Microstructural evolution and mechanical properties of a nickel-based superalloy through long-term service

Microstructures and mechanical properties of nickel-based superalloy (GH783 alloy) bolts used in a power plant 660 MW Ultra supercritical unit were investigated. The results showed that the material's strength increased monotonically but the elongation and impact toughness both decrease as an exponential decay trend with the service time. Microstructural examinations revealed that γ′ phase within the alloy matrix coarsened significantly leading to an enhancement in strength. Meanwhile, additional γ′ precipitated gradually within the grain-boundary β phase, causing the weakening of the grain boundaries, thus a reduction in ductility. The evolution tendency here obtained could be used to approximately predict the variation tendency of mechanical properties with the service time for GH783 alloy used as bolt material.