Study on the microstructural degradation and rejuvenation heat treatment of directionally solidified turbine blades

The directionally solidified Ni-based superalloy is used in gas turbine blades for advanced aircraft engines because of its high stability and favorable properties at elevated temperatures. The microstructures of superalloys degrade when they are exposed to long-term services, and the recovery of these damaged microstructures in a real turbine blade remains a considerable challenge. In this paper, the microstructural degradation in the leading edge of the serviced turbine blades and its microstructural evolution during the rejuvenation heat treatment are analyzed using scanning transmission electron microscopy. The results show that the service-exposed turbine blade suffers from significant microstructural changes, including the coalescence and coarsening of the γ′-phase, transformation of carbides, and precipitation of a topological close-packed phase. The microstructure of the γ′-phase varies between the dendritic arm and the interdendritic region, while the carbide evolution displays regional differences caused by the complex cooling patterns in blades. After the improved rejuvenation heat treatment, most degraded γ′-phases are restored in size, morphology, and volume fraction, accompanied by an increase in microhardness. Despite the variability in microstructural degradation, it is possible to recover the microstructure and microhardness of a severely overaged blade to meet the requirements of continued service.