Microstructural evolution and stress rupture behaviour of a Ni-Based wrought superalloy during thermal exposure

The microstructural evolution of a wrought Ni-based superalloy GH742 during long-term thermal exposure at 1023 K for durations up to 10,000 h was investigated. The influence of the microstructure degradation on the stress rupture properties at 923 K/823 MPa and 1023 K/510 MPa was systematically analysed. Scanning electron microscopy and electron probe microanalysis were utilised to study the material microstructures before and after the thermal exposure. The results show that small flower-like secondary γ′ precipitates changed their morphology from flower-like to cubic with rounded corners, while large flower-like secondary γ′ precipitates started to split. Tertiary spherical γ′ precipitates in the fine-equiaxed-grain region dissolved into the γ matrix. However, coarsening of tertiary γ′ precipitates occurred in large-fibrous-grain regions. The coarsening kinetics appeared to well conform to the Lifshitz–Slyozov–Wagner theory. Stress rupture tests at 923 K/823 MPa and 1023 K/510 MPa showed a sharp decrease in time to failure within the first 1,000 h of prior thermal exposure, which was then almost constant for the next 9,000 h of thermal exposure. This was attributed to the evolution of γ′ precipitates.