Fig. 1. The microstructures of GH984G alloy after standard heat treatment: (a) equiaxed grains observed by OM, (b) the carbides observed by SEM, (c, d) the confirmation of MC and M23C6 by TEM and selected area diffraction patterns (SADPs) and (e) γ′ particles observed by TEM and confirmed by SADP.

Fig. 2. γ′ morphologies after standard heat treatment (a) and thermal exposure at 700 °C for 1000 h (b), 5000 h (c), 10,000 h (d), 650 °C for 10,000 h (e) and 750 °C for 10,000 h (f).

Fig. 3. Evolution of γ′ diameter with the thermal exposure time at different temperature (a) and the linear relationship between r3- r03 and thermal exposure time (b).

Fig. 4. The relationship between the thermal exposure temperature and coarsening rate of γ′.

Fig. 5. The microstructures after thermal exposure at 650 °C (a), 700 °C (b) and 750 °C (c) for 10,000 h.

Fig. 6. The morphologies of M23C6 at grain boundaries after standard heat treatment (a) and thermal exposure at 700 °C for 1000 h (b), 5000 h (c), 10,000 h (d), 650 °C for 10,000 h (e) and 750 °C for 10,000 h (f).

Fig. 7. The impact toughness at room temperature after standard heat treatment and thermal exposure at 650 °C, 700 °C and 750 °C.

Fig. 8. The impact fractures at room temperature after standard heat treatment (a) and thermal exposure at 700 °C for 1000 h (b), 5000 h (c), 10,000 h (d), 650 °C for 10,000 h (e) and 750 °C for 10,000 h (f).

Fig. 9. The local misorientation maps of the longitudinal section near the v-notched and impact fracture surface after standard heat treatment (a) and thermal exposure at 700 °C for 1000 h (b) and 10,000 h (c).

Fig. 10. The geometrically necessary dislocation densities (ρGND) of the longitudinal section near the v-notched and impact fracture surface after standard heat treatment and thermal exposure at 700 °C.

Fig. 11. The cracks appear near the M23C6 carbides in the longitudinal section near the v-notched and impact fracture surface after thermal exposure at 700 °C for 1000 h.

Fig. 12. The relationship of the theoretical critical resolved shear stress vs. γ′ diameter (solid lines) and the yield strength vs. γ′ diameter (hollow squares).

Fig. 13. The interaction between the dislocations and γ′ particles of 700 °C tensile test in the alloy after thermal exposure at 700 °C (a) and 750 °C (b) for 10,000 h.