Investigation of microstructural stability and tensile deformation behavior of a novel PM Ni-based superalloy during long-term thermal exposure

Thermal stability of powder metallurgy (PM) superalloys has a critical impact on their service life. In this study, the microstructure, tensile properties, and fracture mechanism of a novel Ni-based PM superalloy were investigated after thermal exposure at 750–850 °C for 50–2000 h. The coarsening mechanism of γ′ phase transitions from matrix-diffusion controlled to interface-diffusion controlled with an increase in exposure temperature and time. The diffusion activation energy for γ′ phase coarsening was 258.65 kJ/mol, which was primarily controlled by Al, Ti, and W diffusion. Meanwhile, the μ phase precipitated because Cr, Co, and Mo desolvated from the γ′ phase and γ matrix. When the alloy was exposed to a temperature of 800 °C for 50 h, the room temperature yield strength (YS) was 1332 MPa and the elongation was 23.5%, which was significantly better than those of the PM superalloys previously reported. After 2000 h, the YS and elongation still exceeded 1.2 GPa and 10%, respectively. The change in tensile properties arose from the increased γ′ phase size, prompting the deformation mechanism to transition from strong-coupling dislocation shearing to stacking-fault shearing and the Orowan looping mechanism. The decrease in elongation is mainly attributed to the precipitation of the μ phase at grain boundaries (GBs), which weakens the GB strength and induces premature fracture.