Microstructural evolution and mechanical properties of solution annealed cast IN617B alloy during long-term thermal exposure

Cast IN617B alloy, a nickel-based superalloy, is considered as a preferential candidate for castings or valve components in developing 700 °C Advanced Ultra Supercritical (A-USC) power plant. Microstructural evolution and mechanical properties of solution annealed cast IN617B alloy during the long-term thermal exposure at 700 ℃ are systematically investigated. The results show that Ti (C, N), M6C and M23C6 are the primary precipitates in as-cast microstructure, and most of them completely dissolved after solution heat treatment. The major precipitates in the thermal exposed microstructure are M23C6, M6C and γʹ phases, and a trace amount of σ phases are also detected within grains. By comparison with previous studies on wrought IN617B alloy, it is identified that the processing method exerts a significant influence on the morphology of M23C6 carbides within grains. Moreover, the microstructure observation reveals that the growth of M23C6 facilitates the formation of γʹ by enriching γʹ-formation elements, and conversely, dense γʹ phase inhibites the coarsening and transformation of M23C6 by constraining the diffusion of M23C6-formation elements. The tensile strength of 700 °C improves strikingly due to the precipitation strengthening of γʹ phases and carbides while the ductility decreases noticeably. The significant coarsening of carbides on grain boundaries (GBs) results in the fracture mechanism evolution during long-term thermal exposure.