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Flow behavior and dynamic recrystallization of the hot isostatically pressed superalloy EP741NP has been investigated. The isothermal compression tests at a moderate strain rate of 0.01 s−1 and sub-solvus temperatures are performed on the thermomechanical simulator. The deformed samples are analyzed by scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy. The hot isostatically pressed EP741NP alloy exhibits abnormal flow behavior with four work hardening stages when deformed below 1140 °C. Due to the preferential local strain around the previous particle boundary, the primary γʹ interacts with dislocations, stimulating local recrystallization and causing abnormal yielding during work hardening. Subsequently, the secondary γʹ phases dissolve with the advancing recrystallization front, releasing the dislocations pinned by γʹ and forming γ nuclei. However, the high solute concentration in the recrystallization front restricts the growth of new γ grains. The dissolved secondary γʹ discontinuously re-precipitates on new γ grain boundaries, forming a special γ + γʹ structure.

The influence of powder particle size on the microstructure of the powder metallurgy superalloy FGH97 was investigated. The powder atomized by the plasma rotating electrode process was sieved to three types: fine powder (0–53 μm), medium-sized powder (53–150 μm), and coarse powder (150–212 μm), which were all characterized by their nearly perfect spherical shape and low oxygen content. The powder was filled, degassed, sealed, and then densified by hot isostatic pressing. Then the microstructures of the fully dense compacts were characterized. The results show that fine powder led to limited plastic deformation of particles during densification, resulting in a minimum of Σ3 boundaries and modest PPB (Previous Particle Boundary) decoration. The fine powder compact showed that low impact toughness was caused by fracture along the PPB. However, the addition of Nb and Hf promoted the formation of the dispersed stable MC particles inside the grain, thus preventing the diffusion of C to the surface of the powder particles to form M6C carbides, and resulting in little tendency to present PPB for this alloy. Therefore, the medium-sized powder and coarse powder compact did not present PPB. Thus, the medium-sized powder is optimal for HIP compact preparation, given that fine powder produced PPB while the coarse powder increased the grain size.