Evaluation of Hot Workability of Powder Metallurgy Ni-Based Superalloy with Different Initial Microstructures

The effect of the initial microstructure on the hot workability of a powder metallurgy Ni-based superalloy was investigated in the high-temperature range of 950 degrees C to 1180 degrees C and strain rate range of 0.001 to 1.0 s(-1). Six samples with different initial microstructures were fabricated by various hot isostatic pressing (HIP) conditions and subsequent treatments such as hot extrusion. The coarse-grained samples exhibited low hot workability regardless of the deformation conditions. In contrast, the hot workability of the fine-grained samples significantly varied depending on the deformation conditions. The hot workability exhibited a peak at the sub-solvus temperature of similar to 1100 degrees C and decreased at temperatures higher and lower than this temperature. In addition, the hot workability decreased monotonically with increasing the strain rate. The prior particle boundaries (PPBs) acted as cavity nucleation sites and crack paths, especially at lower temperatures and higher strain rates, resulting in early fracture and low hot workability. With decreasing the grain size, the hot workability at the peak temperature improved. The extruded sample with the smallest grain size exhibited the best hot workability, owing to the avoidance of PPB fracture and the acceleration of dynamic recrystallization.