The effect of long-term service exposure on the stability of carbides in Co-Cr-Ni-W (X-45) superalloy

The present study was conducted to investigate the effect of long-term thermal exposure during service on the microstructure and tensile properties of Co-Cr-W alloy (known as X-45 alloy) used as the first stage nozzle in a gas turbine engine. Samples for the microstructure and tensile properties were cut from the side wall and trailing edge exposed to 700 °C and 800 °C, respectively, for various durations, i.e. 12,000 h, 16,000 h, and 18,000 h. At temperature around 800 °C, the irregular coarse M23C6 carbides precipitate at the grain boundaries and interior of the grains from the decomposition of M6C → M23C6 whereas M23C6 carbides precipitation occurs directly on the stacking faults and grain boundaries at 700 °C due to the reaction of 23 M + 6C → M23C6. The tensile results indicate the correlation of the exposure time to morphology and dispersion of carbides. Small and shallow dimples were the main fracture mode of the exposed samples to 800 °C while intergranular fracture mode was observed in the samples exposed to 700 °C.