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Effects of long-term thermal exposure on γ′ particles evolution and impact toughness in the weld joint of Nimonic 263 (N263) superalloy were deeply studied at 750 °C. Results showed that the precipitates in the weld metal were mainly composed of fine γ′ particles, bulky MC carbides, and small M 23 C 6 carbides. With the thermal exposure time increasing from 0 to 3000 h, γ′ particles in the weld metal grew up from 19.7 nm to 90.1 nm at an extremely slow rate. After being exposed for 1000 h, γ′ particles coarsened and some of them transformed into acicular η phase. At the same time, MC carbides decomposed to form η phase and γ′ particles. This dynamic transition ensured the slight reduction in impact toughness of the weld metal after the thermal exposure, which indicated the stable serving performance of N263 weld joint.

The correlated mechanism of microstructural evolution and creep failure of N263 weld joint was studied in this paper. Results show that the creep failure presented in weld metal (WM), which initiated in the solute poor area near coarsened MC and/or M 23 C 6 where Mo and Ti elements emigrated and merged with small MC/M 23 C 6 to larger ones. Moreover, the crack propagated along the primary dendrite wall and second dendrite arm in which Mo and Ti are poor due to the precipitation and coarsening of MC/M 23 C 6 , which is caused by the different partition coefficient of main alloy elements during non-equilibrium solidification. However, the transition of decomposed MC to fine η phase during creep in WM is not responsible to crack initiation and propagation.