Understanding the Fracture Mechanisms of Ni–Co–Cr-type Superalloys: Role of Precipitate Evolution and Strength Degradation

Microstructure stability directly affects the performance degradation of hot-end structural parts such as turbine discs. In this research, long-term (10–10000 h) thermal exposure experiments were conducted on a Ni–Co–Cr type superalloy at 800 °C. The mechanical properties were obtained through high-temperature (750 °C) tensile tests. Subsequently, the microstructure evolution process, including intragranular γ′ and μ phases, and the initiation and propagation of defects were systematically and thoroughly studied. The results showed that intragranular γ′ precipitates coarsened under thermal exposure. After 100 h of thermal exposure, μ phases were observed near the intergranular γ′ precipitates and grain boundaries. The size and volume fraction of the μ phase initially increased, and then remained relatively stable. Numerous cracks formed on the plane of the μ phases and intergranular γ′ precipitates. With an increase in tensile stress, these cracks interconnected and propagated along the intergranular γ′ precipitates, leading to the formation of pores. Moreover, because the size of the intragranular γ′ precipitates exceeded 80 nm, the critical resolved shear stress (CRSS) was primarily activated by the Orowan bypassing mechanism. Notably, the CRSS of dislocation motion, such as stacking fault, strong-coupled shearing, and Orowan bypassing, decreased with increasing intragranular γ′ precipitate size. Therefore, the intragranular strength also weakened and the intragranular γ′ precipitates coarsened under continuous thermal conditions. This research on precipitate evolution and failure mechanisms will contribute to a better understanding of failure mechanisms for Ni–Co–Cr-based superalloys.