Tracing the creep lifetime degradation of IN738LC superalloy based on the primary carbide decomposition reaction and composition changes in the carbide regions

IN738LC nickel-based superalloy is widely used in different high-stress and high-temperature applications. During the service exposure, the microstructure of this alloy undergoes various damage mechanisms such as coarsening of γ′ precipitates, formation of continuous carbide network at grain boundaries, decomposition of MC carbides, and precipitation of detrimental phases. Decomposition of primary MC carbides usually occurs during the long-term high-temperature exposure and results in the reduction of mechanical properties. In the current study, the reaction of MC carbides decomposition is investigated in the microstructure of different gas turbine blades with different heat treatment and operational history. The changes in the composition of carbide regions are analyzed for different blades and compared with their remained creep lives. Since the decomposition of MC carbides to M23C6/M6C carbides is a mostly irreversible reaction, it is less affected by heat treatment and rejuvenation processes, and thereby it provides more valuable information for lifetime assessment and damage analysis of the exposed IN738LC samples. Moreover, the results of current work suggest that analyzing the multi-phase regions around primary MC carbides can be considered as an efficient method to evaluate the microstructural and mechanical degradation of in-service components.