nickel-based superalloy are studied under the deformation temperature range of 920-1040 oC and strain rate range of 0.001-1 1/s. Based on the experimental data, the processing maps are developed and correlated with the deformed microstructures of the studied nickel-based superalloy. The effects of initial aging time on the processing map and microstructures are discussed in detail. It is found that the processing map and microstructures are sensitive to the initial aging time. When the initial aging time is shorter than 12 h, the spherical and short needle-shaped δ phases can stimulate the occurrence of dynamic recrystallization and improve the hot workability, as well as decrease the final forging temperature of the studied nickel-based superalloy. However, when the initial aging time is increased to 24 h, the excessive long needle-shaped δ phases appear and become the potential locations of wedge cracking, which easily leads to flow instability during hot deformation. The aged superalloy under 900 oC for 9 h or 12 h is suitable for the hammer forging process. The optimum deformation parameters for hammer forging process are 1010-1040 oC and 0.1-1 1/s. The aged superalloy under 900 oC for 9 h can be used for the conventional die forging. Furthermore, the forging temperature should be controlled in the range of 980-1040 oC, and the strain rate should be lower than 0.1 1/s. The solution-treated superalloy or the aged superalloy under 900 oC for 6 h or 9 h is suitable for the isothermal die forging, and the optimum hot deformation parameters is 980-1040 oC and near 0.001 1/s.

The dynamic recrystallization (DRX) behavior of a typical nickel-based superalloy is investigated by the hot compression tests. Based on the conventional DRX kinetics model, the volume fractions of DRX are firstly estimated. Results show that there is an obvious deviation between the experimental and predicted volume fractions of DRX when the forming temperature is below 980 oC , which is induced by the slow dynamic recrystallization rate under low forming temperatures. Therefore, the segmented models are proposed to describe the kinetics of DRX for the studied superalloy. Comparisons between the experimental and predicted results indicate that the proposed segmented models can give an accurate and precise estimation of the volume fractions of DRX for the studied superalloy. In addition, the optical observation of the deformed microstructure confirms that the dynamically recrystallized grain size can be well characterized by a power function of Zener-Hollumon parameter.

Uniaxial tensile tests of a typical Ni-based superalloy are conducted under the deformation temperature range of 920-1010 oC and strain rate range of 0.01-0.001 1/s. The effects of initial δ phase ( ) on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The results show that: (1) For the studied Ni-based superalloy with a large amount of δ phase, the flow stress curves are composed of three distinct stages, i.e., work hardening stage, flow softening stage and the final fracture stage. (2) The initial δ phase has significant effects on the deformation behaviors of the studied superalloy. δ phase can cause the obvious work hardening at the beginning of hot deformation, and then accelerates the flow softening by promoting the dynamic recrystallization with further straining. With the increase of initial δ phase, the strain rate sensitivity coefficient decreases firstly and then increases. (3) The combined effects of localized necking and microvoid coalescence cause the final fracture of specimens. The increase of initial δ phase increases the density of nucleus for the formation of microvoids, and promotes the nucleation and coalescence of microvoids.

Due to their excellent properties, nickel-based superalloys are extensively used in critical parts of modern aero engine and gas turbine. The hot deformation behaviors of a typical nickel-based superalloy are investigated by hot compression tests with strain rate of (0.001-1) 1/s and forming temperature of (920-1040) oC. Results show that the flow stress is sensitive to the forming temperature and strain rate. With the increase of forming temperature or the decrease of strain rate, the flow stress decreases significantly. Under the high forming temperature and low strain rate, the flow stress-strain curves show the obvious dynamic recrystallization. Based on the stress-dislocation relation and kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of the studied nickel-based superalloy. Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformation behaviors for the studied nickel-based superalloy.

The hot compressive deformation behaviors of a typical Ni-based superalloy are investigated over wide ranges of forming temperature and strain rate. Based on the experimental data, the efficiencies of power dissipation and instability parameters are evaluated and processing maps are developed to optimize the hot working processing. The microstructures of the studied Ni-based superalloy are analyzed to correlate with the processing maps. It can be found that the flow stress is sensitive to the forming temperature and strain rate. With the increase of forming temperature or the decrease of strain rate, the flow stress significantly decreases. The changes of instability domains may be related to the adiabatic shear bands and the evolution of δ phase during the hot formation. Three optimum hot deformation domains for different forming processes (ingot cogging, conventional die forging and isothermal die forging) are identified, which are validated by the microstructural features and adiabatic shear bands. The optimum window for the ingot cogging processing is identified as the temperature range of 1010-1040 oC and strain rate range of 0.1-1 1/s. The temperature range of 980-1040 and strain rate range of 0.01-0.1 1/s can be selected for the conventional die forging. Additionally, the optimum hot working domain for the isothermal die forging is 1010-1040 oC and near/below 0.001 1/s.